Correct some spelling mistakes

[ganeti-local] / doc / design-2.0.rst

=================
Ganeti 2.0 design
=================

This document describes the major changes in Ganeti 2.0 compared to
the 1.2 version.

The 2.0 version will constitute a rewrite of the 'core' architecture,
paving the way for additional features in future 2.x versions.

.. contents:: :depth: 3

Objective
=========

Ganeti 1.2 has many scalability issues and restrictions due to its
roots as software for managing small and 'static' clusters.

Version 2.0 will attempt to remedy first the scalability issues and
then the restrictions.

Background
==========

While Ganeti 1.2 is usable, it severely limits the flexibility of the
cluster administration and imposes a very rigid model. It has the
following main scalability issues:

- only one operation at a time on the cluster [#]_
- poor handling of node failures in the cluster
- mixing hypervisors in a cluster not allowed

It also has a number of artificial restrictions, due to historical
design:

- fixed number of disks (two) per instance
- fixed number of NICs

.. [#] Replace disks will release the lock, but this is an exception
       and not a recommended way to operate

The 2.0 version is intended to address some of these problems, and
create a more flexible code base for future developments.

Among these problems, the single-operation at a time restriction is
biggest issue with the current version of Ganeti. It is such a big
impediment in operating bigger clusters that many times one is tempted
to remove the lock just to do a simple operation like start instance
while an OS installation is running.

Scalability problems
--------------------

Ganeti 1.2 has a single global lock, which is used for all cluster
operations.  This has been painful at various times, for example:

- It is impossible for two people to efficiently interact with a cluster
  (for example for debugging) at the same time.
- When batch jobs are running it's impossible to do other work (for
  example failovers/fixes) on a cluster.

This poses scalability problems: as clusters grow in node and instance
size it's a lot more likely that operations which one could conceive
should run in parallel (for example because they happen on different
nodes) are actually stalling each other while waiting for the global
lock, without a real reason for that to happen.

One of the main causes of this global lock (beside the higher
difficulty of ensuring data consistency in a more granular lock model)
is the fact that currently there is no long-lived process in Ganeti
that can coordinate multiple operations. Each command tries to acquire
the so called *cmd* lock and when it succeeds, it takes complete
ownership of the cluster configuration and state.

Other scalability problems are due the design of the DRBD device
model, which assumed at its creation a low (one to four) number of
instances per node, which is no longer true with today's hardware.

Artificial restrictions
-----------------------

Ganeti 1.2 (and previous versions) have a fixed two-disks, one-NIC per
instance model. This is a purely artificial restrictions, but it
touches multiple areas (configuration, import/export, command line)
that it's more fitted to a major release than a minor one.

Architecture issues
-------------------

The fact that each command is a separate process that reads the
cluster state, executes the command, and saves the new state is also
an issue on big clusters where the configuration data for the cluster
begins to be non-trivial in size.

Overview
========

In order to solve the scalability problems, a rewrite of the core
design of Ganeti is required. While the cluster operations themselves
won't change (e.g. start instance will do the same things, the way
these operations are scheduled internally will change radically.

The new design will change the cluster architecture to:

.. image:: arch-2.0.png

This differs from the 1.2 architecture by the addition of the master
daemon, which will be the only entity to talk to the node daemons.


Detailed design
===============

The changes for 2.0 can be split into roughly three areas:

- core changes that affect the design of the software
- features (or restriction removals) but which do not have a wide
  impact on the design
- user-level and API-level changes which translate into differences for
  the operation of the cluster

Core changes
------------

The main changes will be switching from a per-process model to a
daemon based model, where the individual gnt-* commands will be
clients that talk to this daemon (see `Master daemon`_). This will
allow us to get rid of the global cluster lock for most operations,
having instead a per-object lock (see `Granular locking`_). Also, the
daemon will be able to queue jobs, and this will allow the individual
clients to submit jobs without waiting for them to finish, and also
see the result of old requests (see `Job Queue`_).

Beside these major changes, another 'core' change but that will not be
as visible to the users will be changing the model of object attribute
storage, and separate that into name spaces (such that an Xen PVM
instance will not have the Xen HVM parameters). This will allow future
flexibility in defining additional parameters. For more details see
`Object parameters`_.

The various changes brought in by the master daemon model and the
read-write RAPI will require changes to the cluster security; we move
away from Twisted and use HTTP(s) for intra- and extra-cluster
communications. For more details, see the security document in the
doc/ directory.

Master daemon
~~~~~~~~~~~~~

In Ganeti 2.0, we will have the following *entities*:

- the master daemon (on the master node)
- the node daemon (on all nodes)
- the command line tools (on the master node)
- the RAPI daemon (on the master node)

The master-daemon related interaction paths are:

- (CLI tools/RAPI daemon) and the master daemon, via the so called
  *LUXI* API
- the master daemon and the node daemons, via the node RPC

There are also some additional interaction paths for exceptional cases:

- CLI tools might access via SSH the nodes (for ``gnt-cluster copyfile``
  and ``gnt-cluster command``)
- master failover is a special case when a non-master node will SSH
  and do node-RPC calls to the current master

The protocol between the master daemon and the node daemons will be
changed from (Ganeti 1.2) Twisted PB (perspective broker) to HTTP(S),
using a simple PUT/GET of JSON-encoded messages. This is done due to
difficulties in working with the Twisted framework and its protocols
in a multithreaded environment, which we can overcome by using a
simpler stack (see the caveats section).

The protocol between the CLI/RAPI and the master daemon will be a
custom one (called *LUXI*): on a UNIX socket on the master node, with
rights restricted by filesystem permissions, the CLI/RAPI will talk to
the master daemon using JSON-encoded messages.

The operations supported over this internal protocol will be encoded
via a python library that will expose a simple API for its
users. Internally, the protocol will simply encode all objects in JSON
format and decode them on the receiver side.

For more details about the RAPI daemon see `Remote API changes`_, and
for the node daemon see `Node daemon changes`_.

The LUXI protocol
+++++++++++++++++

As described above, the protocol for making requests or queries to the
master daemon will be a UNIX-socket based simple RPC of JSON-encoded
messages.

The choice of UNIX was in order to get rid of the need of
authentication and authorisation inside Ganeti; for 2.0, the
permissions on the Unix socket itself will determine the access
rights.

We will have two main classes of operations over this API:

- cluster query functions
- job related functions

The cluster query functions are usually short-duration, and are the
equivalent of the ``OP_QUERY_*`` opcodes in Ganeti 1.2 (and they are
internally implemented still with these opcodes). The clients are
guaranteed to receive the response in a reasonable time via a timeout.

The job-related functions will be:

- submit job
- query job (which could also be categorized in the query-functions)
- archive job (see the job queue design doc)
- wait for job change, which allows a client to wait without polling

For more details of the actual operation list, see the `Job Queue`_.

Both requests and responses will consist of a JSON-encoded message
followed by the ``ETX`` character (ASCII decimal 3), which is not a
valid character in JSON messages and thus can serve as a message
delimiter. The contents of the messages will be a dictionary with two
fields:

:method:
  the name of the method called
:args:
  the arguments to the method, as a list (no keyword arguments allowed)

Responses will follow the same format, with the two fields being:

:success:
  a boolean denoting the success of the operation
:result:
  the actual result, or error message in case of failure

There are two special value for the result field:

- in the case that the operation failed, and this field is a list of
  length two, the client library will try to interpret is as an
  exception, the first element being the exception type and the second
  one the actual exception arguments; this will allow a simple method of
  passing Ganeti-related exception across the interface
- for the *WaitForChange* call (that waits on the server for a job to
  change status), if the result is equal to ``nochange`` instead of the
  usual result for this call (a list of changes), then the library will
  internally retry the call; this is done in order to differentiate
  internally between master daemon hung and job simply not changed

Users of the API that don't use the provided python library should
take care of the above two cases.


Master daemon implementation
++++++++++++++++++++++++++++

The daemon will be based around a main I/O thread that will wait for
new requests from the clients, and that does the setup/shutdown of the
other thread (pools).

There will two other classes of threads in the daemon:

- job processing threads, part of a thread pool, and which are
  long-lived, started at daemon startup and terminated only at shutdown
  time
- client I/O threads, which are the ones that talk the local protocol
  (LUXI) to the clients, and are short-lived

Master startup/failover
+++++++++++++++++++++++

In Ganeti 1.x there is no protection against failing over the master
to a node with stale configuration. In effect, the responsibility of
correct failovers falls on the admin. This is true both for the new
master and for when an old, offline master startup.

Since in 2.x we are extending the cluster state to cover the job queue
and have a daemon that will execute by itself the job queue, we want
to have more resilience for the master role.

The following algorithm will happen whenever a node is ready to
transition to the master role, either at startup time or at node
failover:

#. read the configuration file and parse the node list
   contained within

#. query all the nodes and make sure we obtain an agreement via
   a quorum of at least half plus one nodes for the following:

    - we have the latest configuration and job list (as
      determined by the serial number on the configuration and
      highest job ID on the job queue)

    - if we are not failing over (but just starting), the
      quorum agrees that we are the designated master

    - if any of the above is false, we prevent the current operation
      (i.e. we don't become the master)

#. at this point, the node transitions to the master role

#. for all the in-progress jobs, mark them as failed, with
   reason unknown or something similar (master failed, etc.)

Since due to exceptional conditions we could have a situation in which
no node can become the master due to inconsistent data, we will have
an override switch for the master daemon startup that will assume the
current node has the right data and will replicate all the
configuration files to the other nodes.

**Note**: the above algorithm is by no means an election algorithm; it
is a *confirmation* of the master role currently held by a node.

Logging
+++++++

The logging system will be switched completely to the standard python
logging module; currently it's logging-based, but exposes a different
API, which is just overhead. As such, the code will be switched over
to standard logging calls, and only the setup will be custom.

With this change, we will remove the separate debug/info/error logs,
and instead have always one logfile per daemon model:

- master-daemon.log for the master daemon
- node-daemon.log for the node daemon (this is the same as in 1.2)
- rapi-daemon.log for the RAPI daemon logs
- rapi-access.log, an additional log file for the RAPI that will be
  in the standard HTTP log format for possible parsing by other tools

Since the :term:`watcher` will only submit jobs to the master for
startup of the instances, its log file will contain less information
than before, mainly that it will start the instance, but not the
results.

Node daemon changes
+++++++++++++++++++

The only change to the node daemon is that, since we need better
concurrency, we don't process the inter-node RPC calls in the node
daemon itself, but we fork and process each request in a separate
child.

Since we don't have many calls, and we only fork (not exec), the
overhead should be minimal.

Caveats
+++++++

A discussed alternative is to keep the current individual processes
touching the cluster configuration model. The reasons we have not
chosen this approach is:

- the speed of reading and unserializing the cluster state
  today is not small enough that we can ignore it; the addition of
  the job queue will make the startup cost even higher. While this
  runtime cost is low, it can be on the order of a few seconds on
  bigger clusters, which for very quick commands is comparable to
  the actual duration of the computation itself

- individual commands would make it harder to implement a
  fire-and-forget job request, along the lines "start this
  instance but do not wait for it to finish"; it would require a
  model of backgrounding the operation and other things that are
  much better served by a daemon-based model

Another area of discussion is moving away from Twisted in this new
implementation. While Twisted has its advantages, there are also many
disadvantages to using it:

- first and foremost, it's not a library, but a framework; thus, if
  you use twisted, all the code needs to be 'twiste-ized' and written
  in an asynchronous manner, using deferreds; while this method works,
  it's not a common way to code and it requires that the entire process
  workflow is based around a single *reactor* (Twisted name for a main
  loop)
- the more advanced granular locking that we want to implement would
  require, if written in the async-manner, deep integration with the
  Twisted stack, to such an extend that business-logic is inseparable
  from the protocol coding; we felt that this is an unreasonable
  request, and that a good protocol library should allow complete
  separation of low-level protocol calls and business logic; by
  comparison, the threaded approach combined with HTTPs protocol
  required (for the first iteration) absolutely no changes from the 1.2
  code, and later changes for optimizing the inter-node RPC calls
  required just syntactic changes (e.g.  ``rpc.call_...`` to
  ``self.rpc.call_...``)

Another issue is with the Twisted API stability - during the Ganeti
1.x lifetime, we had to to implement many times workarounds to changes
in the Twisted version, so that for example 1.2 is able to use both
Twisted 2.x and 8.x.

In the end, since we already had an HTTP server library for the RAPI,
we just reused that for inter-node communication.


Granular locking
~~~~~~~~~~~~~~~~

We want to make sure that multiple operations can run in parallel on a
Ganeti Cluster. In order for this to happen we need to make sure
concurrently run operations don't step on each other toes and break the
cluster.

This design addresses how we are going to deal with locking so that:

- we preserve data coherency
- we prevent deadlocks
- we prevent job starvation

Reaching the maximum possible parallelism is a Non-Goal. We have
identified a set of operations that are currently bottlenecks and need
to be parallelised and have worked on those. In the future it will be
possible to address other needs, thus making the cluster more and more
parallel one step at a time.

This section only talks about parallelising Ganeti level operations, aka
Logical Units, and the locking needed for that. Any other
synchronization lock needed internally by the code is outside its scope.

Library details
+++++++++++++++

The proposed library has these features:

- internally managing all the locks, making the implementation
  transparent from their usage
- automatically grabbing multiple locks in the right order (avoid
  deadlock)
- ability to transparently handle conversion to more granularity
- support asynchronous operation (future goal)

Locking will be valid only on the master node and will not be a
distributed operation. Therefore, in case of master failure, the
operations currently running will be aborted and the locks will be
lost; it remains to the administrator to cleanup (if needed) the
operation result (e.g. make sure an instance is either installed
correctly or removed).

A corollary of this is that a master-failover operation with both
masters alive needs to happen while no operations are running, and
therefore no locks are held.

All the locks will be represented by objects (like
``lockings.SharedLock``), and the individual locks for each object
will be created at initialisation time, from the config file.

The API will have a way to grab one or more than one locks at the same
time.  Any attempt to grab a lock while already holding one in the wrong
order will be checked for, and fail.


The Locks
+++++++++

At the first stage we have decided to provide the following locks:

- One "config file" lock
- One lock per node in the cluster
- One lock per instance in the cluster

All the instance locks will need to be taken before the node locks, and
the node locks before the config lock. Locks will need to be acquired at
the same time for multiple instances and nodes, and internal ordering
will be dealt within the locking library, which, for simplicity, will
just use alphabetical order.

Each lock has the following three possible statuses:

- unlocked (anyone can grab the lock)
- shared (anyone can grab/have the lock but only in shared mode)
- exclusive (no one else can grab/have the lock)

Handling conversion to more granularity
+++++++++++++++++++++++++++++++++++++++

In order to convert to a more granular approach transparently each time
we split a lock into more we'll create a "metalock", which will depend
on those sub-locks and live for the time necessary for all the code to
convert (or forever, in some conditions). When a metalock exists all
converted code must acquire it in shared mode, so it can run
concurrently, but still be exclusive with old code, which acquires it
exclusively.

In the beginning the only such lock will be what replaces the current
"command" lock, and will acquire all the locks in the system, before
proceeding. This lock will be called the "Big Ganeti Lock" because
holding that one will avoid any other concurrent Ganeti operations.

We might also want to devise more metalocks (eg. all nodes, all
nodes+config) in order to make it easier for some parts of the code to
acquire what it needs without specifying it explicitly.

In the future things like the node locks could become metalocks, should
we decide to split them into an even more fine grained approach, but
this will probably be only after the first 2.0 version has been
released.

Adding/Removing locks
+++++++++++++++++++++

When a new instance or a new node is created an associated lock must be
added to the list. The relevant code will need to inform the locking
library of such a change.

This needs to be compatible with every other lock in the system,
especially metalocks that guarantee to grab sets of resources without
specifying them explicitly. The implementation of this will be handled
in the locking library itself.

When instances or nodes disappear from the cluster the relevant locks
must be removed. This is easier than adding new elements, as the code
which removes them must own them exclusively already, and thus deals
with metalocks exactly as normal code acquiring those locks. Any
operation queuing on a removed lock will fail after its removal.

Asynchronous operations
+++++++++++++++++++++++

For the first version the locking library will only export synchronous
operations, which will block till the needed lock are held, and only
fail if the request is impossible or somehow erroneous.

In the future we may want to implement different types of asynchronous
operations such as:

- try to acquire this lock set and fail if not possible
- try to acquire one of these lock sets and return the first one you
  were able to get (or after a timeout) (select/poll like)

These operations can be used to prioritize operations based on available
locks, rather than making them just blindly queue for acquiring them.
The inherent risk, though, is that any code using the first operation,
or setting a timeout for the second one, is susceptible to starvation
and thus may never be able to get the required locks and complete
certain tasks. Considering this providing/using these operations should
not be among our first priorities.

Locking granularity
+++++++++++++++++++

For the first version of this code we'll convert each Logical Unit to
acquire/release the locks it needs, so locking will be at the Logical
Unit level.  In the future we may want to split logical units in
independent "tasklets" with their own locking requirements. A different
design doc (or mini design doc) will cover the move from Logical Units
to tasklets.

Code examples
+++++++++++++

In general when acquiring locks we should use a code path equivalent
to::

  lock.acquire()
  try:
    ...
    # other code
  finally:
    lock.release()

This makes sure we release all locks, and avoid possible deadlocks. Of
course extra care must be used not to leave, if possible locked
structures in an unusable state. Note that with Python 2.5 a simpler
syntax will be possible, but we want to keep compatibility with Python
2.4 so the new constructs should not be used.

In order to avoid this extra indentation and code changes everywhere in
the Logical Units code, we decided to allow LUs to declare locks, and
then execute their code with their locks acquired. In the new world LUs
are called like this::

  # user passed names are expanded to the internal lock/resource name,
  # then known needed locks are declared
  lu.ExpandNames()
  ... some locking/adding of locks may happen ...
  # late declaration of locks for one level: this is useful because sometimes
  # we can't know which resource we need before locking the previous level
  lu.DeclareLocks() # for each level (cluster, instance, node)
  ... more locking/adding of locks can happen ...
  # these functions are called with the proper locks held
  lu.CheckPrereq()
  lu.Exec()
  ... locks declared for removal are removed, all acquired locks released ...

The Processor and the LogicalUnit class will contain exact documentation
on how locks are supposed to be declared.

Caveats
+++++++

This library will provide an easy upgrade path to bring all the code to
granular locking without breaking everything, and it will also guarantee
against a lot of common errors. Code switching from the old "lock
everything" lock to the new system, though, needs to be carefully
scrutinised to be sure it is really acquiring all the necessary locks,
and none has been overlooked or forgotten.

The code can contain other locks outside of this library, to synchronise
other threaded code (eg for the job queue) but in general these should
be leaf locks or carefully structured non-leaf ones, to avoid deadlock
race conditions.


Job Queue
~~~~~~~~~

Granular locking is not enough to speed up operations, we also need a
queue to store these and to be able to process as many as possible in
parallel.

A Ganeti job will consist of multiple ``OpCodes`` which are the basic
element of operation in Ganeti 1.2 (and will remain as such). Most
command-level commands are equivalent to one OpCode, or in some cases
to a sequence of opcodes, all of the same type (e.g. evacuating a node
will generate N opcodes of type replace disks).


Job execution—“Life of a Ganeti job”
++++++++++++++++++++++++++++++++++++

#. Job gets submitted by the client. A new job identifier is generated
   and assigned to the job. The job is then automatically replicated
   [#replic]_ to all nodes in the cluster. The identifier is returned to
   the client.
#. A pool of worker threads waits for new jobs. If all are busy, the job
   has to wait and the first worker finishing its work will grab it.
   Otherwise any of the waiting threads will pick up the new job.
#. Client waits for job status updates by calling a waiting RPC
   function. Log message may be shown to the user. Until the job is
   started, it can also be canceled.
#. As soon as the job is finished, its final result and status can be
   retrieved from the server.
#. If the client archives the job, it gets moved to a history directory.
   There will be a method to archive all jobs older than a a given age.

.. [#replic] We need replication in order to maintain the consistency
   across all nodes in the system; the master node only differs in the
   fact that now it is running the master daemon, but it if fails and we
   do a master failover, the jobs are still visible on the new master
   (though marked as failed).

Failures to replicate a job to other nodes will be only flagged as
errors in the master daemon log if more than half of the nodes failed,
otherwise we ignore the failure, and rely on the fact that the next
update (for still running jobs) will retry the update. For finished
jobs, it is less of a problem.

Future improvements will look into checking the consistency of the job
list and jobs themselves at master daemon startup.


Job storage
+++++++++++

Jobs are stored in the filesystem as individual files, serialized
using JSON (standard serialization mechanism in Ganeti).

The choice of storing each job in its own file was made because:

- a file can be atomically replaced
- a file can easily be replicated to other nodes
- checking consistency across nodes can be implemented very easily,
  since all job files should be (at a given moment in time) identical

The other possible choices that were discussed and discounted were:

- single big file with all job data: not feasible due to difficult
  updates
- in-process databases: hard to replicate the entire database to the
  other nodes, and replicating individual operations does not mean wee
  keep consistency


Queue structure
+++++++++++++++

All file operations have to be done atomically by writing to a temporary
file and subsequent renaming. Except for log messages, every change in a
job is stored and replicated to other nodes.

::

  /var/lib/ganeti/queue/
    job-1 (JSON encoded job description and status)
    […]
    job-37
    job-38
    job-39
    lock (Queue managing process opens this file in exclusive mode)
    serial (Last job ID used)
    version (Queue format version)


Locking
+++++++

Locking in the job queue is a complicated topic. It is called from more
than one thread and must be thread-safe. For simplicity, a single lock
is used for the whole job queue.

A more detailed description can be found in doc/locking.rst.


Internal RPC
++++++++++++

RPC calls available between Ganeti master and node daemons:

jobqueue_update(file_name, content)
  Writes a file in the job queue directory.
jobqueue_purge()
  Cleans the job queue directory completely, including archived job.
jobqueue_rename(old, new)
  Renames a file in the job queue directory.


Client RPC
++++++++++

RPC between Ganeti clients and the Ganeti master daemon supports the
following operations:

SubmitJob(ops)
  Submits a list of opcodes and returns the job identifier. The
  identifier is guaranteed to be unique during the lifetime of a
  cluster.
WaitForJobChange(job_id, fields, […], timeout)
  This function waits until a job changes or a timeout expires. The
  condition for when a job changed is defined by the fields passed and
  the last log message received.
QueryJobs(job_ids, fields)
  Returns field values for the job identifiers passed.
CancelJob(job_id)
  Cancels the job specified by identifier. This operation may fail if
  the job is already running, canceled or finished.
ArchiveJob(job_id)
  Moves a job into the …/archive/ directory. This operation will fail if
  the job has not been canceled or finished.


Job and opcode status
+++++++++++++++++++++

Each job and each opcode has, at any time, one of the following states:

Queued
  The job/opcode was submitted, but did not yet start.
Waiting
  The job/opcode is waiting for a lock to proceed.
Running
  The job/opcode is running.
Canceled
  The job/opcode was canceled before it started.
Success
  The job/opcode ran and finished successfully.
Error
  The job/opcode was aborted with an error.

If the master is aborted while a job is running, the job will be set to
the Error status once the master started again.


History
+++++++

Archived jobs are kept in a separate directory,
``/var/lib/ganeti/queue/archive/``.  This is done in order to speed up
the queue handling: by default, the jobs in the archive are not
touched by any functions. Only the current (unarchived) jobs are
parsed, loaded, and verified (if implemented) by the master daemon.


Ganeti updates
++++++++++++++

The queue has to be completely empty for Ganeti updates with changes
in the job queue structure. In order to allow this, there will be a
way to prevent new jobs entering the queue.


Object parameters
~~~~~~~~~~~~~~~~~

Across all cluster configuration data, we have multiple classes of
parameters:

A. cluster-wide parameters (e.g. name of the cluster, the master);
   these are the ones that we have today, and are unchanged from the
   current model

#. node parameters

#. instance specific parameters, e.g. the name of disks (LV), that
   cannot be shared with other instances

#. instance parameters, that are or can be the same for many
   instances, but are not hypervisor related; e.g. the number of VCPUs,
   or the size of memory

#. instance parameters that are hypervisor specific (e.g. kernel_path
   or PAE mode)


The following definitions for instance parameters will be used below:

:hypervisor parameter:
  a hypervisor parameter (or hypervisor specific parameter) is defined
  as a parameter that is interpreted by the hypervisor support code in
  Ganeti and usually is specific to a particular hypervisor (like the
  kernel path for :term:`PVM` which makes no sense for :term:`HVM`).

:backend parameter:
  a backend parameter is defined as an instance parameter that can be
  shared among a list of instances, and is either generic enough not
  to be tied to a given hypervisor or cannot influence at all the
  hypervisor behaviour.

  For example: memory, vcpus, auto_balance

  All these parameters will be encoded into constants.py with the prefix
  "BE\_" and the whole list of parameters will exist in the set
  "BES_PARAMETERS"

:proper parameter:
  a parameter whose value is unique to the instance (e.g. the name of a
  LV, or the MAC of a NIC)

As a general rule, for all kind of parameters, “None” (or in
JSON-speak, “nil”) will no longer be a valid value for a parameter. As
such, only non-default parameters will be saved as part of objects in
the serialization step, reducing the size of the serialized format.

Cluster parameters
++++++++++++++++++

Cluster parameters remain as today, attributes at the top level of the
Cluster object. In addition, two new attributes at this level will
hold defaults for the instances:

- hvparams, a dictionary indexed by hypervisor type, holding default
  values for hypervisor parameters that are not defined/overridden by
  the instances of this hypervisor type

- beparams, a dictionary holding (for 2.0) a single element 'default',
  which holds the default value for backend parameters

Node parameters
+++++++++++++++

Node-related parameters are very few, and we will continue using the
same model for these as previously (attributes on the Node object).

There are three new node flags, described in a separate section "node
flags" below.

Instance parameters
+++++++++++++++++++

As described before, the instance parameters are split in three:
instance proper parameters, unique to each instance, instance
hypervisor parameters and instance backend parameters.

The “hvparams” and “beparams” are kept in two dictionaries at instance
level. Only non-default parameters are stored (but once customized, a
parameter will be kept, even with the same value as the default one,
until reset).

The names for hypervisor parameters in the instance.hvparams subtree
should be choosen as generic as possible, especially if specific
parameters could conceivably be useful for more than one hypervisor,
e.g. ``instance.hvparams.vnc_console_port`` instead of using both
``instance.hvparams.hvm_vnc_console_port`` and
``instance.hvparams.kvm_vnc_console_port``.

There are some special cases related to disks and NICs (for example):
a disk has both Ganeti-related parameters (e.g. the name of the LV)
and hypervisor-related parameters (how the disk is presented to/named
in the instance). The former parameters remain as proper-instance
parameters, while the latter value are migrated to the hvparams
structure. In 2.0, we will have only globally-per-instance such
hypervisor parameters, and not per-disk ones (e.g. all NICs will be
exported as of the same type).

Starting from the 1.2 list of instance parameters, here is how they
will be mapped to the three classes of parameters:

- name (P)
- primary_node (P)
- os (P)
- hypervisor (P)
- status (P)
- memory (BE)
- vcpus (BE)
- nics (P)
- disks (P)
- disk_template (P)
- network_port (P)
- kernel_path (HV)
- initrd_path (HV)
- hvm_boot_order (HV)
- hvm_acpi (HV)
- hvm_pae (HV)
- hvm_cdrom_image_path (HV)
- hvm_nic_type (HV)
- hvm_disk_type (HV)
- vnc_bind_address (HV)
- serial_no (P)


Parameter validation
++++++++++++++++++++

To support the new cluster parameter design, additional features will
be required from the hypervisor support implementations in Ganeti.

The hypervisor support  implementation API will be extended with the
following features:

:PARAMETERS: class-level attribute holding the list of valid parameters
  for this hypervisor
:CheckParamSyntax(hvparams): checks that the given parameters are
  valid (as in the names are valid) for this hypervisor; usually just
  comparing ``hvparams.keys()`` and ``cls.PARAMETERS``; this is a class
  method that can be called from within master code (i.e. cmdlib) and
  should be safe to do so
:ValidateParameters(hvparams): verifies the values of the provided
  parameters against this hypervisor; this is a method that will be
  called on the target node, from backend.py code, and as such can
  make node-specific checks (e.g. kernel_path checking)

Default value application
+++++++++++++++++++++++++

The application of defaults to an instance is done in the Cluster
object, via two new methods as follows:

- ``Cluster.FillHV(instance)``, returns 'filled' hvparams dict, based on
  instance's hvparams and cluster's ``hvparams[instance.hypervisor]``

- ``Cluster.FillBE(instance, be_type="default")``, which returns the
  beparams dict, based on the instance and cluster beparams

The FillHV/BE transformations will be used, for example, in the
RpcRunner when sending an instance for activation/stop, and the sent
instance hvparams/beparams will have the final value (noded code doesn't
know about defaults).

LU code will need to self-call the transformation, if needed.

Opcode changes
++++++++++++++

The parameter changes will have impact on the OpCodes, especially on
the following ones:

- ``OpInstanceCreate``, where the new hv and be parameters will be sent
  as dictionaries; note that all hv and be parameters are now optional,
  as the values can be instead taken from the cluster
- ``OpInstanceQuery``, where we have to be able to query these new
  parameters; the syntax for names will be ``hvparam/$NAME`` and
  ``beparam/$NAME`` for querying an individual parameter out of one
  dictionary, and ``hvparams``, respectively ``beparams``, for the whole
  dictionaries
- ``OpModifyInstance``, where the the modified parameters are sent as
  dictionaries

Additionally, we will need new OpCodes to modify the cluster-level
defaults for the be/hv sets of parameters.

Caveats
+++++++

One problem that might appear is that our classification is not
complete or not good enough, and we'll need to change this model. As
the last resort, we will need to rollback and keep 1.2 style.

Another problem is that classification of one parameter is unclear
(e.g. ``network_port``, is this BE or HV?); in this case we'll take
the risk of having to move parameters later between classes.

Security
++++++++

The only security issue that we foresee is if some new parameters will
have sensitive value. If so, we will need to have a way to export the
config data while purging the sensitive value.

E.g. for the drbd shared secrets, we could export these with the
values replaced by an empty string.

Node flags
~~~~~~~~~~

Ganeti 2.0 adds three node flags that change the way nodes are handled
within Ganeti and the related infrastructure (iallocator interaction,
RAPI data export).

*master candidate* flag
+++++++++++++++++++++++

Ganeti 2.0 allows more scalability in operation by introducing
parallelization. However, a new bottleneck is reached that is the
synchronization and replication of cluster configuration to all nodes
in the cluster.

This breaks scalability as the speed of the replication decreases
roughly with the size of the nodes in the cluster. The goal of the
master candidate flag is to change this O(n) into O(1) with respect to
job and configuration data propagation.

Only nodes having this flag set (let's call this set of nodes the
*candidate pool*) will have jobs and configuration data replicated.

The cluster will have a new parameter (runtime changeable) called
``candidate_pool_size`` which represents the number of candidates the
cluster tries to maintain (preferably automatically).

This will impact the cluster operations as follows:

- jobs and config data will be replicated only to a fixed set of nodes
- master fail-over will only be possible to a node in the candidate pool
- cluster verify needs changing to account for these two roles
- external scripts will no longer have access to the configuration
  file (this is not recommended anyway)


The caveats of this change are:

- if all candidates are lost (completely), cluster configuration is
  lost (but it should be backed up external to the cluster anyway)

- failed nodes which are candidate must be dealt with properly, so
  that we don't lose too many candidates at the same time; this will be
  reported in cluster verify

- the 'all equal' concept of ganeti is no longer true

- the partial distribution of config data means that all nodes will
  have to revert to ssconf files for master info (as in 1.2)

Advantages:

- speed on a 100+ nodes simulated cluster is greatly enhanced, even
  for a simple operation; ``gnt-instance remove`` on a diskless instance
  remove goes from ~9seconds to ~2 seconds

- node failure of non-candidates will be less impacting on the cluster

The default value for the candidate pool size will be set to 10 but
this can be changed at cluster creation and modified any time later.

Testing on simulated big clusters with sequential and parallel jobs
show that this value (10) is a sweet-spot from performance and load
point of view.

*offline* flag
++++++++++++++

In order to support better the situation in which nodes are offline
(e.g. for repair) without altering the cluster configuration, Ganeti
needs to be told and needs to properly handle this state for nodes.

This will result in simpler procedures, and less mistakes, when the
amount of node failures is high on an absolute scale (either due to
high failure rate or simply big clusters).

Nodes having this attribute set will not be contacted for inter-node
RPC calls, will not be master candidates, and will not be able to host
instances as primaries.

Setting this attribute on a node:

- will not be allowed if the node is the master
- will not be allowed if the node has primary instances
- will cause the node to be demoted from the master candidate role (if
  it was), possibly causing another node to be promoted to that role

This attribute will impact the cluster operations as follows:

- querying these nodes for anything will fail instantly in the RPC
  library, with a specific RPC error (RpcResult.offline == True)

- they will be listed in the Other section of cluster verify

The code is changed in the following ways:

- RPC calls were be converted to skip such nodes:

  - RpcRunner-instance-based RPC calls are easy to convert

  - static/classmethod RPC calls are harder to convert, and were left
    alone

- the RPC results were unified so that this new result state (offline)
  can be differentiated

- master voting still queries in repair nodes, as we need to ensure
  consistency in case the (wrong) masters have old data, and nodes have
  come back from repairs

Caveats:

- some operation semantics are less clear (e.g. what to do on instance
  start with offline secondary?); for now, these will just fail as if
  the flag is not set (but faster)
- 2-node cluster with one node offline needs manual startup of the
  master with a special flag to skip voting (as the master can't get a
  quorum there)

One of the advantages of implementing this flag is that it will allow
in the future automation tools to automatically put the node in
repairs and recover from this state, and the code (should/will) handle
this much better than just timing out. So, future possible
improvements (for later versions):

- watcher will detect nodes which fail RPC calls, will attempt to ssh
  to them, if failure will put them offline
- watcher will try to ssh and query the offline nodes, if successful
  will take them off the repair list

Alternatives considered: The RPC call model in 2.0 is, by default,
much nicer - errors are logged in the background, and job/opcode
execution is clearer, so we could simply not introduce this. However,
having this state will make both the codepaths clearer (offline
vs. temporary failure) and the operational model (it's not a node with
errors, but an offline node).


*drained* flag
++++++++++++++

Due to parallel execution of jobs in Ganeti 2.0, we could have the
following situation:

- gnt-node migrate + failover is run
- gnt-node evacuate is run, which schedules a long-running 6-opcode
  job for the node
- partway through, a new job comes in that runs an iallocator script,
  which finds the above node as empty and a very good candidate
- gnt-node evacuate has finished, but now it has to be run again, to
  clean the above instance(s)

In order to prevent this situation, and to be able to get nodes into
proper offline status easily, a new *drained* flag was added to the
nodes.

This flag (which actually means "is being, or was drained, and is
expected to go offline"), will prevent allocations on the node, but
otherwise all other operations (start/stop instance, query, etc.) are
working without any restrictions.

Interaction between flags
+++++++++++++++++++++++++

While these flags are implemented as separate flags, they are
mutually-exclusive and are acting together with the master node role
as a single *node status* value. In other words, a flag is only in one
of these roles at a given time. The lack of any of these flags denote
a regular node.

The current node status is visible in the ``gnt-cluster verify``
output, and the individual flags can be examined via separate flags in
the ``gnt-node list`` output.

These new flags will be exported in both the iallocator input message
and via RAPI, see the respective man pages for the exact names.

Feature changes
---------------

The main feature-level changes will be:

- a number of disk related changes
- removal of fixed two-disk, one-nic per instance limitation

Disk handling changes
~~~~~~~~~~~~~~~~~~~~~

The storage options available in Ganeti 1.x were introduced based on
then-current software (first DRBD 0.7 then later DRBD 8) and the
estimated usage patters. However, experience has later shown that some
assumptions made initially are not true and that more flexibility is
needed.

One main assumption made was that disk failures should be treated as
'rare' events, and that each of them needs to be manually handled in
order to ensure data safety; however, both these assumptions are false:

- disk failures can be a common occurrence, based on usage patterns or
  cluster size
- our disk setup is robust enough (referring to DRBD8 + LVM) that we
  could automate more of the recovery

Note that we still don't have fully-automated disk recovery as a goal,
but our goal is to reduce the manual work needed.

As such, we plan the following main changes:

- DRBD8 is much more flexible and stable than its previous version
  (0.7), such that removing the support for the ``remote_raid1``
  template and focusing only on DRBD8 is easier

- dynamic discovery of DRBD devices is not actually needed in a cluster
  that where the DRBD namespace is controlled by Ganeti; switching to a
  static assignment (done at either instance creation time or change
  secondary time) will change the disk activation time from O(n) to
  O(1), which on big clusters is a significant gain

- remove the hard dependency on LVM (currently all available storage
  types are ultimately backed by LVM volumes) by introducing file-based
  storage

Additionally, a number of smaller enhancements are also planned:
- support variable number of disks
- support read-only disks

Future enhancements in the 2.x series, which do not require base design
changes, might include:

- enhancement of the LVM allocation method in order to try to keep
  all of an instance's virtual disks on the same physical
  disks

- add support for DRBD8 authentication at handshake time in
  order to ensure each device connects to the correct peer

- remove the restrictions on failover only to the secondary
  which creates very strict rules on cluster allocation

DRBD minor allocation
+++++++++++++++++++++

Currently, when trying to identify or activate a new DRBD (or MD)
device, the code scans all in-use devices in order to see if we find
one that looks similar to our parameters and is already in the desired
state or not. Since this needs external commands to be run, it is very
slow when more than a few devices are already present.

Therefore, we will change the discovery model from dynamic to
static. When a new device is logically created (added to the
configuration) a free minor number is computed from the list of
devices that should exist on that node and assigned to that
device.

At device activation, if the minor is already in use, we check if
it has our parameters; if not so, we just destroy the device (if
possible, otherwise we abort) and start it with our own
parameters.

This means that we in effect take ownership of the minor space for
that device type; if there's a user-created DRBD minor, it will be
automatically removed.

The change will have the effect of reducing the number of external
commands run per device from a constant number times the index of the
first free DRBD minor to just a constant number.

Removal of obsolete device types (MD, DRBD7)
++++++++++++++++++++++++++++++++++++++++++++

We need to remove these device types because of two issues. First,
DRBD7 has bad failure modes in case of dual failures (both network and
disk - it cannot propagate the error up the device stack and instead
just panics. Second, due to the asymmetry between primary and
secondary in MD+DRBD mode, we cannot do live failover (not even if we
had MD+DRBD8).

File-based storage support
++++++++++++++++++++++++++

Using files instead of logical volumes for instance storage would
allow us to get rid of the hard requirement for volume groups for
testing clusters and it would also allow usage of SAN storage to do
live failover taking advantage of this storage solution.

Better LVM allocation
+++++++++++++++++++++

Currently, the LV to PV allocation mechanism is a very simple one: at
each new request for a logical volume, tell LVM to allocate the volume
in order based on the amount of free space. This is good for
simplicity and for keeping the usage equally spread over the available
physical disks, however it introduces a problem that an instance could
end up with its (currently) two drives on two physical disks, or
(worse) that the data and metadata for a DRBD device end up on
different drives.

This is bad because it causes unneeded ``replace-disks`` operations in
case of a physical failure.

The solution is to batch allocations for an instance and make the LVM
handling code try to allocate as close as possible all the storage of
one instance. We will still allow the logical volumes to spill over to
additional disks as needed.

Note that this clustered allocation can only be attempted at initial
instance creation, or at change secondary node time. At add disk time,
or at replacing individual disks, it's not easy enough to compute the
current disk map so we'll not attempt the clustering.

DRBD8 peer authentication at handshake
++++++++++++++++++++++++++++++++++++++

DRBD8 has a new feature that allow authentication of the peer at
connect time. We can use this to prevent connecting to the wrong peer
more that securing the connection. Even though we never had issues
with wrong connections, it would be good to implement this.


LVM self-repair (optional)
++++++++++++++++++++++++++

The complete failure of a physical disk is very tedious to
troubleshoot, mainly because of the many failure modes and the many
steps needed. We can safely automate some of the steps, more
specifically the ``vgreduce --removemissing`` using the following
method:

#. check if all nodes have consistent volume groups
#. if yes, and previous status was yes, do nothing
#. if yes, and previous status was no, save status and restart
#. if no, and previous status was no, do nothing
#. if no, and previous status was yes:
    #. if more than one node is inconsistent, do nothing
    #. if only one node is inconsistent:
        #. run ``vgreduce --removemissing``
        #. log this occurrence in the Ganeti log in a form that
           can be used for monitoring
        #. [FUTURE] run ``replace-disks`` for all
           instances affected

Failover to any node
++++++++++++++++++++

With a modified disk activation sequence, we can implement the
*failover to any* functionality, removing many of the layout
restrictions of a cluster:

- the need to reserve memory on the current secondary: this gets reduced
  to a must to reserve memory anywhere on the cluster

- the need to first failover and then replace secondary for an
  instance: with failover-to-any, we can directly failover to
  another node, which also does the replace disks at the same
  step

In the following, we denote the current primary by P1, the current
secondary by S1, and the new primary and secondaries by P2 and S2. P2
is fixed to the node the user chooses, but the choice of S2 can be
made between P1 and S1. This choice can be constrained, depending on
which of P1 and S1 has failed.

- if P1 has failed, then S1 must become S2, and live migration is not
  possible
- if S1 has failed, then P1 must become S2, and live migration could be
  possible (in theory, but this is not a design goal for 2.0)

The algorithm for performing the failover is straightforward:

- verify that S2 (the node the user has chosen to keep as secondary) has
  valid data (is consistent)

- tear down the current DRBD association and setup a DRBD pairing
  between P2 (P2 is indicated by the user) and S2; since P2 has no data,
  it will start re-syncing from S2

- as soon as P2 is in state SyncTarget (i.e. after the resync has
  started but before it has finished), we can promote it to primary role
  (r/w) and start the instance on P2

- as soon as the P2?S2 sync has finished, we can remove
  the old data on the old node that has not been chosen for
  S2

Caveats: during the P2?S2 sync, a (non-transient) network error
will cause I/O errors on the instance, so (if a longer instance
downtime is acceptable) we can postpone the restart of the instance
until the resync is done. However, disk I/O errors on S2 will cause
data loss, since we don't have a good copy of the data anymore, so in
this case waiting for the sync to complete is not an option. As such,
it is recommended that this feature is used only in conjunction with
proper disk monitoring.


Live migration note: While failover-to-any is possible for all choices
of S2, migration-to-any is possible only if we keep P1 as S2.

Caveats
+++++++

The dynamic device model, while more complex, has an advantage: it
will not reuse by mistake the DRBD device of another instance, since
it always looks for either our own or a free one.

The static one, in contrast, will assume that given a minor number N,
it's ours and we can take over. This needs careful implementation such
that if the minor is in use, either we are able to cleanly shut it
down, or we abort the startup. Otherwise, it could be that we start
syncing between two instance's disks, causing data loss.


Variable number of disk/NICs per instance
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Variable number of disks
++++++++++++++++++++++++

In order to support high-security scenarios (for example read-only sda
and read-write sdb), we need to make a fully flexibly disk
definition. This has less impact that it might look at first sight:
only the instance creation has hard coded number of disks, not the disk
handling code. The block device handling and most of the instance
handling code is already working with "the instance's disks" as
opposed to "the two disks of the instance", but some pieces are not
(e.g. import/export) and the code needs a review to ensure safety.

The objective is to be able to specify the number of disks at
instance creation, and to be able to toggle from read-only to
read-write a disk afterward.

Variable number of NICs
+++++++++++++++++++++++

Similar to the disk change, we need to allow multiple network
interfaces per instance. This will affect the internal code (some
function will have to stop assuming that ``instance.nics`` is a list
of length one), the OS API which currently can export/import only one
instance, and the command line interface.

Interface changes
-----------------

There are two areas of interface changes: API-level changes (the OS
interface and the RAPI interface) and the command line interface
changes.

OS interface
~~~~~~~~~~~~

The current Ganeti OS interface, version 5, is tailored for Ganeti 1.2.
The interface is composed by a series of scripts which get called with
certain parameters to perform OS-dependent operations on the cluster.
The current scripts are:

create
  called when a new instance is added to the cluster
export
  called to export an instance disk to a stream
import
  called to import from a stream to a new instance
rename
  called to perform the os-specific operations necessary for renaming an
  instance

Currently these scripts suffer from the limitations of Ganeti 1.2: for
example they accept exactly one block and one swap devices to operate
on, rather than any amount of generic block devices, they blindly assume
that an instance will have just one network interface to operate, they
can not be configured to optimise the instance for a particular
hypervisor.

Since in Ganeti 2.0 we want to support multiple hypervisors, and a
non-fixed number of network and disks the OS interface need to change to
transmit the appropriate amount of information about an instance to its
managing operating system, when operating on it. Moreover since some old
assumptions usually used in OS scripts are no longer valid we need to
re-establish a common knowledge on what can be assumed and what cannot
be regarding Ganeti environment.


When designing the new OS API our priorities are:
- ease of use
- future extensibility
- ease of porting from the old API
- modularity

As such we want to limit the number of scripts that must be written to
support an OS, and make it easy to share code between them by uniforming
their input.  We also will leave the current script structure unchanged,
as far as we can, and make a few of the scripts (import, export and
rename) optional. Most information will be passed to the script through
environment variables, for ease of access and at the same time ease of
using only the information a script needs.


The Scripts
+++++++++++

As in Ganeti 1.2, every OS which wants to be installed in Ganeti needs
to support the following functionality, through scripts:

create:
  used to create a new instance running that OS. This script should
  prepare the block devices, and install them so that the new OS can
  boot under the specified hypervisor.
export (optional):
  used to export an installed instance using the given OS to a format
  which can be used to import it back into a new instance.
import (optional):
  used to import an exported instance into a new one. This script is
  similar to create, but the new instance should have the content of the
  export, rather than contain a pristine installation.
rename (optional):
  used to perform the internal OS-specific operations needed to rename
  an instance.

If any optional script is not implemented Ganeti will refuse to perform
the given operation on instances using the non-implementing OS. Of
course the create script is mandatory, and it doesn't make sense to
support the either the export or the import operation but not both.

Incompatibilities with 1.2
__________________________

We expect the following incompatibilities between the OS scripts for 1.2
and the ones for 2.0:

- Input parameters: in 1.2 those were passed on the command line, in 2.0
  we'll use environment variables, as there will be a lot more
  information and not all OSes may care about all of it.
- Number of calls: export scripts will be called once for each device
  the instance has, and import scripts once for every exported disk.
  Imported instances will be forced to have a number of disks greater or
  equal to the one of the export.
- Some scripts are not compulsory: if such a script is missing the
  relevant operations will be forbidden for instances of that OS. This
  makes it easier to distinguish between unsupported operations and
  no-op ones (if any).


Input
_____

Rather than using command line flags, as they do now, scripts will
accept inputs from environment variables. We expect the following input
values:

OS_API_VERSION
  The version of the OS API that the following parameters comply with;
  this is used so that in the future we could have OSes supporting
  multiple versions and thus Ganeti send the proper version in this
  parameter
INSTANCE_NAME
  Name of the instance acted on
HYPERVISOR
  The hypervisor the instance should run on (e.g. 'xen-pvm', 'xen-hvm',
  'kvm')
DISK_COUNT
  The number of disks this instance will have
NIC_COUNT
  The number of NICs this instance will have
DISK_<N>_PATH
  Path to the Nth disk.
DISK_<N>_ACCESS
  W if read/write, R if read only. OS scripts are not supposed to touch
  read-only disks, but will be passed them to know.
DISK_<N>_FRONTEND_TYPE
  Type of the disk as seen by the instance. Can be 'scsi', 'ide',
  'virtio'
DISK_<N>_BACKEND_TYPE
  Type of the disk as seen from the node. Can be 'block', 'file:loop' or
  'file:blktap'
NIC_<N>_MAC
  Mac address for the Nth network interface
NIC_<N>_IP
  Ip address for the Nth network interface, if available
NIC_<N>_BRIDGE
  Node bridge the Nth network interface will be connected to
NIC_<N>_FRONTEND_TYPE
  Type of the Nth NIC as seen by the instance. For example 'virtio',
  'rtl8139', etc.
DEBUG_LEVEL
  Whether more out should be produced, for debugging purposes. Currently
  the only valid values are 0 and 1.

These are only the basic variables we are thinking of now, but more
may come during the implementation and they will be documented in the
:manpage:`ganeti-os-api` man page. All these variables will be
available to all scripts.

Some scripts will need a few more information to work. These will have
per-script variables, such as for example:

OLD_INSTANCE_NAME
  rename: the name the instance should be renamed from.
EXPORT_DEVICE
  export: device to be exported, a snapshot of the actual device. The
  data must be exported to stdout.
EXPORT_INDEX
  export: sequential number of the instance device targeted.
IMPORT_DEVICE
  import: device to send the data to, part of the new instance. The data
  must be imported from stdin.
IMPORT_INDEX
  import: sequential number of the instance device targeted.

(Rationale for INSTANCE_NAME as an environment variable: the instance
name is always needed and we could pass it on the command line. On the
other hand, though, this would force scripts to both access the
environment and parse the command line, so we'll move it for
uniformity.)


Output/Behaviour
________________

As discussed scripts should only send user-targeted information to
stderr. The create and import scripts are supposed to format/initialise
the given block devices and install the correct instance data. The
export script is supposed to export instance data to stdout in a format
understandable by the the import script. The data will be compressed by
Ganeti, so no compression should be done. The rename script should only
modify the instance's knowledge of what its name is.

Other declarative style features
++++++++++++++++++++++++++++++++

Similar to Ganeti 1.2, OS specifications will need to provide a
'ganeti_api_version' containing list of numbers matching the
version(s) of the API they implement. Ganeti itself will always be
compatible with one version of the API and may maintain backwards
compatibility if it's feasible to do so. The numbers are one-per-line,
so an OS supporting both version 5 and version 20 will have a file
containing two lines. This is different from Ganeti 1.2, which only
supported one version number.

In addition to that an OS will be able to declare that it does support
only a subset of the Ganeti hypervisors, by declaring them in the
'hypervisors' file.


Caveats/Notes
+++++++++++++

We might want to have a "default" import/export behaviour that just
dumps all disks and restores them. This can save work as most systems
will just do this, while allowing flexibility for different systems.

Environment variables are limited in size, but we expect that there will
be enough space to store the information we need. If we discover that
this is not the case we may want to go to a more complex API such as
storing those information on the filesystem and providing the OS script
with the path to a file where they are encoded in some format.



Remote API changes
~~~~~~~~~~~~~~~~~~

The first Ganeti remote API (RAPI) was designed and deployed with the
Ganeti 1.2.5 release.  That version provide read-only access to the
cluster state. Fully functional read-write API demands significant
internal changes which will be implemented in version 2.0.

We decided to go with implementing the Ganeti RAPI in a RESTful way,
which is aligned with key features we looking. It is simple,
stateless, scalable and extensible paradigm of API implementation. As
transport it uses HTTP over SSL, and we are implementing it with JSON
encoding, but in a way it possible to extend and provide any other
one.

Design
++++++

The Ganeti RAPI is implemented as independent daemon, running on the
same node with the same permission level as Ganeti master
daemon. Communication is done through the LUXI library to the master
daemon. In order to keep communication asynchronous RAPI processes two
types of client requests:

- queries: server is able to answer immediately
- job submission: some time is required for a useful response

In the query case requested data send back to client in the HTTP
response body. Typical examples of queries would be: list of nodes,
instances, cluster info, etc.

In the case of job submission, the client receive a job ID, the
identifier which allows one to query the job progress in the job queue
(see `Job Queue`_).

Internally, each exported object has an version identifier, which is
used as a state identifier in the HTTP header E-Tag field for
requests/responses to avoid race conditions.


Resource representation
+++++++++++++++++++++++

The key difference of using REST instead of others API is that REST
requires separation of services via resources with unique URIs. Each
of them should have limited amount of state and support standard HTTP
methods: GET, POST, DELETE, PUT.

For example in Ganeti's case we can have a set of URI:

 - ``/{clustername}/instances``
 - ``/{clustername}/instances/{instancename}``
 - ``/{clustername}/instances/{instancename}/tag``
 - ``/{clustername}/tag``

A GET request to ``/{clustername}/instances`` will return the list of
instances, a POST to ``/{clustername}/instances`` should create a new
instance, a DELETE ``/{clustername}/instances/{instancename}`` should
delete the instance, a GET ``/{clustername}/tag`` should return get
cluster tags.

Each resource URI will have a version prefix. The resource IDs are to
be determined.

Internal encoding might be JSON, XML, or any other. The JSON encoding
fits nicely in Ganeti RAPI needs. The client can request a specific
representation via the Accept field in the HTTP header.

REST uses HTTP as its transport and application protocol for resource
access. The set of possible responses is a subset of standard HTTP
responses.

The statelessness model provides additional reliability and
transparency to operations (e.g. only one request needs to be analyzed
to understand the in-progress operation, not a sequence of multiple
requests/responses).


Security
++++++++

With the write functionality security becomes a much bigger an issue.
The Ganeti RAPI uses basic HTTP authentication on top of an
SSL-secured connection to grant access to an exported resource. The
password is stored locally in an Apache-style ``.htpasswd`` file. Only
one level of privileges is supported.

Caveats
+++++++

The model detailed above for job submission requires the client to
poll periodically for updates to the job; an alternative would be to
allow the client to request a callback, or a 'wait for updates' call.

The callback model was not considered due to the following two issues:

- callbacks would require a new model of allowed callback URLs,
  together with a method of managing these
- callbacks only work when the client and the master are in the same
  security domain, and they fail in the other cases (e.g. when there is
  a firewall between the client and the RAPI daemon that only allows
  client-to-RAPI calls, which is usual in DMZ cases)

The 'wait for updates' method is not suited to the HTTP protocol,
where requests are supposed to be short-lived.

Command line changes
~~~~~~~~~~~~~~~~~~~~

Ganeti 2.0 introduces several new features as well as new ways to
handle instance resources like disks or network interfaces. This
requires some noticeable changes in the way command line arguments are
handled.

- extend and modify command line syntax to support new features
- ensure consistent patterns in command line arguments to reduce
  cognitive load

The design changes that require these changes are, in no particular
order:

- flexible instance disk handling: support a variable number of disks
  with varying properties per instance,
- flexible instance network interface handling: support a variable
  number of network interfaces with varying properties per instance
- multiple hypervisors: multiple hypervisors can be active on the same
  cluster, each supporting different parameters,
- support for device type CDROM (via ISO image)

As such, there are several areas of Ganeti where the command line
arguments will change:

- Cluster configuration

  - cluster initialization
  - cluster default configuration

- Instance configuration

  - handling of network cards for instances,
  - handling of disks for instances,
  - handling of CDROM devices and
  - handling of hypervisor specific options.

There are several areas of Ganeti where the command line arguments
will change:

- Cluster configuration

  - cluster initialization
  - cluster default configuration

- Instance configuration

  - handling of network cards for instances,
  - handling of disks for instances,
  - handling of CDROM devices and
  - handling of hypervisor specific options.

Notes about device removal/addition
+++++++++++++++++++++++++++++++++++

To avoid problems with device location changes (e.g. second network
interface of the instance becoming the first or third and the like)
the list of network/disk devices is treated as a stack, i.e. devices
can only be added/removed at the end of the list of devices of each
class (disk or network) for each instance.

gnt-instance commands
+++++++++++++++++++++

The commands for gnt-instance will be modified and extended to allow
for the new functionality:

- the add command will be extended to support the new device and
  hypervisor options,
- the modify command continues to handle all modifications to
  instances, but will be extended with new arguments for handling
  devices.

Network Device Options
++++++++++++++++++++++

The generic format of the network device option is:

  --net $DEVNUM[:$OPTION=$VALUE][,$OPTION=VALUE]

:$DEVNUM: device number, unsigned integer, starting at 0,
:$OPTION: device option, string,
:$VALUE: device option value, string.

Currently, the following device options will be defined (open to
further changes):

:mac: MAC address of the network interface, accepts either a valid
  MAC address or the string 'auto'. If 'auto' is specified, a new MAC
  address will be generated randomly. If the mac device option is not
  specified, the default value 'auto' is assumed.
:bridge: network bridge the network interface is connected
  to. Accepts either a valid bridge name (the specified bridge must
  exist on the node(s)) as string or the string 'auto'. If 'auto' is
  specified, the default brigde is used. If the bridge option is not
  specified, the default value 'auto' is assumed.

Disk Device Options
+++++++++++++++++++

The generic format of the disk device option is:

  --disk $DEVNUM[:$OPTION=$VALUE][,$OPTION=VALUE]

:$DEVNUM: device number, unsigned integer, starting at 0,
:$OPTION: device option, string,
:$VALUE: device option value, string.

Currently, the following device options will be defined (open to
further changes):

:size: size of the disk device, either a positive number, specifying
  the disk size in mebibytes, or a number followed by a magnitude suffix
  (M for mebibytes, G for gibibytes). Also accepts the string 'auto' in
  which case the default disk size will be used. If the size option is
  not specified, 'auto' is assumed. This option is not valid for all
  disk layout types.
:access: access mode of the disk device, a single letter, valid values
  are:

  - *w*: read/write access to the disk device or
  - *r*: read-only access to the disk device.

  If the access mode is not specified, the default mode of read/write
  access will be configured.
:path: path to the image file for the disk device, string. No default
  exists. This option is not valid for all disk layout types.

Adding devices
++++++++++++++

To add devices to an already existing instance, use the device type
specific option to gnt-instance modify. Currently, there are two
device type specific options supported:

:--net: for network interface cards
:--disk: for disk devices

The syntax to the device specific options is similar to the generic
device options, but instead of specifying a device number like for
gnt-instance add, you specify the magic string add. The new device
will always be appended at the end of the list of devices of this type
for the specified instance, e.g. if the instance has disk devices 0,1
and 2, the newly added disk device will be disk device 3.

Example: gnt-instance modify --net add:mac=auto test-instance

Removing devices
++++++++++++++++

Removing devices from and instance is done via gnt-instance
modify. The same device specific options as for adding instances are
used. Instead of a device number and further device options, only the
magic string remove is specified. It will always remove the last
device in the list of devices of this type for the instance specified,
e.g. if the instance has disk devices 0, 1, 2 and 3, the disk device
number 3 will be removed.

Example: gnt-instance modify --net remove test-instance

Modifying devices
+++++++++++++++++

Modifying devices is also done with device type specific options to
the gnt-instance modify command. There are currently two device type
options supported:

:--net: for network interface cards
:--disk: for disk devices

The syntax to the device specific options is similar to the generic
device options. The device number you specify identifies the device to
be modified.

Example::

  gnt-instance modify --disk 2:access=r

Hypervisor Options
++++++++++++++++++

Ganeti 2.0 will support more than one hypervisor. Different
hypervisors have various options that only apply to a specific
hypervisor. Those hypervisor specific options are treated specially
via the ``--hypervisor`` option. The generic syntax of the hypervisor
option is as follows::

  --hypervisor $HYPERVISOR:$OPTION=$VALUE[,$OPTION=$VALUE]

:$HYPERVISOR: symbolic name of the hypervisor to use, string,
  has to match the supported hypervisors. Example: xen-pvm

:$OPTION: hypervisor option name, string
:$VALUE: hypervisor option value, string

The hypervisor option for an instance can be set on instance creation
time via the ``gnt-instance add`` command. If the hypervisor for an
instance is not specified upon instance creation, the default
hypervisor will be used.

Modifying hypervisor parameters
+++++++++++++++++++++++++++++++

The hypervisor parameters of an existing instance can be modified
using ``--hypervisor`` option of the ``gnt-instance modify``
command. However, the hypervisor type of an existing instance can not
be changed, only the particular hypervisor specific option can be
changed. Therefore, the format of the option parameters has been
simplified to omit the hypervisor name and only contain the comma
separated list of option-value pairs.

Example::

  gnt-instance modify --hypervisor cdrom=/srv/boot.iso,boot_order=cdrom:network test-instance

gnt-cluster commands
++++++++++++++++++++

The command for gnt-cluster will be extended to allow setting and
changing the default parameters of the cluster:

- The init command will be extend to support the defaults option to
  set the cluster defaults upon cluster initialization.
- The modify command will be added to modify the cluster
  parameters. It will support the --defaults option to change the
  cluster defaults.

Cluster defaults

The generic format of the cluster default setting option is:

  --defaults $OPTION=$VALUE[,$OPTION=$VALUE]

:$OPTION: cluster default option, string,
:$VALUE: cluster default option value, string.

Currently, the following cluster default options are defined (open to
further changes):

:hypervisor: the default hypervisor to use for new instances,
  string. Must be a valid hypervisor known to and supported by the
  cluster.
:disksize: the disksize for newly created instance disks, where
  applicable. Must be either a positive number, in which case the unit
  of megabyte is assumed, or a positive number followed by a supported
  magnitude symbol (M for megabyte or G for gigabyte).
:bridge: the default network bridge to use for newly created instance
  network interfaces, string. Must be a valid bridge name of a bridge
  existing on the node(s).

Hypervisor cluster defaults
+++++++++++++++++++++++++++

The generic format of the hypervisor cluster wide default setting
option is::

  --hypervisor-defaults $HYPERVISOR:$OPTION=$VALUE[,$OPTION=$VALUE]

:$HYPERVISOR: symbolic name of the hypervisor whose defaults you want
  to set, string
:$OPTION: cluster default option, string,
:$VALUE: cluster default option value, string.

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