Synnefo » snf-ganeti

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Ganeti reason trail

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This is a design document detailing the implementation of a way for Ganeti to

track the origin and the reason of every executed command, from its starting

point (command line, remote API, some htool, etc.) to its actual execution

time.

Current state and shortcomings

==============================

There is currently no way to track why a job and all the operations part of it

were executed, and who or what triggered the execution.

This is an inconvenience in general, and also it makes impossible to have

certain information, such as finding the reason why an instance last changed its

status (i.e.: why it was started/stopped/rebooted/etc.), or distinguishing

an admin request from a scheduled maintenance or an automated tool's work.

Proposed changes

================

We propose to introduce a new piece of information, that will be called "reason

trail", to track the path from the issuing of a command to its execution.

The reason trail will be a list of 3-tuples ``(source, reason, timestamp)``,

with:

``source``

  The entity deciding to perform (or forward) a command.

  It is represented by an arbitrary string, but strings prepended by "gnt:"

  are reserved for Ganeti components, and they will be refused by the

  interfaces towards the external world.

``reason``

  The reason why the entity decided to perform the operation.

  It is represented by an arbitrary string. The string might possibly be empty,

  because certain components of the system might just "pass on" the operation

  (therefore wanting to be recorded in the trail) but without an explicit

  reason.

``timestamp``

  The time when the element was added to the reason trail. It has to be

  expressed in nanoseconds since the unix epoch (0:00:00 January 01, 1970).

  If not enough precision is available (or needed) it can be padded with

  zeroes.

The reason trail will be attached at the OpCode level. When it has to be

serialized externally (such as on the RAPI interface), it will be serialized in

JSON format. Specifically, it will be serialized as a list of elements.

Each element will be a list with two strings (for ``source`` and ``reason``)

and one integer number (the ``timestamp``).

Any component the operation goes through is allowed (but not required) to append

it's own reason to the list. Other than this, the list shouldn't be modified.

As an example here is the reason trail for a shutdown operation invoked from

the command line through the gnt-instance tool::

  [("user", "Cleanup of unused instances", 1363088484000000000),

   ("gnt:client:gnt-instance", "stop", 1363088484020000000),

   ("gnt:opcode:shutdown", "job=1234;index=0", 1363088484026000000),

   ("gnt:daemon:noded:shutdown", "", 1363088484135000000)]

where the first 3-tuple is determined by a user-specified message, passed to

gnt-instance through a command line parameter.

The same operation, launched by an external GUI tool, and executed through the

remote API, would have a reason trail like::

  [("user", "Cleanup of unused instances", 1363088484000000000),

   ("other-app:tool-name", "gui:stop", 1363088484000300000),

   ("gnt:client:rapi:shutdown", "", 1363088484020000000),

   ("gnt:library:rlib2:shutdown", "", 1363088484023000000),

   ("gnt:opcode:shutdown", "job=1234;index=0", 1363088484026000000),

   ("gnt:daemon:noded:shutdown", "", 1363088484135000000)]

Implementation

==============

The OpCode base class will be modified to include a new field, OP_REASON.

This will receive the reason trail as built by all the previous steps.

When an OpCode is added to a job (in jqueue.py) the job number and the opcode

index will be recorded as the reason for the existence of that opcode.

The implementation of this design will start from the operations that affect the

instance status. They will be changed so that the "reason" is passed to them.

They will then export the new expected instance status, together

with the associated reason for the monitoring daemon.

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