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========================== |
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Ganeti daemons refactoring |
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========================== |
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.. contents:: :depth: 2 |
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This is a design document detailing the plan for refactoring the internal |
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structure of Ganeti, and particularly the set of daemons it is divided into. |
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|
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Current state and shortcomings |
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============================== |
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|
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Ganeti is comprised of a growing number of daemons, each dealing with part of |
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the tasks the cluster has to face, and communicating with the other daemons |
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using a variety of protocols. |
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|
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Specifically, as of Ganeti 2.8, the situation is as follows: |
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|
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``Master daemon (MasterD)`` |
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It is responsible for managing the entire cluster, and it's written in Python. |
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It is executed on a single node (the master node). It receives the commands |
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given by the cluster administrator (through the remote API daemon or the |
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command line tools) over the LUXI protocol. The master daemon is responsible |
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for creating and managing the jobs that will execute such commands, and for |
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managing the locks that ensure the cluster will not incur in race conditions. |
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|
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Each job is managed by a separate Python thread, that interacts with the node |
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daemons via RPC calls. |
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|
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The master daemon is also responsible for managing the configuration of the |
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cluster, changing it when required by some job. It is also responsible for |
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copying the configuration to the other master candidates after updating it. |
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|
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``RAPI daemon (RapiD)`` |
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It is written in Python and runs on the master node only. It waits for |
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requests issued remotely through the remote API protocol. Then, it forwards |
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them, using the LUXI protocol, to the master daemon (if they are commands) or |
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to the query daemon if they are queries about the configuration (including |
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live status) of the cluster. |
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|
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``Node daemon (NodeD)`` |
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It is written in Python. It runs on all the nodes. It is responsible for |
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receiving the master requests over RPC and execute them, using the appropriate |
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backend (hypervisors, DRBD, LVM, etc.). It also receives requests over RPC for |
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the execution of queries gathering live data on behalf of the query daemon. |
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|
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``Configuration daemon (ConfD)`` |
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It is written in Haskell. It runs on all the master candidates. Since the |
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configuration is replicated only on the master node, this daemon exists in |
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order to provide information about the configuration to nodes needing them. |
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The requests are done through ConfD's own protocol, HMAC signed, |
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implemented over UDP, and meant to be used by parallely querying all the |
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master candidates (or a subset thereof) and getting the most up to date |
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answer. This is meant as a way to provide a robust service even in case master |
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is temporarily unavailable. |
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|
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``Query daemon (QueryD)`` |
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It is written in Haskell. It runs on all the master candidates. It replies |
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to Luxi queries about the current status of the system, including live data it |
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obtains by querying the node daemons through RPCs. |
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|
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``Monitoring daemon (MonD)`` |
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It is written in Haskell. It runs on all nodes, including the ones that are |
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not vm-capable. It is meant to provide information on the status of the |
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system. Such information is related only to the specific node the daemon is |
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running on, and it is provided as JSON encoded data over HTTP, to be easily |
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readable by external tools. |
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The monitoring daemon communicates with ConfD to get information about the |
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configuration of the cluster. The choice of communicating with ConfD instead |
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of MasterD allows it to obtain configuration information even when the cluster |
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is heavily degraded (e.g.: when master and some, but not all, of the master |
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candidates are unreachable). |
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|
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The current structure of the Ganeti daemons is inefficient because there are |
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many different protocols involved, and each daemon needs to be able to use |
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multiple ones, and has to deal with doing different things, thus making |
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sometimes unclear which daemon is responsible for performing a specific task. |
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Also, with the current configuration, jobs are managed by the master daemon |
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using python threads. This makes terminating a job after it has started a |
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difficult operation, and it is the main reason why this is not possible yet. |
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The master daemon currently has too many different tasks, that could be handled |
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better if split among different daemons. |
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Proposed changes |
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================ |
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In order to improve on the current situation, a new daemon subdivision is |
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proposed, and presented hereafter. |
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|
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.. digraph:: "new-daemons-structure" |
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|
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{rank=same; RConfD LuxiD;} |
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{rank=same; Jobs rconfigdata;} |
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node [shape=box] |
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RapiD [label="RapiD [M]"] |
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LuxiD [label="LuxiD [M]"] |
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WConfD [label="WConfD [M]"] |
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Jobs [label="Jobs [M]"] |
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RConfD [label="RConfD [MC]"] |
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MonD [label="MonD [All]"] |
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NodeD [label="NodeD [All]"] |
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Clients [label="gnt-*\nclients [M]"] |
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p1 [shape=none, label=""] |
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p2 [shape=none, label=""] |
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p3 [shape=none, label=""] |
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p4 [shape=none, label=""] |
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configdata [shape=none, label="config.data"] |
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rconfigdata [shape=none, label="config.data\n[MC copy]"] |
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locksdata [shape=none, label="locks.data"] |
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|
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RapiD -> LuxiD [label="LUXI"] |
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LuxiD -> WConfD [label="WConfD\nproto"] |
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LuxiD -> Jobs [label="fork/exec"] |
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Jobs -> WConfD [label="WConfD\nproto"] |
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Jobs -> NodeD [label="RPC"] |
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LuxiD -> NodeD [label="RPC"] |
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rconfigdata -> RConfD |
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configdata -> rconfigdata [label="sync via\nNodeD RPC"] |
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WConfD -> NodeD [label="RPC"] |
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WConfD -> configdata |
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WConfD -> locksdata |
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MonD -> RConfD [label="RConfD\nproto"] |
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Clients -> LuxiD [label="LUXI"] |
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p1 -> MonD [label="MonD proto"] |
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p2 -> RapiD [label="RAPI"] |
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p3 -> RConfD [label="RConfD\nproto"] |
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p4 -> Clients [label="CLI"] |
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``LUXI daemon (LuxiD)`` |
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It will be written in Haskell. It will run on the master node and it will be |
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the only LUXI server, replying to all the LUXI queries. These includes both |
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the queries about the live configuration of the cluster, previously served by |
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QueryD, and the commands actually changing the status of the cluster by |
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submitting jobs. Therefore, this daemon will also be the one responsible with |
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managing the job queue. When a job needs to be executed, the LuxiD will spawn |
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a separate process tasked with the execution of that specific job, thus making |
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it easier to terminate the job itself, if needeed. When a job requires locks, |
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LuxiD will request them from WConfD. |
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In order to keep availability of the cluster in case of failure of the master |
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node, LuxiD will replicate the job queue to the other master candidates, by |
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RPCs to the NodeD running there (the choice of RPCs for this task might be |
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reviewed at a second time, after implementing this design). |
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``Configuration management daemon (WConfD)`` |
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It will run on the master node and it will be responsible for the management |
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of the authoritative copy of the cluster configuration (that is, it will be |
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the daemon actually modifying the ``config.data`` file). All the requests of |
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configuration changes will have to pass through this daemon, and will be |
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performed using a LUXI-like protocol ("WConfD proto" in the graph. The exact |
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protocol will be defined in the separate design document that will detail the |
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WConfD separation). Having a single point of configuration management will |
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also allow Ganeti to get rid of possible race conditions due to concurrent |
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modifications of the configuration. When the configuration is updated, it |
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will have to push the received changes to the other master candidates, via |
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RPCs, so that RConfD daemons and (in case of a failure on the master node) |
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the WConfD daemon on the new master can access an up-to-date version of it |
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(the choice of RPCs for this task might be reviewed at a second time). This |
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daemon will also be the one responsible for managing the locks, granting them |
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to the jobs requesting them, and taking care of freeing them up if the jobs |
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holding them crash or are terminated before releasing them. In order to do |
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this, each job, after being spawned by LuxiD, will open a local unix socket |
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that will be used to communicate with it, and will be destroyed when the job |
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terminates. LuxiD will be able to check, after a timeout, whether the job is |
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still running by connecting here, and to ask WConfD to forcefully remove the |
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locks if the socket is closed. |
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Also, WConfD should hold a serialized list of the locks and their owners in a |
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file (``locks.data``), so that it can keep track of their status in case it |
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crashes and needs to be restarted (by asking LuxiD which of them are still |
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running). |
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Interaction with this daemon will be performed using Unix sockets. |
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``Configuration query daemon (RConfD)`` |
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It is written in Haskell, and it corresponds to the old ConfD. It will run on |
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all the master candidates and it will serve information about the the static |
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configuration of the cluster (the one contained in ``config.data``). The |
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provided information will be highly available (as in: a response will be |
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available as long as a stable-enough connection between the client and at |
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least one working master candidate is available) and its freshness will be |
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best effort (the most recent reply from any of the master candidates will be |
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returned, but it might still be older than the one available through WConfD). |
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The information will be served through the ConfD protocol. |
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``Rapi daemon (RapiD)`` |
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It remains basically unchanged, with the only difference that all of its LUXI |
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query are directed towards LuxiD instead of being split between MasterD and |
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QueryD. |
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``Monitoring daemon (MonD)`` |
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It remains unaffected by the changes in this design document. It will just get |
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some of the data it needs from RConfD instead of the old ConfD, but the |
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interfaces of the two are identical. |
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``Node daemon (NodeD)`` |
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It remains unaffected by the changes proposed in the design document. The only |
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difference being that it will receive its RPCs from LuxiD (for job queue |
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replication), from WConfD (for configuration replication) and for the |
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processes executing single jobs (for all the operations to be performed by |
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nodes) instead of receiving them just from MasterD. |
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This restructuring will allow us to reorganize and improve the codebase, |
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introducing cleaner interfaces and giving well defined and more restricted tasks |
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to each daemon. |
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Furthermore, having more well-defined interfaces will allow us to have easier |
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upgrade procedures, and to work towards the possibility of upgrading single |
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components of a cluster one at a time, without the need for immediately |
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upgrading the entire cluster in a single step. |
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Implementation |
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============== |
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While performing this refactoring, we aim to increase the amount of |
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Haskell code, thus benefiting from the additional type safety provided by its |
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wide compile-time checks. In particular, all the job queue management and the |
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configuration management daemon will be written in Haskell, taking over the role |
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currently fulfilled by Python code executed as part of MasterD. |
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The changes describe by this design document are quite extensive, therefore they |
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will not be implemented all at the same time, but through a sequence of steps, |
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leaving the codebase in a consistent and usable state. |
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#. Rename QueryD to LuxiD. |
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A part of LuxiD, the one replying to configuration |
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queries including live information about the system, already exists in the |
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form of QueryD. This is being renamed to LuxiD, and will form the first part |
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of the new daemon. NB: this is happening starting from Ganeti 2.8. At the |
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beginning, only the already existing queries will be replied to by LuxiD. |
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More queries will be implemented in the next versions. |
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#. Let LuxiD be the interface for the queries and MasterD be their executor. |
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Currently, MasterD is the only responsible for receiving and executing LUXI |
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queries, and for managing the jobs they create. |
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Receiving the queries and managing the job queue will be extracted from |
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MasterD into LuxiD. |
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Actually executing jobs will still be done by MasterD, that contains all the |
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logic for doing that and for properly managing locks and the configuration. |
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A separate design document will detail how the system will decide which jobs |
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to send over for execution, and how to rate-limit them. |
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#. Extract WConfD from MasterD. |
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The logic for managing the configuration file is factored out to the |
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dedicated WConfD daemon. All configuration changes, currently executed |
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directly by MasterD, will be changed to be IPC requests sent to the new |
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daemon. |
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|
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#. Extract locking management from MasterD. |
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The logic for managing and granting locks is extracted to WConfD as well. |
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Locks will not be taken directly anymore, but asked via IPC to WConfD. |
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This step can be executed on its own or at the same time as the previous one. |
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|
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#. Jobs are executed as processes. |
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The logic for running jobs is rewritten so that each job can be managed by an |
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independent process. LuxiD will spawn a new (Python) process for every single |
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job. The RPCs will remain unchanged, and the LU code will stay as is as much |
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as possible. |
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MasterD will cease to exist as a deamon on its own at this point, but not |
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before. |
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Further considerations |
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====================== |
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|
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There is a possibility that a job will finish performing its task while LuxiD |
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and/or WConfD will not be available. |
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In order to deal with this situation, each job will write the results of its |
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execution on a file. The name of this file will be known to LuxiD before |
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starting the job, and will be stored together with the job ID, and the |
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name of the job-unique socket. |
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The job, upon ending its execution, will signal LuxiD (through the socket), so |
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that it can read the result of the execution and release the locks as needed. |
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In case LuxiD is not available at that time, the job will just terminate without |
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signalling it, and writing the results on file as usual. When a new LuxiD |
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becomes available, it will have the most up-to-date list of running jobs |
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(received via replication from the former LuxiD), and go through it, cleaning up |
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all the terminated jobs. |
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.. vim: set textwidth=72 : |
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.. Local Variables: |
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.. mode: rst |
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.. fill-column: 72 |
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.. End: |