This describes the future planned structure of Ganeti daemons.
Signed-off-by: Michele Tartara <[email protected]>
---
Makefile.am | 1 +
doc/design-daemons.rst | 261 ++++++++++++++++++++++++++++++++++++++++++++++++
doc/design-draft.rst | 1 +
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diff --git a/Makefile.am b/Makefile.am
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--- a/Makefile.am
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@@ -422,6 +422,7 @@ docinput = \
doc/design-cpu-pinning.rst \
doc/design-device-uuid-name.rst \
doc/design-draft.rst \
+ doc/design-daemons.rst \
doc/design-htools-2.3.rst \
doc/design-http-server.rst \
doc/design-impexp2.rst \
diff --git a/doc/design-daemons.rst b/doc/design-daemons.rst
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+==========================
+Ganeti daemons refactoring
+==========================
+
+.. contents:: :depth: 2
+
+This is a design document detailing the plan for refactoring the internal
+structure of Ganeti, and particularly the set of daemons it is divided into.
+
+
+Current state and shortcomings
+==============================
+
+Ganeti is comprised of a growing number of daemons, each dealing with part of
+the tasks the cluster has to face, and communicating with the other daemons
+using a variety of protocols.
+
+Specifically, as of Ganeti 2.8, the situation is as follows:
+
+``Master daemon (MasterD)``
+ It is responsible for managing the entire cluster, and it's written in
Python.
+ It is executed on a single node (the master node). It receives the commands
+ given by the cluster administrator (through the remote API daemon or the
+ command line tools) over the LUXI protocol. The master daemon is responsible
+ for creating and managing the jobs that will execute such commands, and for
+ managing the locks that ensure the cluster will not incur in race conditions.
+
+ Each job is managed by a separate Python thread, that interacts with the node
+ daemons via RPC calls.
+
+ The master daemon is also responsible for managing the configuration of the
+ cluster, changing it when required by some job. It is also responsible for
+ copying the configuration to the other master candidates after updating it.
+
+``RAPI daemon (RapiD)``
+ It is written in Python and runs on the master node only. It waits for
+ requests issued remotely through the remote API protocol. Then, it forwards
+ them, using the LUXI protocol, to the master daemon (if they are commands) or
+ to the query daemon if they are queries about the configuration (including
+ live status) of the cluster.
+
+``Node daemon (NodeD)``
+ It is written in Python. It runs on all the nodes. It is responsible for
+ receiving the master requests over RPC and execute them, using the
appropriate
+ backend (hypervisors, DRBD, LVM, etc.). It also receives requests over RPC
for
+ the execution of queries gathering live data on behalf of the query daemon.
+
+``Configuration daemon (ConfD)``
+ It is written in Haskell. It runs on all the master candidates. Since the
+ configuration is replicated only on the master node, this daemon exists in
+ order to provide information about the configuration to nodes needing them.
+ The requests are done through ConfD's own protocol, HMAC signed,
+ implemented over UDP, and meant to be used by parallely querying all the
+ master candidates (or a subset thereof) and getting the most up to date
+ answer. This is meant as a way to provide a robust service even in case
master
+ is temporarily unavailable.
+
+``Query daemon (QueryD)``
+ It is written in Haskell. It runs on all the master candidates. It replies
+ to Luxi queries about the current status of the system, including live data
it
+ obtains by querying the node daemons through RPCs.
+
+``Monitoring daemon (MonD)``
+ It is written in Haskell. It runs on all nodes, including the ones that are
+ not vm-capable. It is meant to provide information on the status of the
+ system. Such information is related only to the specific node the daemon is
+ running on, and it is provided as JSON encoded data over HTTP, to be easily
+ readable by external tools.
+ The monitoring daemon communicates with ConfD to get information about the
+ configuration of the cluster. The choice of communicating with ConfD instead
+ of MasterD allows it to obtain configuration information even when the
cluster
+ is heavily degraded (e.g.: when master and some, but not all, of the master
+ candidates are unreachable).
+
+The current structure of the Ganeti daemons is inefficient because there are
+many different protocols involved, and each daemon needs to be able to use
+multiple ones, and has to deal with doing different things, thus making
+sometimes unclear which daemon is responsible for performing a specific task.
+
+Also, with the current configuration, jobs are managed by the master daemon
+using python threads. This makes terminating a job after it has started a
+difficult operation, and it is the main reason why this is not possible yet.
+
+The master daemon currently has too many different tasks, that could be handled
+better if split among different daemons.
+
+
+Proposed changes
+================
+
+In order to improve on the current situation, a new daemon subdivision is
+proposed, and presented hereafter.
+
+.. digraph:: "new-daemons-structure"
+
+ {rank=same; RConfD LuxiD;}
+ {rank=same; Jobs rconfigdata;}
+ node [shape=box]
+ RapiD [label="RapiD [M]"]
+ LuxiD [label="LuxiD [M]"]
+ WConfD [label="WConfD [M]"]
+ Jobs [label="Jobs [M]"]
+ RConfD [label="RConfD [MC]"]
+ MonD [label="MonD [All]"]
+ NodeD [label="NodeD [All]"]
+ Clients [label="gnt-*\nclients [M]"]
+ p1 [shape=none, label=""]
+ p2 [shape=none, label=""]
+ p3 [shape=none, label=""]
+ p4 [shape=none, label=""]
+ configdata [shape=none, label="config.data"]
+ rconfigdata [shape=none, label="config.data\n[MC copy]"]
+ locksdata [shape=none, label="locks.data"]
+
+ RapiD -> LuxiD [label="LUXI"]
+ LuxiD -> WConfD [label="WConfD\nproto"]
+ LuxiD -> Jobs [label="fork/exec"]
+ Jobs -> WConfD [label="WConfD\nproto"]
+ Jobs -> NodeD [label="RPC"]
+ LuxiD -> NodeD [label="RPC"]
+ rconfigdata -> RConfD
+ configdata -> rconfigdata [label="sync via\nNodeD RPC"]
+ WConfD -> NodeD [label="RPC"]
+ WConfD -> configdata
+ WConfD -> locksdata
+ MonD -> RConfD [label="RConfD\nproto"]
+ Clients -> LuxiD [label="LUXI"]
+ p1 -> MonD [label="MonD proto"]
+ p2 -> RapiD [label="RAPI"]
+ p3 -> RConfD [label="RConfD\nproto"]
+ p4 -> Clients [label="CLI"]
+
+``LUXI daemon (LuxiD)``
+ It will be written in Haskell. It will run on the master node and it will be
+ the only LUXI server, replying to all the LUXI queries. These includes both
+ the queries about the live configuration of the cluster, previously served by
+ QueryD, and the commands actually changing the status of the cluster by
+ submitting jobs. Therefore, this daemon will also be the one responsible with
+ managing the job queue. When a job needs to be executed, the LuxiD will spawn
+ a separate process tasked with the execution of that specific job, thus
making
+ it easier to terminate the job itself, if needeed. When a job requires
locks,
+ LuxiD will request them from WConfD.
+ In order to keep availability of the cluster in case of failure of the master
+ node, LuxiD will replicate the job queue to the other master candidates, by
+ RPCs to the NodeD running there.
+
+``Configuration management daemon (WConfD)``
+ It will run on the master node and it will be responsible for the management
+ of the authoritative copy of the cluster configuration (that is, it will be
+ the daemon actually modifying the ``config.data`` file). All the requests of
+ configuration changes will have to pass through this daemon, and will be
+ performed using a LUXI-like protocol ("WConfD proto" in the graph. The exact
+ protocol will be defined in the separate design document that will detail the
+ WConfD separation). Having a single point of configuration management will
+ also allow Ganeti to get rid of possible race conditions due to concurrent
+ modifications of the configuration. When the configuration is updated, it
+ will have to push the received changes to the other master candidates, via
+ RPC, so that RConfD daemons and (in case of a failure on the master node) the
+ WConfD daemon on the new master can access an up-to-date version of it. This
+ daemon will also be the one responsible for managing the locks, granting them
+ to the jobs requesting them, and taking care of freeing them up if the jobs
+ holding them crash or are terminated before releasing them. In order to do
+ this, each job, after being spawned by LuxiD, will open a local unix socket
+ that will be used to communicate with it, and will be destroyed when the job
+ terminates. LuxiD will be able to check, after a timeout, whether the job is
+ still running by connecting here, and to ask WConfD to forcefully remove the
+ locks if the socket is closed.
+ Also, WConfD should hold a serialized list of the locks and their owners in a
+ file (``locks.data``), so that it can keep track of their status in case it
+ crashes and needs to be restarted (by asking LuxiD which of them are still
+ running).
+ Interaction with this daemon will be performed using Unix sockets.
+
+``Configuration query daemon (RConfD)``
+ It is written in Haskell, and it corresponds to the old ConfD. It will run on
+ all the master candidates and it will serve information about the the static
+ configuration of the cluster (the one contained in ``config.data``). The
+ provided information will be highly available (as in: a response will be
+ available as long as a stable-enough connection between the client and at
+ least one working master candidate is available) and its freshness will be
+ best effort (the most recent reply from any of the master candidates will be
+ returned, but it might still be older than the one available through WConfD).
+ The information will be served through the ConfD protocol.
+
+``Rapi daemon (RapiD)``
+ It remains basically unchanged, with the only difference that all of its LUXI
+ query are directed towards LuxiD instead of being split between MasterD and
+ QueryD.
+
+``Monitoring daemon (MonD)``
+ It remains unaffected by the changes in this design document. It will just
get
+ some of the data it needs from RConfD instead of the old ConfD, but the
+ interfaces of the two are identical.
+
+``Node daemon (NodeD)``
+ It remains unaffected by the changes proposed in the design document. The
only
+ difference being that it will receive its RPCs from LuxiD (for job queue
+ replication), from WConfD (for configuration replication) and for the
+ processes executing single jobs (for all the operations to be performed by
+ nodes) instead of receiving them just from MasterD.
+
+This restructuring will allow us to reorganize and improve the codebase,
+introducing cleaner interfaces and giving well defined and more restricted
tasks
+to each daemon.
+
+Furthermore, having more well-defined interfaces will allow us to have easier
+upgrade procedures, and to work towards the possibility of upgrading single
+components of a cluster one at a time, without the need for immediately
+upgrading the entire cluster in a single step.
+
+
+Implementation
+==============
+
+While performing this refactoring, we aim to increase the amount of
+Haskell code, thus benefiting from the additional type safety provided by its
+wide compile-time checks. In particular, all the job queue management and the
+configuration management daemon will be written in Haskell, taking over the
role
+currently fulfilled by Python code executed as part of MasterD.
+
+The changes describe by this design document are quite extensive, therefore
they
+will not be implemented all at the same time, but through a sequence of steps,
+leaving the codebase in a consistent and usable state.
+
+#. Rename QueryD to LuxiD.
+ A part of LuxiD, the one replying to configuration
+ queries including live information about the system, already exists in the
+ form of QueryD. This is being renamed to LuxiD, and will form the first part
+ of the new daemon. NB: this is happening in Ganeti 2.8.
+
+#. Let LuxiD be the interface for the queries and MasterD be their executor.
+ Currently, MasterD is the only responsible for receiving and executing LUXI
+ queries, and for managing the jobs they create.
+ Receiving the queries and managing the job queue will be extracted from
+ MasterD into LuxiD.
+ Actually executing jobs will still be done by MasterD, that contains all the
+ logic for doing that and for properly managing locks and the configuration.
+
+#. Extract WConfD from MasterD.
+ The logic for managing the configuration file is factored out to the
+ dedicated WConfD daemon. All configuration changes, currently executed
+ directly by MasterD, will be changed to be IPC requests sent to the new
+ daemon.
+
+#. Extract locking management from MasterD.
+ The logic for managing and granting locks is extracted to WConfD as well.
+ Locks will not be taken directly anymore, but asked via IPC to WConfD.
+ This step can be executed on its own or at the same time as the previous
one.
+
+#. Jobs are executed as processes.
+ The logic for running jobs is rewritten so that each job can be managed by
an
+ independent process. LuxiD will spawn a new (Python) process for every
single
+ job. The RPCs will remain unchanged, and the LU code will stay as is as much
+ as possible.
+
+
+.. vim: set textwidth=72 :
+.. Local Variables:
+.. mode: rst
+.. fill-column: 72
+.. End:
diff --git a/doc/design-draft.rst b/doc/design-draft.rst
index 8c2eff2..04dce6b 100644
--- a/doc/design-draft.rst
+++ b/doc/design-draft.rst
@@ -16,6 +16,7 @@ Design document drafts
design-monitoring-agent.rst
design-hroller.rst
design-storagetypes.rst
+ design-daemons.rst
.. vim: set textwidth=72 :
.. Local Variables:
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