Ganeti daemons refactoring

Created:

2013-Sep-27

Status:

Implemented

Ganeti-Version:

2.12.0

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.

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 needed. 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 (the choice of RPCs for this task might be reviewed at a second time, after implementing this design).

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 RPCs, 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 (the choice of RPCs for this task might be reviewed at a second time). 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 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.

1. 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 starting from Ganeti 2.8. At the beginning, only the already existing queries will be replied to by LuxiD. More queries will be implemented in the next versions.

2. 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. At this stage, scheduling will simply consist in starting jobs until a fixed maximum number of simultaneously running jobs is reached.

3. 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.

4. 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.

5. 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. MasterD will cease to exist as a daemon on its own at this point, but not before.

6. Improve job scheduling algorithm. The simple algorithm for scheduling jobs will be replaced by a more intelligent one. Also, the implementation of Filtering of jobs for the Ganeti job queue can be started.

Job death detection

Requirements:

• It must be possible to reliably detect a death of a process even under uncommon conditions such as very heavy system load.

• A daemon must be able to detect a death of a process even if the daemon is restarted while the process is running.

• The solution must not rely on being able to communicate with a process.

• The solution must work for the current situation where multiple jobs run in a single process.

• It must be POSIX compliant.

These conditions rule out simple solutions like checking a process ID (because the process might be eventually replaced by another process with the same ID) or keeping an open connection to a process.

Solution: As a job process is spawned, before attempting to communicate with any other process, it will create a designated empty lock file, open it, acquire an exclusive lock on it, and keep it open. When connecting to a daemon, the job process will provide it with the path of the file. If the process dies unexpectedly, the operating system kernel automatically cleans up the lock.

Therefore, daemons can check if a process is dead by trying to acquire a shared lock on the lock file in a non-blocking mode:

• If the locking operation succeeds, it means that the exclusive lock is missing, therefore the process has died, but the lock file hasn’t been cleaned up yet. The daemon should release the lock immediately. Optionally, the daemon may delete the lock file.

• If the file is missing, the process has died and the lock file has been cleaned up.

• If the locking operation fails due to a lock conflict, it means the process is alive.

Using shared locks for querying lock files ensures that the detection works correctly even if multiple daemons query a file at the same time.

A job should close and remove its lock file when completely finishes. The WConfD daemon will be responsible for removing stale lock files of jobs that didn’t remove its lock files themselves.

Statelessness of the protocol: To keep our protocols stateless, the job id and the path the to lock file are sent as part of every request that deals with resources, in particular the Ganeti Locks. All resources are owned by the pair (job id, lock file). In this way, several jobs can live in the same process (as it will be in the transition period), but owner death detection still only depends on the owner of the resource. In particular, no additional lookup table is needed to obtain the lock file for a given owner.

Considered alternatives: An alternative to creating a separate lock file would be to lock the job status file. However, file locks are kept only as long as the file is open. Therefore any operation followed by closing the file would cause the process to release the lock. In particular, with jobs as threads, the master daemon wouldn’t be able to keep locks and operate on job files at the same time.

Job execution

As the Luxi daemon will be responsible for executing jobs, it needs to start jobs in such a way that it can properly detect if the job dies under any circumstances (such as Luxi daemon being restarted in the process).

The name of the lock file will be stored in the corresponding job file so that anybody is able to check the status of the process corresponding to a job.

The proposed procedure:

1. The Luxi daemon saves the name of its own lock file into the job file.

2. The Luxi daemon forks, creating a bi-directional pipe with the child process.

3. The child process creates and locks its own, proper lock file and handles its name to the Luxi daemon through the pipe.

4. The Luxi daemon saves the name of the lock file into the job file and confirms it to the child process.

5. Only then the child process can replace itself by the actual job process.

If the child process detects that the pipe is broken before receiving the confirmation, it must terminate, not starting the actual job. This way, the actual job is only started if it is ensured that its lock file name is written to the job file.

If the Luxi daemon detects that the pipe is broken before successfully sending the confirmation in step 4., it assumes that the job has failed. If the pipe gets broken after sending the confirmation, no further action is necessary. If the child doesn’t receive the confirmation, it will die and its death will be detected by Luxid eventually.

If the Luxi daemon dies before completing the procedure, the job will not be started. If the job file contained the daemon’s lock file name, it will be detected as dead (because the daemon process died). If the job file already contained its proper lock file, it will also be detected as dead (because the child process won’t start the actual job and die).

WConfD details

WConfD will communicate with its clients through a Unix domain socket for both configuration management and locking. Clients can issue multiple RPC calls through one socket. For each such a call the client sends a JSON request document with a remote function name and data for its arguments. The server replies with a JSON response document containing either the result of signalling a failure.

Any state associated with client processes will be mirrored on persistent storage and linked to the identity of processes so that the WConfD daemon will be able to resume its operation at any point after a restart or a crash. WConfD will track each client’s process start time along with its process ID to be able detect if a process dies and it’s process ID is reused. WConfD will clear all locks and other state associated with a client if it detects it’s process no longer exists.

Configuration management

The new configuration management protocol will be implemented in the following steps:

Step 1:

1. Implement the following functions in WConfD and export them through RPC:

   • Obtain a single internal lock, either in shared or exclusive mode. This lock will substitute the current lock _config_lock in config.py.

   • Release the lock.

   • Return the whole configuration data to a client.

   • Receive the whole configuration data from a client and replace the current configuration with it. Distribute it to master candidates and distribute the corresponding ssconf.

   WConfD must detect deaths of its clients (see Job death detection) and release locks automatically.

2. In config.py modify public methods that access configuration:

   • Instead of acquiring a local lock, obtain a lock from WConfD using the above functions

   • Fetch the current configuration from WConfD.

   • Use it to perform the method’s task.

   • If the configuration was modified, send it to WConfD at the end.

   • Release the lock to WConfD.

This will decouple the configuration management from the master daemon, even though the specific configuration tasks will still performed by individual jobs.

After this step it’ll be possible access the configuration from separate processes.

Step 2:

1. Reimplement all current methods of ConfigWriter for reading and writing the configuration of a cluster in WConfD.

2. Expose each of those functions in WConfD as a separate RPC function. This will allow easy future extensions or modifications.

3. Replace ConfigWriter with a stub (preferably automatically generated from the Haskell code) that will contain the same methods as the current ConfigWriter and delegate all calls to its methods to WConfD.

Step 3:

In a later step, the impact of the config lock will be reduced by moving it more and more into an internal detail of WConfD. This forthcoming process of Removal of the Config Lock Overhead is described separately.

Locking

The new locking protocol will be implemented as follows:

Re-implement the current locking mechanism in WConfD and expose it for RPC calls. All current locks will be mapped into a data structure that will uniquely identify them (storing lock’s level together with it’s name).

WConfD will impose a linear order on locks. The order will be compatible with the current ordering of lock levels so that existing code will work without changes.

WConfD will keep the set of currently held locks for each client. The protocol will allow the following operations on the set:

Update:

Update the current set of locks according to a given list. The list contains locks and their desired level (release / shared / exclusive). To prevent deadlocks, WConfD will check that all newly requested locks (or already held locks requested to be upgraded to exclusive) are greater in the sense of the linear order than all currently held locks, and fail the operation if not. Only the locks in the list will be updated, other locks already held will be left intact. If the operation fails, the client’s lock set will be left intact.

Opportunistic union:

Add as much as possible locks from a given set to the current set within a given timeout. WConfD will again check the proper order of locks and acquire only the ones that are allowed wrt. the current set. Returns the set of acquired locks, possibly empty. Immediate. Never fails. (It would also be possible to extend the operation to try to wait until a given number of locks is available, or a given timeout elapses.)

List:

List the current set of held locks. Immediate, never fails.

Intersection:

Retain only a given set of locks in the current one. This function is provided for convenience, it’s redundant wrt. list and update. Immediate, never fails.

Additional restrictions due to lock implications:

Ganeti supports locks that act as if a lock on a whole group (like all nodes) were held. To avoid dead locks caused by the additional blockage of those group locks, we impose certain restrictions. Whenever A is a group lock and B belongs to A, then the following holds.

• A is in lock order before B.

• All locks that are in the lock order between A and B also belong to A.

• It is considered a lock-order violation to ask for an exclusive lock on B while holding a shared lock on A.

After this step it’ll be possible to use locks from jobs as separate processes.

The above set of operations allows the clients to use various work-flows. In particular:

Pessimistic strategy:

Lock all potentially relevant resources (for example all nodes), determine which will be needed, and release all the others.

Optimistic strategy:

Determine what locks need to be acquired without holding any. Lock the required set of locks. Determine the set of required locks again and check if they are all held. If not, release everything and restart.

Future aims:

• Add more fine-grained locks to prevent unnecessary blocking of jobs. This could include locks on parameters of entities or locks on their states (so that a node remains online, but otherwise can change, etc.). In particular, adding, moving and removing instances currently blocks the whole node.

• Add checks that all modified configuration parameters belong to entities the client has locked and log violations.

• Make the above checks mandatory.

• Automate optimistic locking and checking the locks in logical units. For example, this could be accomplished by allowing some of the initial phases of LogicalUnit (such as ExpandNames and DeclareLocks) to be run repeatedly, checking if the set of locks requested the second time is contained in the set acquired after the first pass.

• Add the possibility for a job to reserve hardware resources such as disk space or memory on nodes. Most likely as a new, special kind of instances that would only block its resources and allow to be converted to a regular instance. This would allow long-running jobs such as instance creation or move to lock the corresponding nodes, acquire the resources and turn the locks into shared ones, keeping an exclusive lock only on the instance.

• Use more sophisticated algorithm for preventing deadlocks such as a wait-for graph. This would allow less union failures and allow more optimistic, scalable acquisition of locks.

Further considerations

There is a possibility that a job will finish performing its task while LuxiD and/or WConfD will not be available. In order to deal with this situation, each job will update its job file in the queue. This is race free, as LuxiD will no longer touch the job file, once the job is started; a corollary of this is that the job also has to take care of replicating updates to the job file. LuxiD will watch job files for changes to determine when a job was cleanly finished. To determine jobs that died without having the chance of updating the job file, the Job death detection mechanism will be used.