nickva commented on a change in pull request #409: RFC for CouchDB background workers URL: https://github.com/apache/couchdb-documentation/pull/409#discussion_r281636484
########## File path: rfcs/007-background-workers.md ########## @@ -0,0 +1,252 @@ +--- +name: Formal RFC +about: Submit a formal Request For Comments for consideration by the team. +title: 'Background workers with FoundationDB backend' +labels: rfc, discussion +assignees: '' + +--- + +[NOTE]: # ( ^^ Provide a general summary of the RFC in the title above. ^^ ) + +# Introduction + +This document describes a data model and behavior of CouchDB background workers. + +## Abstract + +CouchDB background workers are used for things like index building and +replication. We present a generalized model that allows creation, running, and +monitoring of these jobs. "Jobs" are represented generically such that both +replication and indexing could take advantage of the same framework. The basic +idea is that of a global job queue for each job type. New jobs are inserted +into the jobs table and enqueued for execution. + +There are a number of workers that attempt to dequeue pending jobs and run +them. "Running" is specific to each job type and would be different for +replication and indexing. Workers are processes which execute jobs. The MAY be +individual Erlang processes, but could also be implemented in Python, Java or +any other environment with a FoundationDB client. The only coordination between +workers happens via the database. Workers can start and stop at any time. +Workers monitor each other for liveliness and in case some workers abruptly +terminate, all the jobs of a dead worker are re-enqueued into the global +pending queue. + +## Requirements Language + +[NOTE]: # ( Do not alter the section below. Follow its instructions. ) + +The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", +"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this +document are to be interpreted as described in +[RFC 2119](https://www.rfc-editor.org/rfc/rfc2119.txt). + +## Terminology + +--- + +`Job`: An unit of work, identified by a `JobId` and also having a `JobType`. + +`Job table`: A subspace holding the list of jobs indexed by `JobId`. + +`Pending queue`: A queue of jobs which are ready to run. Workers may pick jobs +from this queue and start running them. + +`Active jobs`: A subspace holding the list of jobs currently run by a +particular worker. + +`Worker`: A job execution unit. Workers could be individual processes or groups +of processes running on remote nodes. + +`Health key` : A key that the worker periodically updates with a timestamp to +indicate that they are "alive" and ready to process jobs. + +`Versionstamp`: a 12 byte, unique, monotonically (but not sequentially) +increasing value for each committed transaction. + +--- + +# Detailed Description + +## Data Model + +The main job table: + - `("couch_workers", "jobs", JobType, JobId) = (JobState, WorkerId, Priority, CancelReq, JobInfo, JobOps)` + +Pending queue: + - `("couch_workers", "pending", JobType, Priority, JobId) = ""` + +Active queue: + - `("couch_workers", "active", WorkerType, Worker, JobId) = (JobState, JobInfo, JobOpts)` + +Worker registration and health: + - `("couch_workers", WorkerType, "workers_vs") = Versionstamp` + - `("couch_workers", WorkerType, "workers", WorkerId) = WorkerOpts` + - `("couch_workers", WorkerType, "health", WorkerId) = (Versionstamp, Timestamp, WorkerTimeout)` + +### Data model fields + +- `JobType`: The job type. It MAY be ``replication`` or ``indexing``, however, + specific types are not in scope of this document. + +- `JobId` : MUST unique identify a job in a cluster. It MAY be a `UUID` or + derived from some other job parameters. + +- `JobState` : MUST be one of `pending`, `running`, or `completed` + +- `Priority` : MAY be any key which allows sorting jobs, such as a timestamp, + or a tag. + +- `WorkerId` : MUST uniquely identify a worker in a cluster. It MAY be + generated each time the worker is restarted, or it could be persisted across + worker restarts, as long as it is unique across the whole cluster. + +- `CancelReq` : A boolean flag indicating a request to the worker to cancel the + job. + +- `JobOps` : Type-specific job options. Represented as a JSON object. This MAY + contain fields like `"source"`, `"target"`, `"dbname"` etc. + +- `JobInfo` : A per-job info object. Represented as JSON. This object will + contain details about a job's current state. It MAY have fields such as + "update_seq", "error_count", etc. + + +## Job Lifecyle + +New jobs are posted to the main jobs table along with an entry in the pending +queue. Priority key assignment is type specific. `WorkerId` is set to `nil` +initially. + +Workers will monitor the pending queue with their matching `JobType`. That is, +`"replication"` workers will monitor the `"replication"` pending queue, +indexing workers the `"indexing"` queue, etc. If there are jobs ready to run, +they will attempt to grab one or more jobs from the pending queue and assign it +to themselves for execution. They do that by setting the `WorkerId` in the jobs +table entry to their worker ID, removing the job from pending queue and create +a new entry in their own `"active"` jobs area. + +When a job finishes running, either because it was a successful completion or a +terminal failure, the worker MUST remove the job from its active queue and +clear its `WorkerId` field. Then, based on type-specific behavior, it MUST do +either one of: + + - Update the `JobState` to `"completed"` and leave the job in the jobs table. + It would be up to the job creation logic to inspect and remove it. + - Delete the job from the system altogether. This may be useful when running + _replicate jobs, for example. + + +If a user decided to cancel a job that is running, they MUST toggle `CancelReq` +to `true`. Then they MUST wait for the job to stop running. If the job is not +running, they may directly remove the job from the pending and the main jobs +table. + + +## Worker Lifecyle + +Workers can be dynamically started and stopped. When workers start, they MUST +register themselves in the `"workers"` subspace based on their `WorkerType`. +Each change to the workers list, should be accompanied by updating the +`"workers_vs"` versionstamp. That versionstamp MAY be efficiently monitored for +changes using a watch. + + +### Worker Liveliness Monitoring + +As long as the workers are active and ready to process jobs, they MUST +periodically ping their health key. The period MUST be less than the +`WorkerTimeout` value. Other worker nodes, for example, one of the "neighbors" +(to left or to the right) of the worker, will monitor that health entry. If a +worker dies unexpectedly, the neighbor MUST remove all of the dead worker's +jobs from the active queue and submit them back into the pending queue. + + +### Liveliness Race Condition + +In case workers come back alive after not updating their health entry often +enough, and another worker had cleared and re-submitted they jobs, workers MUST +monitor their active queue and if they detect that it was cleared or `WorkerId` +was updated and they are no longer the current `WorkerId`, they MUST stop +running the job and go back to the pending queue to pick new jobs to run. + +### Clean Worker Shutdown + +If a worker is stopped cleanly, they MUST stop accepting new jobs from the +pending queue, stop running their jobs, and then resubmit those jobs back into +the pending queue. Finally, they MUST clear their health entry and remove +themselves from workers membership list. + +### User Job Cancellation + +If a user decided to a cancel an active job. They MUST update the `CancelReq` +field to `true`. Workers should periodically monitor that field and if they +field is true, they should stop running the job, update its jobs state and +remove it from their active queue. + +## `/_active_tasks` API Compatibility + +The shape of the active queue was deliberately constructed with the hopes of +using it to implement the `/_active_tasks` API endpoint. + +When the HTTP API is access, a single `get_range(...)` request for +`("couch_workers", "active")` subspace MAY be used to generate the results. Review comment: Yeah that is a good idea. I couldn't think of a good shape for the API yet and left it out. Once we get a feel for it, (play with a prototype), it might be easier to see what it might look like. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: [email protected] With regards, Apache Git Services
