doc: Background tasks #4352
Open
doc: Background tasks #4352
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This patch implements the whole support ecosystem for server-side background tasks, in order to help lessen the load (and blocking) of API handlers in the web-server for long-running operations. A **Task** is represented by two things in strict co-existence: a lightweight, `pickle`-able implementation in the server's code (a subclass of `AbstractTask`) and a corresponding `BackgroundTask` database entity, which resides in the "configuration" database (shared across all products). A Task is created by API request handlers and then the user is instructed to retain the `TaskToken`: the task's unique identifier. Following, the server will dispatch execution of the object into a background worker process, and keep status synchronisation via the database. Even in a service cluster deployment, load balancing will not interfere with users' ability to query a task's status. While normal users can only query the status of a single task (which is usually automatically done by client code, and not the user manually executing something); product administrators, and especially server administrators have the ability to query an arbitrary set of tasks using the potential filters, with a dedicated API function (`getTasks()`) for this purpose. Tasks can be cancelled only by `SUPERUSER`s, at which point a special binary flag is set in the status record. However, to prevent complicating inter-process communication, cancellation is supposed to be implemented by `AbstractTask` subclasses in a co-operative way. The execution of tasks in a process and a `Task`'s ability to "communicate" with its execution environment is achieved through the new `TaskManager` instance, which is created for every process of a server's deployment. Unfortunately, tasks can die gracelessly if the server is terminated (either internally, or even externally). For this reason, the `DROPPED` status will indicate that the server has terminated prior to, or during a task's execution, and it was unable to produce results. The server was refactored significantly around the handling of subprocesses in order to support various server shutdown scenarios. Servers will start `background_worker_processes` number of task handling subprocesses, which are distinct from the already existing "API handling" subprocesses. By default, if unconfigured, `background_worker_processes` is equal to `worker_processes` (the number of API processes to spawn), which is equal to `$(nproc)` (CPU count in the system). This patch includes a `TestingDummyTask` demonstrative subclass of `AbstractTask` which counts up to an input number of seconds, and each second it gracefully checks whether it is being killed. The corresponding testing API endpoint, `createDummyTask()` can specify whether the task should simulate a failing status. This endpoint can only be used from, but is used extensively, the unit testing of the project. This patch does not include "nice" or "ergonomic" facilities for admins to manage the tasks, and so far, only the server-side of the corresponding API calls are supported.
whisperity
added
enhancement 🌟
documentation 📖
whisperity
requested review from
dkrupp,
bruntib and
vodorok
as code owners
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Important
⛔ Blocked by #4317.
Part of the Asynchronous Store Protocol (#3672) support ecosystem.
Developer documentation as a short guide what to pay attention for when developing new background task subclasses/implementations.