Jumpstarter

draft/jumpstarter.rst

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+.. vale off
+
+======================
+CEP XXXX: Jumpstarter
+======================
+
+:CEP: XXXX
+:Author: Micah Lyle
+:Implementation Team: Omer Katz
+:Shepherd: Omer Katz
+:Status: Draft
+:Type: Feature
+:Created: 2021-05-09
+:Last-Modified: 2021-05-09
+
+.. contents:: Table of Contents
+   :depth: 3
+   :local:
+
+Abstract
+========
+
+`Next-Gen Celery`_ aims to:
+
+1. Move Celery from the past to the present. 
+
+2. Resolve long-standing design bugs in our implementation. 
+
+3. Modernize the code to support Python3+. 
+
+4. Do a number of other things not entirely in the scope of this document (see `Next Gen Rationale`_). 
+
+Before work is to begin on this next generation of Celery, it's crucial to have
+a discussion about and carve out the building blocks that will form the
+primitives for the upcoming Celery. These building blocks should fulfill 1, 2,
+and 3 (especially) above, and also help fulfill the goals from the linked
+`Next-Gen Rationale`_.
+
+For Celery to move forward into the modern age, and the future of computing
+with increasingly decentralized systems, it's important for Celery to follow a
+standardized asynchronous execution model. Namely, we want to define things
+like: 
+
+1. How does Celery interact with itself (its own internal components)? 
+
+2. How does Celery interact with others (and how do others interact with it)? 
+
+3. How do Celery's internal components interact with each other and their outside environment? 
+
+Also, we want Celery to work with the emerging and maturing asynchronous python
+landscape.  Celery is *very asynchronous* by nature, dealing with brokers,
+queues, results, timeouts, chord-like joins, task groups, you name it.
+Modern asynchronous python frameworks have provided new, innovative, and robust
+solutions to many things that overlap with Celery's goals.
+
+To do all of this, we propose modeling next-gen Celery off of the `Actor Model`_. Namely,
+we propose that the Celery Worker will be modeled as an `Actor System`_ of sorts.
+
+By the end of this initial implementation, we should have a proposal and reference implementation that: 
+
+1. Models Tasks and other key Celery primitives (some to be implemented and
+further specced-out down the line) as Actors. 
+
+2. Can give a clear overview about how these various Celery primitives (as
+Actors) communicate with each other and pass messages (referencing the Actor
+model). 
+
+3. Can enable the development of a future version of Celery where it can be run
+embedded with ``async/await`` (if desired by the programmer, not necessary),
+and can have synchronous-friendly workers and asynchronous-friendsly/purely
+asynchronous workers. 
+
+
+Specification
+=============
+
+TODO
+
+This section should contain a complete, detailed technical specification should
+describe the syntax and semantics of any new feature.  The specification should
+be detailed enough to allow implementation -- that is, developers other than the
+author should (given the right experience) be able to independently implement
+the feature, given only the CEP.
+
+Motivation
+==========
+
+TODO (WIP)
+
+This section should explain *why* this CEP is needed. The motivation is critical
+for CEPs that want to add substantial new features or materially refactor
+existing ones.  It should clearly explain why the existing solutions are
+inadequate to address the problem that the CEP solves.  CEP submissions without
+sufficient motivation may be rejected outright.
+
+Let's start with an example, and then generalize. A quick search in Celery's
+GitHub issue tracker for `chord
+<https://github.com/celery/celery/issues?q=is%3Aissue+is%3Aopen+chord>`_ shows
+a many issues. A number of them (of which some have been fixed) involve complex
+cases involving non-trivial Celery workflows. While fixes have been released
+over time as these have come up, many issues stand unresolved to this day. The
+Celery team could continue to try and fix these issues, or we could take a look
+back and think about how we want to design Celery for the future. although
+there have been a number of difficulties with getting started on fixing these
+issues in the first place! I'll try and list some of what I see these to be:
+
+1. Celery has had to support, for a long time, both Python 2 and Python 3.
+While the codebase has been modernized to a degree with Python 3 nowadays, it
+wasn't originally conceived or written that way. It was built back in the
+Python 2 days, and had (and still continues to have), a number of features and
+ideas. It was (and still is), incredibly pluggible, and supports a number of
+use cases and very high throughout if configured correctly. However, Python as
+a language has changed significantly in the last 10 years. Namely, proper
+support for coroutines in the form of `async/await`. 
+
+2. Celery, as a whole, involves a number of different working parts that are
+integrated together. In the current scheme of things, we have workers, brokers,
+result stores, workflows (``canvas`` primitives), rate limiting, policies,
+signatures, settings, periodic tasks, scheduling, ..., and many other things.
+Many of the primitives for these integrated parts were built almost a decade
+ago, when both the language landscape (as mentioned above), but also the
+*library landscape* was quite a bit different than today. Celery came with a
+custom CLI parser, custom event loop implementation (which it still has at the
+time of writing), and support for a number of other things that are today
+implemented in multiple different solid third party libraries. 
+
+3. Given the complexity of the mentioned issues, it's not easy to get to the
+root cause of what's happening in the first place. Because Celery is very
+*asynchronous* by nature, many of the tests are wrapped with a ``flaky``
+decorator which means, sometimes (depending on the arrival of messages, speed
+of IPC, speed of the network, and many other factors), maybe the test won't
+pass in an expected amount of time or might not always succeed. Testing and
+modeling asynchronous processes is definitely not easy, but the python
+language, tooling, and libraries have evolved considerably in the last
+*decade*, and Celery is well positioned for a large restructuring that allows
+it both internally take advantage of the latest Python features, and also
+provide external integration with them as well. 
+
+With all of these mentioned, and given the chord example, again, we could
+continue to try and improve and fix things as is, or we could take a step back
+and reflect as to *why* there are so many subleties and quirks with the
+implementation. I think one of the ultimate reasons boils down to a lack of
+simplicity. In Celery, certain aspects of the codebase are responsible for
+a number of things...
+
+^ TODO: I want to elborate on this more, but I'm not sure if it's appropriate.
+I think the Actor Model and modeling the worker as an Actor System gives us a
+number of benefits, which I want to succintly summarize in the beginning and
+give a lot more detail as a part of the CEP. Specifically, I'd like to
+explain/say why modeling things as an Actor system will make Celery a lot
+easier to reason about, maintain, and add new, for example, Canvas primitives
+to (or even re-build/design them, etc. if that's something that's desired).
+There are clear advantages to keeping state internal to the worker, for
+example, adding cancel groups, and other things that make some of the more
+asynchronous parts of Celery easier to work with, test, and reason about. The
+main thing also, though, is to really separate concerns and responsibilities.
+It *seems* to me that some of the current Celery code is not very SRP, in the
+sense that it's responsible for calling code at a number of different layers
+and doing a number of complex things. With the actor system, each actor gets a
+lot more specialized and I think that makes the implementation a lot easier to
+both reason about and extend.  Composition (vs., for example, inheritance
+currently present with ``Signature`` objects for example to then create the
+canvas objects) then becomes a lot more attractive and possible.  To give an
+example, a ``chord`` then simply becomes responsible for passing a message to
+the underlying ``Task`` (s) or ``result`` (s) (to define more) that would
+essentially make them then send off another "message" (we'll call it right now)
+or messages to other components in the actor system that are then responsible
+for handling that message (think of ``on_chord_part_return`` here).
+
+
+
+Rationale
+=========
+
+TODO
+
+This section should flesh out the specification by describing what motivated
+the specific design design and why particular design decisions were made.  It
+should describe alternate designs that were considered and related work.
+
+The rationale should provide evidence of consensus within the community and
+discuss important objections or concerns raised during discussion.
+
+Backwards Compatibility
+=======================
+
+TODO
+
+If this CEP introduces backwards incompatibilities, you must must include this
+section. It should describe these incompatibilities and their severity, and what
+mitigation you plan to take to deal with these incompatibilities.
+
+Reference Implementation
+========================
+
+The `Reference Implementation`_ has a nice sketch of how actors might look in
+`Jumpstarter`_. Some of the kinks and details are still being worked out, but
+that's the place to go and start taking a look at the time of writing. Further
+buildout of certain aspects of the reference implementation (which are also
+related to `Celery Next-Gen`_) may be blocked or waiting on some third-party
+library support. One example is we're waiting for an `APScheduler 4.0
+Release`_.
+
+Copyright
+=========
+
+This document has been placed in the public domain per the Creative Commons
+CC0 1.0 Universal license (https://creativecommons.org/publicdomain/zero/1.0/deed).
+
+(All CEPs must include this exact copyright statement.)
+
+.. Next-Gen Celery https://github.com/celery/ceps/blob/master/draft/high-level-architecture.rst
+.. Jumpstarter https://github.com/celery/jumpstarter
+.. Reference Implementation https://github.com/celery/jumpstarter/tree/actor
+.. AP Scheduler 4.0 Release https://github.com/agronholm/apscheduler/issues/465
+.. Next-Gen Rationale https://github.com/celery/ceps/blob/master/draft/high-level-architecture.rst#rationale
+.. Actor Model https://en.wikipedia.org/wiki/Actor_model
+.. Actor System https://doc.akka.io/docs/akka/current/general/actor-systems.html