Async web responses in Rails - discussion - inspired by Rails Enqueue API

Originally posted in Rails Talk (!topic/rubyonrails-talk/DHqc9sQu0h8) but it was suggested to me to post in Rails-Core, so apologies for the double post!

**The Problem**

I wonder how many people (I'm one of them) started with basic Rails applications serving HTML, JSON, or both, and eventually ran into a point where certain parts of the application became too slow, or were re-factored to consume some 3rd party services, and ended up not being able to synchronously serve the response in a timely manner, requiring switching to asynchronous responses.

**The Pattern**

At an abstract level, the behaviour is as follows:

1. Get request (could be either HTML or JSON)
1. Initiate some kind of async job ("job" here is interpreted widely, could be delayed/enqueue job or some other paradigm, point is, it's asynchronous to the request and has no guaranteed completion time)
1. Respond to requestor with 202 Accepted status, or some other status signifying "we accepted your request but do not have a response yet)
1. Include in response the URL client should check to visit response. Note that this isn't necessarily always mapping nicely to CRUD in sense of returning standard RESTful id. For example, nature of the job could be something like a very complex search/report query, of the form /items/123?conditions=..., but we can't just tell client to visit /items/123 for their result, because different clients doing this search may request same resource but with different filter conditions.
1. Client will poll the URL returned at last step, which will either return "check back later" status if response is not done, or the actual response if it's finished (or, alternatively, 3rd URL to visit the finished response once it's complete, which client will then visit to get their actual data).
1. If response jobs need to be stored on server side, need some mechanism to eventually clear them out.

**The Rails Way**

You may look at above and say "well, you have a custom requirement, so write yourself a custom solution, Rails can't read your mind". And you might be right. But, on the other hand:

1. Over many projects I've been on, this has been a very common requirement. For many applications which scale beyond a certain point both load-wise and 3rd-party-integration-wise, response times are often not guaranteed because of dependencies you have no direct control over, and we can't just hang the request until the job is done.
1. I don't _know_ from the beginning when, and for which resources, I'll need async request/response handling. I want to be able to Just Code stuff using the basic simple Rails as I need it, and switch to async processing later for needed endpoints only, as my application evolves. I want to be able to do this with minimal changes on both API and internal implementation. For example, if my regular controller uses current_user (from session), current_account (from request), and other such variables, I want to be able to continue using them in async controllers and not have to re-write the whole controller/view after switching to async.

I was inspired to start this discussion by the latest Enqueue work added in Rails 4.2. After many years and many competing async job processors (delayed_job, sidekiq, resque, etc.) Rails decided it made sense for them to provide a wrapper API so that code can be written in a consistent way and the implementation be relatively easy to change with no external impact. Just as importantly, it now becomes possible to write code that is synchronous yet uses the Enqueue API (using the "inline" adapter), and later pick and choose which parts should become async based on the application evolution.

The Enqueue API makes the backend job processing easier to make async, but the controller-level request-response handling is still a sore point:

1. The URL pattern for async responses is different from standard REST, making migrations from sync to async requests painful and existing APIs changed.
1. Rendering a RESTful response (either HTML or JSON) synchronously is trivial in normal Rails controllers; rendering it async is not. Even seemingly simple things like rendering an existing model/view is not easy without the familiar controller context. There are some gems that try to encapsulate it by constructing a custom controller and stubbing or caching, some examples Google found. Unfortunately, doing this is tedious work and makes it difficult to use existing session or request-based helpers like current_user or other methods from controller or application helpers. A lot of session/request caching and method re-definitions are needed.
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- This just doesn't seem a "Rails Way" to solve my problem, it makes me feel like I'm fighting against the MVC/REST instead of leveraging it. :-(

**The Vision**

What do you think of being able to do something like:

class ItemsController < ApplicationController::Base
  respond_to_async :show

  def show


class ItemsController < ApplicationController::Base

  def show
    render 'show', async: true

This would provide the ground work (e.g. REST/URL structure) to handle requests in a way which would be possible to make asynchronous if and when needed. Similar to Enqueue, there could be an "inline" pattern that behaves synchronously, but allows smooth transition to true async later, e.g. "redirect" would provide a response URL for client to visit.

Similar to the "enqueue" philosophy, the main purpose of the async request/response API would not to actually force a specific implementation, but to provide a wrapper API that the actual implementation can fit in. Users can either write their custom async implementation or use a 3rd party gem, but any such implementation should conform to the expectations set by the API.

The above snippets are just hypothetical examples of what such an API _might_ be like, I'm open to totally different ideas to solve this problem too.

**The Discussion**

Have you previously worked with implementing SOA or other requests which cannot be responded to immediately with final result because the job is too slow or distributed? I'd love to hear your opinion about this! Some factors to consider would be:

1. What was the response type of your application? HTML or JSON? Did it support normal forms, front-end JS frameworks, mobile APIs, etc?
1. How did you handle such a problem? Was it similar to above or did you have a drastically different way?
1. Did you design your application to have asynchronous responses from day 1, or did you start with a basic Rails application and had to make all or parts of it respond asynchronously later? What was the migration like?
1. Did you ever think that Rails could provide a more consistent standard and easier migration path from sync to async responses?
Or perhaps you didn't have to build such systems? Perhaps you think they don't even make sense and are not The Rails Way and don't belong in Rails? I'd love to hear from you too - if you think Rails helping to solve this isn't the right approach, then what might the right solution be/look like?

All feedback welcome!