add async_redis Action Cable adapter (async-redis), docs, tests, and middleware hardening

[darkamenosa]

Hi Samuel,

As discussed in previous thread, so I prose async_redis adapter for this. People just need to change 1 line on cable.yml, then done.

I tested on a few projects, it worked well. However, I haven't finished the load test. So not so sure, but I stand for correct.

Summary:

• Introduces a fiber‑safe async_redis pub/sub adapter for Action Cable built on async-redis.
• Matches Rails’ Redis adapter semantics (dynamic subscribe/unsubscribe, reconnect + resubscribe) while using Async reactors.
• Adds a small middleware hardening for nil app (404 fallback in tests).
• Documents usage and configuration; adds tests and decode coverage.

Motivation:

• Provide a first‑class, non‑blocking Redis adapter that plays well with Falcon/Async and actioncable-next.
• Avoids thread/contention pitfalls by owning Redis clients within Async reactors.
• Makes it trivial for apps to enable: adapter: async_redis in cable.yml.

Implementation Details:

• Adapter: lib/action_cable/subscription_adapter/async_redis.rb
  • Listener
    • Runs in a dedicated OS thread with an Async reactor.
    • Uses Async::Redis::Client#subscribe to obtain a subscription context.
    • Supports dynamic SUBSCRIBE/UNSUBSCRIBE via a when_connected queue.
    • On error: reset → backoff → reconnect → resubscribe all known channels.
  • Publisher
    • Dedicated Async reactor thread.
    • Owns a single Async::Redis::Client.
    • Backpressure via SizedQueue (default 10,000); optionally drop_oldest/drop_newest when not blocking.
  • success_callback
    • Invoked immediately after issuing SUBSCRIBE (async-redis does not surface subscribe ACK events).
  • ChannelPrefix
    • Uses the existing prefixing module to maintain parity with Rails adapter behavior.
• Middleware: lib/async/cable/middleware.rb
  • Keeps default mount path “/cable”.
  • If @app is nil (as in tests), returns a minimal 404 instead of calling @app.
• Dependency
  • async-redis ~> 0.13 added to async-cable.gemspec.

Configuration (cable.yml)

cable.yml

development:
  adapter: async_redis
  url: redis://127.0.0.1:6379/0
  reconnect_attempts: [1, 2, 5]  # (or integer N → that many attempts with 0s sleep)
  async_redis_publish_queue_size: 10000
  async_redis_publish_block_when_full: true
  async_redis_publish_drop_policy: :drop_oldest | :drop_newest (used only if not blocking)
  async_redis_publish_drop_warn_every: 100

Compatibility & Behavior Notes

• No breaking changes for existing middleware behavior; default remains “/cable”.
• The adapter’s immediate success_callback differs from Rails’ redis gem where ACK hooks are available; this mirrors actioncable-next’s Inline/Test semantics and
  works reliably with async-redis.
• Backpressure defaults to blocking when full, protecting memory under Redis slowdown; drop policy is opt‑in.

Tests

• Unit (Sus):
  • test/async/cable/async_redis_adapter.rb: verifies adapter resolution when adapter: "async_redis" is configured (no Redis required).
  • test/async/cable/middleware.rb: connects on “/cable” to match middleware default.
• External (guarded):
  • test/external/async_redis_pubsub.rb: runs only if REDIS_URL is set; subscribes, broadcasts, and asserts delivery.

Docs

• README: “Async Redis Adapter” usage + cable.yml options.
• In‑code API docs added for adapter, publisher, listener, middleware, socket.
• decode coverage: 34/34 documented public definitions.

How To Test Locally

• Install: bundle install
• Unit tests: bundle exec sus
• External Redis pub/sub (requires Redis):

  REDIS_URL=redis://127.0.0.1:6379/0 bundle exec bake test:external

• Documentation coverage: bundle exec bake decode:index:coverage lib
• Lint: bundle exec rubocop -A --fail-level W

Risks / Tradeoffs:

• Immediate success_callback (no subscribe ACKs in async-redis) — chosen for reliability and consistency across adapters.
• Backpressure queue defaults to blocking; if non‑blocking + drop policy is chosen, messages can be dropped under sustained pressure (warns at a configurable
  interval).

Alternatives Considered:

• Per‑publish client creation (simpler but higher overhead).
• Sharing a single client across threads (risks contention and misuse of Async::Pool outside reactors).

Future Work

• Optional metrics hooks (publish queue depth, reconnects).
• Extended external tests for dynamic subscribe/unsubscribe and reconnect scenarios using Async fixtures.

Checklist

• Adapter implemented and namespaced under ActionCable::SubscriptionAdapter.
• Tests updated/passing locally (unit and guarded external).
• Documentation updated (README and inline).
• decode documentation coverage passing.
• RuboCop warnings addressed.

Release Notes:

• Add async_redis adapter (async-redis) for Action Cable; configurable backpressure; docs and tests included. Enable via adapter: async_redis in cable.yml.

Thanks for reviewing! Happy to adjust naming, defaults (queue size/behavior), or test coverage based on maintainer feedback.