Add custom subscription store interface

[TikiTDO]

Summary

Adds a custom subscription store interface to graphql-anycable while keeping Redis as the built-in default store.

This trims the PR down to the refactoring/interface layer requested in review. The PostgreSQL implementation has been moved to a standalone gem owned under TikiTDO: https://github.com/TikiTDO/graphql-anycable_postgresql-store

What Changed

• Added GraphQL::AnyCable.register_subscription_store(name) { ... } for external store adapters.
• Kept GraphQL::AnyCable.subscription_store = store for direct store injection.
• Routed subscription reads/writes/lookups/deletes through with_subscription_store.
• Preserved the existing Redis behavior behind GraphQL::AnyCable::SubscriptionStores::Redis.
• Kept Redis stats working after moving Redis key-prefix constants into the Redis store.
• Removed the bundled PostgreSQL store, pg development dependency, and PostgreSQL-specific core config.
• Added README documentation for the custom store protocol and registration flow.
• Clarified that Redis is only required when using the built-in Redis subscription store; external stores such as the PostgreSQL store do not require Redis for graphql-anycable subscription state.
• Updated the changelog and added focused interface specs.

Store Interface

A custom store object implements:

• stream_for(fingerprint)
• fingerprints_for_topic(topic)
• subscription_ids_for_fingerprints(fingerprints)
• subscription_exists?(subscription_id)
• write_subscription(subscription_id, channel_id:, data:, events:, expiration_seconds:)
• read_subscription(subscription_id)
• delete_channel_subscriptions(channel_id)
• delete_subscription(subscription_id)

Reviewer Focus

• Redis should remain the default and compatible behavior for existing users.
• The core gem no longer depends on or references PostgreSQL.
• External stores can register by name without monkey-patching GraphQL::AnyCable.
• Unknown configured stores now raise an actionable registration error.
• Redis-specific stats still work with the extracted Redis store class.

Validation

Passed against a temporary isolated Redis container:

REDIS_URL=redis://localhost:56379/6 bundle exec rspec

Passed syntax/diff checks:

bundle exec ruby -Ilib -e 'require "graphql-anycable"; store = Object.new; GraphQL::AnyCable.register_subscription_store(:custom) { store }; GraphQL::AnyCable.config.subscription_store = :custom; abort "custom store failed" unless GraphQL::AnyCable.subscription_store.equal?(store); puts "custom store registration ok"'
ruby -c lib/graphql-anycable.rb
ruby -c lib/graphql/anycable/stats.rb
ruby -c spec/graphql/anycable_spec.rb
git diff --check

Related Follow-Up

• Standalone PostgreSQL subscription store gem: https://github.com/TikiTDO/graphql-anycable_postgresql-store
• anycable-go Postgres consumer/pubsub: Feature: Postgres signalling anycable#306
• anycable-rb producer adapter: Feature: Postgres signalling anycable-rb#9
• anycable-rails setup generator: Feature: Postgres signalling anycable-rails#223

Checklist

• Preserved Redis store behavior behind a storage abstraction
• Added a public custom store registration API
• Removed PostgreSQL implementation from the core gem
• Added a changelog entry
• Updated documentation

[palkan]

Thanks!

Finally, we can make graphql-anycable Redis-independent! That's a good idea.

However, I'm not sure that we should add Postgres adapter implementation to the core gem. I'd prefer to have it as a standalone plugin (well, tbh, and not to maintain ourself).

Let's keep only the refactoring part with a clear interface for defining and using custom adapters in this PR (so we can release it independently and quicker), and then you can create a standalone graphql-anycable_postgresql-store gem (or whatever).

[TikiTDO]

Split it out into it's own gem, should be ready in a bit. I'll take a look at the go lib at the end of the day.

[palkan]

Thanks! Almost there.

Left some comments.

Note: this feature deserves a major upgrade (2.0), so introducing minor incompatible changes in the interface is fine.

[palkan]

Looks good 👍

One more thing we've missed—Cleaner: https://github.com/anycable/graphql-anycable/blob/master/lib/graphql/anycable/cleaner.rb

We should also make it adapter-agnostic.

In the end, I think we should have the following structure for the Redis store:

graphql/anycable/subscription_stores/
  redis/
    cleaner.rb
    stats.rb
  redis.rb

That is, let's extract Stats object from the store (just re-organize the code, keep the interface) and add the cleaner object to be used by the Rake tasks.

[TikiTDO]

@palkan Done

[TikiTDO]

Ah, sorry about that. We normally explicitly test delegations so it inherited our rules. Should be cleaned up now