[ActiveRecord] Accept pool_class option

[kirs]

Context: #42271

There's lots of exciting work going in Ruby 3.0 with fibers, non-blocking IO and support for custom schedulers. Eventually we'd like to run Rails apps with ActiveRecord in non-blocking servers like Falcon. Those servers would require the code not to rely on thread-local variables, which ActiveRecord connection pool does right now.

This PR is an attempt to allow AR to take pool_class as an option to DB configuration. It will allow @ioquatix and the async ruby team to implement a fiber-local connection pool, and to run ActiveRecord on non-blocking servers.

main...kirs:async-rb is a quick and dirty example how a custom connection pool implementation might look like.

[kirs]

Ideally, this should come not as a configuration option but as a method on the DB adapter class. That way we could have async_mysql adapter that would specify that it wants to use "async" pool. Something like:

module ActiveRecord module ConnectionAdapters class AbstractAdapter def pool_class ConnectionAdapters::ConnectionPool end module ActiveRecord module ConnectionAdapters class AsyncMysqlAdapter < AbstractMysqlAdapter def pool_class AsyncConnectionPool end 

However, we initialize the pool object before we resolve mysql2 to Mysql2Adapter as a class. That makes us unable to call something like adapter.pool_class as early as when initializing the pool.

Hence the reason why I made this an option of the connection config.

[eileencodes]

I don't know how I feel about this implementation. There is already a class in the pool which is the class that established the connection. Now we're introducing another class that you then would need to re-implement all the methods on pool to use. That feels confusing to explain and implement. Is there another way to accomplish this?

[ioquatix]

Maybe worth investigating the design of Sequel, since they have got this part of the design working really well?

[rails-bot]

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