Rails 6 provides a framework for auto-routing incoming requests to either the primary database connection, or a read replica.
By default, this new functionality allows your app to automatically route read requests (GET, HEAD) to a read-relica database if it has been at least 2 seconds since the last write request (any request that is not a GET or HEAD request) was made.
The logic that specifies when a read request should be routed to a replica is specified in a resolver class, ActiveRecord::Middleware::DatabaseSelector::Resolver by default, which you would override if you wanted custom behavior.
The middleware also provides a session class, ActiveRecord::Middleware::DatabaseSelector::Resolver::Session that is tasked with keeping track of when the last write request was made. Like the resolver, this class can also be overridden.
To enable the default behavior, you would add the following configuration options to one of your app's environment files - config/environments/production.rb for example:
config.active_record.database_selector = { delay: 2.seconds }
config.active_record.database_resolver =
ActiveRecord::Middleware::DatabaseSelector::Resolver
config.active_record.database_operations =
ActiveRecord::Middleware::DatabaseSelector::Resolver::Session
If you decide to override the default functionality, you can use these configuration options to specify the delay you'd like to use, the name of your custom resolver class, and the name of your custom session class, both of which should be descendants of the default classes
