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README.md

ActiveReplicas

Build Status

Drop-in read-replica querying in ActiveRecord. It proxies around ActiveRecord's connection to send read-only queries to replica databases; writes will automatically go to the primary and "stick" the request into using the primary for any further queries.

This is heavily inspired by Kickstarter's replica_pools gem. It seeks to improve on that gem by better interfacing with ActiveRecord's connection pools.

Installation & usage

ActiveReplicas injects itself into ActiveRecord. To start you'll want to add it to your application's Gemfile:

gem 'active_replicas'

You then need to instruct it as to which connection to use for the primary and which connection(s) to use for the read replicas:

# config/initializers/active_replicas.rb
ActiveReplicas::Railtie.hijack_active_record primary: { url: 'mysql2://user@primary/my_app' },
                                             replicas: {
                                               replica0: { url: 'mysql2://user@replica/my_app' }
                                             }

Note: ActiveReplicas does not do anything automatically. It only injects itself into ActiveRecord when you tell it do so (see above).

How it works

A few things happen when you call hijack_active_record:

  • It defines delegations for the connection methods that can be sent to replicas and the methods that must be sent to a primary.
  • It sets up a ConnectionHandler that will act in place of ActiveRecord's normal connection handler.
  • It replaces ActiveRecord's log subscriber with its own LogSubscriber which adds information about replica/primary status to logging messages.

The new connection handler manages connection pools for each configured primary and replica database(s). Externally it provides instances of ProxyingConnectionPool in place of normal connection pools. Those proxying pools then forward connection methods to primary or replica pools.

Using it with Ruby on Rails

ActiveReplicas works seamlessly within the Rails request lifecycle. It extends the clear_active_connections! called in ActiveRecord's middleware to not only clear the active connections (releasing them back to their respective pool) but also resets the primary/replica state. That way the next connection will start out using a replica.

Contributing

Bug reports and pull requests are welcome on GitHub. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

Licensed under the 3-clause BSD license. See LICENSE for details.

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Smart read replicas in ActiveRecord

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