High Performance PostgreSQL for Rails

Switching Shards Automatically

Let’s look at the basics of automatic shard switching.

To switch automatically, Active Record uses a ShardSelector class. A shard selector runs as a middleware and maps a HTTP request to a shard.

To set this up, you’ll need a ShardSelector and a Resolver class. Following is an example Resolver class for the shard selector, with a lock parameter value.

The lock parameter indicates whether the shard can be changed within the block where it’s used or not.

The Resolver class processes the request host to identify a tenant and then finds the shard for that tenant.

ruby/database_shard_resolver.rb

Rails.application.configure do
  config.active_record.shard_selector = { lock: true }

  config.active_record ...