class ActiveRecord::Middleware::ShardSelector

The ShardSelector Middleware provides a framework for automatically swapping shards. Rails provides a basic framework to determine which shard to switch to and allows for applications to write custom strategies for swapping if needed.

The ShardSelector takes a set of options (currently only ‘lock` is supported) that can be used by the middleware to alter behavior. `lock` is true by default and will prohibit the request from switching shards once inside the block. If `lock` is false, then shard swapping will be allowed. For tenant based sharding, `lock` should always be true to prevent application code from mistakenly switching between tenants.

Options can be set in the config:

config.active_record.shard_selector = { lock: true }

Applications must also provide the code for the resolver as it depends on application specific models. An example resolver would look like this:

config.active_record.shard_resolver = ->(request) {
  subdomain = request.subdomain
  tenant = Tenant.find_by_subdomain!(subdomain)
  tenant.shard
}

Attributes

options[R]
resolver[R]

Public Class Methods

new(app, resolver, options = {}) click to toggle source
# File lib/active_record/middleware/shard_selector.rb, line 30
def initialize(app, resolver, options = {})
  @app = app
  @resolver = resolver
  @options = options
end

Public Instance Methods

call(env) click to toggle source
# File lib/active_record/middleware/shard_selector.rb, line 38
def call(env)
  request = ActionDispatch::Request.new(env)

  shard = selected_shard(request)

  set_shard(shard) do
    @app.call(env)
  end
end

Private Instance Methods

selected_shard(request) click to toggle source
# File lib/active_record/middleware/shard_selector.rb, line 49
def selected_shard(request)
  resolver.call(request)
end
set_shard(shard, &block) click to toggle source
# File lib/active_record/middleware/shard_selector.rb, line 53
def set_shard(shard, &block)
  ActiveRecord::Base.connected_to(shard: shard.to_sym) do
    ActiveRecord::Base.prohibit_shard_swapping(options.fetch(:lock, true), &block)
  end
end