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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)


[R] options
[R] resolver

Class Public methods

new(app, resolver, options = {})

# File activerecord/lib/active_record/middleware/shard_selector.rb, line 30
def initialize(app, resolver, options = {})
  @app = app
  @resolver = resolver
  @options = options

Instance Public methods


# File activerecord/lib/active_record/middleware/shard_selector.rb, line 38
def call(env)
  request =

  shard = selected_shard(request)

  set_shard(shard) do