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Class: ActiveRecord::Middleware::ShardSelector

Relationships & Source Files
Inherits: Object
Defined in: activerecord/lib/active_record/middleware/shard_selector.rb

Overview

Shard Selector Middleware

The ShardSelector ::ActiveRecord::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.

Setup

Applications must provide a resolver that will provide application-specific logic for selecting the appropriate shard. Setting config.active_record.shard_resolver will cause ::Rails to add ShardSelector to the default middleware stack.

The resolver, along with any configuration options, can be set in the application configuration using an initializer like so:

Rails.application.configure do
  config.active_record.shard_selector = { lock: false, class_name: "AnimalsRecord" }
  config.active_record.shard_resolver = ->(request) {
    subdomain = request.subdomain
    tenant = Tenant.find_by_subdomain!(subdomain)
    tenant.shard
  }
end

Configuration

The behavior of ShardSelector can be altered through some configuration options.

lock:

lock is true by default and will prohibit the request from switching shards once inside the block. If lock is false, then shard switching will be allowed. For tenant based sharding, lock should always be true to prevent application code from mistakenly switching between tenants.

class_name:

class_name is the name of the abstract connection class to switch. By default, the ShardSelector will use ActiveRecord::Base, but if the application has multiple databases, then this option should be set to the name of the sharded database’s abstract connection class.

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Constructor Details

.new(app, resolver, options = {}) ⇒ ShardSelector

[ GitHub ] 

  


# File 'activerecord/lib/active_record/middleware/shard_selector.rb', line 47



def initialize(app, resolver, options = {})
  @app = app
  @resolver = resolver
  @options = options
end


  







Instance Attribute Details



  

    #options   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/middleware/shard_selector.rb', line 53



attr_reader :resolver, :options


  








  

    #resolver   (readonly)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/middleware/shard_selector.rb', line 53



attr_reader :resolver, :options


  







Instance Method Details



  

    #call(env)  
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/middleware/shard_selector.rb', line 55



def call(env)
  request = ActionDispatch::Request.new(env)

  shard = selected_shard(request)

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


  








  

    #selected_shard(request)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/middleware/shard_selector.rb', line 66



def selected_shard(request)
  resolver.call(request)
end


  








  

    #set_shard(shard, &block)   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/middleware/shard_selector.rb', line 70



def set_shard(shard, &block)
  klass = options[:class_name]&.constantize || ActiveRecord::Base

  klass.connected_to(shard: shard.to_sym) do
    klass.prohibit_shard_swapping(options.fetch(:lock, true), &block)
  end
end