= ActiveRecord::Turntable

ActiveRecord::Turntable is a database sharding extension for ActiveRecord.

== Dependencies
activerecord(>=3.0.0)

== Supported Database
Currently supports mysql only.

== Installation

Add 'activerecord-turntable' to Gemfile:

    gem 'activerecord-turntable', '1.0.0'

Run a bundle install:

    bundle install

Run install generator:

    bundle exec rails g active_record:turntable:install

generator creates Rails.root/config/turntable.yml

== ActiveRecord::Turntable terms

*Shard
*Cluster - a cluster of shards.
*Master - default ActiveRecord::Base's connection
*Sequencer - a module that creates global sequence number

== Example

=== Example Databases Structure
one main database(default ActiveRecord::Base connection) and
three user databases sharded by user_id.

                         +-------+
                         |  App  |
                         +-------+
                             |
       +---------+-----------+---------+---------+
       |         |           |         |         |
  +--------+ +---------+ +-------+ +-------+ +-------+
  | Master | |Sequencer| |UserDB1| |UserDB2| |UserDB3|
  +--------+ +---------+ +-------+ +-------+ +-------+

=== Example Configuration

Edit turntable.yml and database.yml. See below example config.

example turntable.yml:

    development:
      clusters:
        user_cluster: # <-- cluster name
          algorithm: range # <-- only range or range_bsearch sharding is supported currently.
          shards:
            - connection: user_shard_1 # <-- shard name
              less_than: 100           # <-- shard range(like mysql partitioning)
            - connection: user_shard_2
              less_than: 200
            - connection: user_shard_3
              less_than: 2000000000

database.yml:
    connection_spec: &spec
      adapter: mysql2
      encoding: utf8
      reconnect: false
      pool: 5
      username: root
      password: root
      socket: /tmp/mysql.sock

    development:
      <<: *spec
      database: sample_app_development
      seq: # <-- sequence definition
        user_seq_1:
          <<: *spec
          database: sample_app_user_seq_development
      shards: # <-- shards definition
        user_shard_1:
          <<: *spec
          database: sample_app_user1_development
        user_shard_2:
          <<: *spec
          database: sample_app_user2_development
        user_shard_3:
          <<: *spec
          database: sample_app_user3_development


=== Migration Example

Generate a model:

  bundle exec rails g model user name:string

Edit migration file:

  class CreateUsers < ActiveRecord::Migration
    clusters :user_cluster # <-- the cluster that executes migration

    def change
      create_table :users do |t|
        t.string :name
        t.timestamps
      end
      create_sequence_for(:users)
    end
  end

then please execute rake tasks:

  bundle exec rake db:create
  bundle exec rake db:migrate

Those rake tasks would be executed to shards too.

=== Model Example
add turntable [shard_key_name] to the model class:

  class User < ActiveRecord::Base
    turntable :user_cluster, :id
    sequencer
    has_one :status
  end

  class Status < ActiveRecord::Base
    turntable :user_cluster, :user_id
    sequencer
    belongs_to :user
  end

== Usage

=== Create
    > User.create(name: "hoge")
      (0.0ms) [Shard: user_seq_1] BEGIN
      (0.3ms) [Shard: user_seq_1] UPDATE `users_id_seq` SET id=LAST_INSERT_ID(id+1)
      (0.8ms) [Shard: user_seq_1] COMMIT
      (0.1ms) [Shard: user_seq_1] SELECT LAST_INSERT_ID()
      (0.1ms) [Shard: user_shard_1] BEGIN
    [ActiveRecord::Turntable] Sending method: insert, sql: #<Arel::InsertManager:0x007f8503685b48>, shards: ["user_shard_1"]
      SQL (0.8ms) [Shard: user_shard_1] INSERT INTO `users` (`created_at`, `id`, `name`, `updated_at`) VALUES ('2012-04-10 03:59:42', 2, 'hoge', '2012-04-10 03:59:42')
      (0.4ms) [Shard: user_shard_1] COMMIT
    => #<User id: 2, name: "hoge", created_at: "2012-04-10 03:59:42", updated_at: "2012-04-10 03:59:42">

=== Retrieve
    > user = User.find(2)
    [ActiveRecord::Turntable] Sending method: select_all, sql: #<Arel::SelectManager:0x007f850466e668>, shards: ["user_shard_1"]
      User Load (0.3ms) [Shard: user_shard_1] SELECT `users`.* FROM `users` WHERE `users`.`id` = 2 LIMIT 1
    => #<User id: 2, name: "hoge", created_at: "2012-04-10 03:59:42", updated_at: "2012-04-10 03:59:42">

=== Update
    > user.update_attributes(name: "hogefoo")
      (0.1ms) [Shard: user_shard_1] BEGIN
    [ActiveRecord::Turntable] Sending method: update, sql: UPDATE `users` SET `name` = 'hogefoo', `updated_at` = '2012-04-10 04:07:52' WHERE `users`.`id` = 2, shards: ["user_shard_1"]
      (0.3ms) [Shard: user_shard_1] UPDATE `users` SET `name` = 'hogefoo', `updated_at` = '2012-04-10 04:07:52' WHERE `users`.`id` = 2
      (0.8ms) [Shard: user_shard_1] COMMIT
    => true

=== Delete
    > user.destroy
      (0.2ms) [Shard: user_shard_1] BEGIN
    [ActiveRecord::Turntable] Sending method: delete, sql: #<Arel::DeleteManager:0x007f8503677ea8>, shards: ["user_shard_1"]
      SQL (0.3ms) [Shard: user_shard_1] DELETE FROM `users` WHERE `users`.`id` = 2
      (1.7ms) [Shard: user_shard_1] COMMIT
    => #<User id: 2, name: "hogefoo", created_at: "2012-04-10 03:59:42", updated_at: "2012-04-10 04:07:52">

=== Count

    > User.count
    [ActiveRecord::Turntable] Sending method: select_value, sql: #<Arel::SelectManager:0x007f9e82ccebb0>, shards: ["user_shard_1", "user_shard_2", "user_shard_3"]
       (0.8ms) [Shard: user_shard_1] SELECT COUNT(*) FROM `users`
       (0.3ms) [Shard: user_shard_2] SELECT COUNT(*) FROM `users`
       (0.2ms) [Shard: user_shard_3] SELECT COUNT(*) FROM `users`
    => 1

=== Sequencer
Sequencer provides generating global IDs.

Add below to the migration:

      create_sequence_for(:users) # <-- this line creates sequence table

This will creates sequence table.

Next, add sequencer to the model:

  class User < ActiveRecord::Base
    turntable :id
    sequencer # <-- this line enables sequencer module
    has_one :status
  end


=== Transaction
    > user = User.find(2)
    > user3 = User.create(name: "hoge3")
    > User.shards_transaction([user, user3]) do
    >   user.name  = "hogehoge"
    >   user3.name = "hogehoge3"
    >   user.save!
    >   user3.save!
    > end

=== Send query to a specific shard.
use with_shard method:

    AR::Base.connection.with_shard(shard1) do
      # something queries to shard1
    end

To access shard objects, use below:
* AR::Base.connection.shards # \\{shard_name => shard_obj,....}
* AR::Base#turntable_shard
* AR::Base.connection.select_shard(shard_key_value) #=> shard

=== Send query to all shards
use with_all method:

  User.connection.with_all do
    User.order("created_at DESC").limit(3).all
  end


=== Migration
If you specify cluster or shard, migration will be executed to the cluster(or shard) and master database.

to specify cluster:

    class CreateUsers < ActiveRecord::Migration
      clusters :user_cluster
      ....
    end

to specify shard:

    class CreateUsers < ActiveRecord::Migration
      shards :user_shard_01
      ....
    end

if you would like to execute a migration to all databases in the environments, use :all symbol:

    class CreateUsers < ActiveRecord::Migration
      clusters :all
      ....
    end

== Connection Management
Rails's ConnectionManagement middleware keeps ActiveRecord's connection during the process is alive, but Turntable keeps more connections.
This may cause flooding max connections on your database. So, we made a middleware that disconnects on each request.
To use turntable's ConnectionManagement middleware, add below line to your initializer.

      app.middleware.swap ActiveRecord::ConnectionAdapters::ConnectionManagement, ActiveRecord::Turntable::Rack::ConnectionManagement

== Inability
* Queries includes "ORDER BY", "GROUP BY" and "LIMIT" clauses cannot be distributed.
* "has many through" and "habtm" relationships causes wrong results. ex) User-Friend-User relation

== Thanks to
ConnectionProxy, Distributed Migration implementation is inspired by Octopus and DataFabric.

== LICENSE
Copyright (c) 2012 Drecom Co.,Ltd.

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
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distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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