ActiveRecord::Turntable

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

Dependencies

activerecord(>=5.0.0, <6.0)

• Current latest stable branch is stable branch.

If you are using with older activerecord versions, use following versions.

• activerecord 4.x - use activerecord-turntable version 2.x.(stable-2-x branch)
• activerecord 3.x - use activerecord-turntable version 1.x.(stable-1-x branch)

Supported DBMS

MySQL only.

Installation

Add to Gemfile:

gem 'activerecord-turntable', '~> 4.4.1'

Run a bundle install:

bundle install

Run install generator:

bundle exec rails g active_record:turntable:install

generator creates

#{Rails.root}/config/turntable.yml

Terminologies

Shard

Shard is a database which is horizontal partitioned.

Cluster

Cluster of shards. i.e) set of userdb1, userdb2, userdb3. Shards in the same cluster should have the same schema structure.

Master

Default ActiveRecord::Base's connection.

Sequencer

Turntable's sequence system for clustered database.

This keeps primary key ids to be unique each shards.

Example

Example Databases Structure

One main database(default ActiveRecord::Base connection) and three user databases sharded by user_id.

                  +-------+
                  |  App  |
                  +-------+
                      |
       +---------+---------+---------+---------+
       |         |         |         |         |
  `--------` `-------` `-------` `-------` `-------`
  | Master | |UserDB1| |UserDB2| |UserDB3| | SeqDB |
  `--------` `-------` `-------` `-------` `-------`

Example Configuration

Edit DSL type configuration file(config/turntable.rb) or YAML configuration file(turntable.yml). And add database connection settings to database.yml.

See below example config.

• example turntable.rb

cluster :user_cluster do
  # algorithm [algorithm name symbol(range|range_bsearch|modulo)]
  algorithm :range_bsearch

sequencer [sequence name] [sequence type] [*options hash]
  sequencer :user_seq, :mysql, connection: :user_seq
shard [range], to: [connection names in database.yml]
  shard   1...100,        to: :user_shard_1
  shard 100...200,        to: :user_shard_2
  shard 200...2000000000, to: :user_shard_3
If you are using modulo algorithm, pass integer sequence start with zero.

shard 0, to: :user_shard_1
shard 1, to: :user_shard_2
shard 2, to: :user_shard_3
end

• example turntable.yml

    development:
      clusters:
        user_cluster: # 

• database.yml

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

development:
  &lt;<: database: sample_app_development seq: sequence database definition user_seq_1: sample_app_user_seq_development shards: shards user_shard_1: sample_app_user1_development user_shard_2: sample_app_user2_development user_shard_3: sample_app_user3_development>
About algorithms


range, range_bsearch - distribute records by key range

  algorithm :range_bsearch

  shard      1...20_000,     to: :user_shard_1
  shard 20_000...40_000,     to: :user_shard_2
  shard 40_000...60_000,     to: :user_shard_1
  shard 60_000...80_000,     to: :user_shard_2
  shard 80_000...10_000_000, to: :user_shard_3


modulo - distribute records by key modulo

  algorithm :modulo

  shard 0, to: :user_shard_1
  shard 1, to: :user_shard_2
  shard 2, to: :user_shard_3


hash_slot - distribute records by key hashes


default hash function is 
Zlib.crc32(key.to_s)
  algorithm :hash_slot
  # Or specify hash function
  # algorithm :hash_slot, hash_func: ->(key) { Zlib.adler32(key.to_s) }

  shard     0...4096,  to: :user_shard_1
  shard  4096...8192,  to: :user_shard_2
  shard  8192...12288, to: :user_shard_3
  shard 12288...16384, to: :user_shard_4

Slave support (experimental)

Slave enabled configuration examples:


config/turntable.rb

cluster :user_cluster do
...

shard [range], to: [connection names in database.yml]
  shard   1...100,        to: :user_shard_1, slaves: [:user_shard_1_1]
  shard 100...200,        to: :user_shard_2, slaves: [:user_shard_2_1]
  shard 200...2000000000, to: :user_shard_3, slaves: [:user_shard_3_1]
end


config/turntable.yml

    development:
      clusters:
        user_cluster: # 


config/database.yml


Add slave connection settings under 
shards
.
...
  shards:
    user_shard_1:
      <<: database: turntable_user_shard_1_test user_shard_1_1: turntable_user_shard_1_1_test user_shard_2: turntable_user_shard_2_test user_shard_2_1: turntable_user_shard_2_1_test user_shard_3: turntable_user_shard_3_test user_shard_3_1: turntable_user_shard_3_1_test>
Slave usage:
User.with_slave {
  # `User` model will use slave databases within this block.
}

User.with_master {
User model will use master database within this block.
}

Example Migration

Generate a model:
bundle exec rails g model user name:string

And Edit migration file:
class CreateUsers < ActiveRecord::Migration
  # Specify cluster executes migration if you need.
  # Default, migration would be executed to all databases.
  # clusters :user_cluster

  def change
    create_table :users do |t|
      t.string :name
      t.timestamps
    end
    create_sequence_for(:users) # 
Then please execute rake tasks:
bundle exec rake db:create
bundle exec rake db:migrate

Those rake tasks would be executed to shards too.

Example Model

Add turntable [shardkeyname] to the model class:
class User < ApplicationRecord
  turntable :user_cluster, :id
  sequencer :user_seq
  has_one :status
end

class Status < ApplicationRecord
  turntable :user_cluster, :user_id
  sequencer :user_seq
  belongs_to :user
end

Usage

Creating
    > 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: #<: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
    => #

Retrieving
    > user = User.find(2)
    [ActiveRecord::Turntable] Sending method: select_all, sql: #<:selectmanager:0x007f850466e668>, shards: ["user_shard_1"]
      User Load (0.3ms) [Shard: user_shard_1] SELECT `users`.* FROM `users` WHERE `users`.`id` = 2 LIMIT 1
    => #

Updating
    > 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: #<: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
    => #

Counting
    > User.count
    [ActiveRecord::Turntable] Sending method: select_value, sql: #<: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.

Turntable has follow 3 sequencers currently:



:mysql - Use database table to generate ids.
:barrage - Use barrage gem to generate ids
:katsubushi - katsubushi sequencer backend


Mysql example

First, add configuration to turntable.yml and database.yml


database.yml

    development:
      ...
      seq: # 


turntable.yml

    development:
      clusters:
        user_cluster: # 
Add below to the migration:
create_sequence_for(:users) # 
Next, add sequencer definition to the model:
  class User < ApplicationRecord
    turntable :id
    sequencer :user_seq # 
Barrage example

First, add barrage gem to your Gemfile:
gem 'barrage'

Then, add configuration to turntable.yml:



turntable.yml

    development:
      clusters:
        user_cluster: # 
Next, add sequencer definition to the model:
  class User < ApplicationRecord
    turntable :id
    sequencer :barrage_seq # 
Katsubushi example

katsubushi is available as a sequence server.
ActiveRecord::Turntable accesses katsubushi via Dalli.

First, add dalli gem to your Gemfile:
gem 'dalli'

Then, add configuration to turntable.yml:



turntable.yml

    development:
      clusters:
        user_cluster: # 
Next, add sequencer definition to the model:
  class User < ApplicationRecord
    turntable :id
    sequencer :katsubushi_seq # 
Transactions

Turntable has some transaction support methods.

shards_transaction

Pass AR::Base instances, 
shards_transaction
 method suitable shards
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

cluster_transaction

When executing transaction on all shards in the cluster, use 
#{cluster_name}_transaction
 method:
User.user_cluster_transaction do
  # Transaction is opened all shards in "user_cluster"
end

Migration

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

Default, migrations will be executed to all databases.

to specify cluster:
    class CreateUsers < ActiveRecord::Migration
      clusters :user_cluster
      ....
    end

to specify shard:
    class CreateUsers < ActiveRecord::Migration
      shards :user_shard_01
      ....
    end

Limitations



Queries includes "ORDER BY", "GROUP BY" and "LIMIT" clauses cannot be distributed.
"has many through" and "habtm" relationships may returns unexpected results. ex) 
User-Friend-User
 relation


TIPS

Send query to a specific shard.

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


with_shard
 method accepts following types to specify a shard:


ActiveRecord::Turntable::Shard object
AcitveRecord::Base object - 
AR::Base#turntable_shard
 will be used
Numeric, String - a shard key value
Symbol - shard name symbol


To access shard objects, use below:


AR::Base.connection.shards # \{shardname => shardobj,....}
AR::Base#turntable_shard # Returns current object's shard
AR::Base.connection.shardfor(shardkey_value) #=> shard


Send query to all shards

Use with_all method:
  User.with_all do
    User.order("created_at DESC").limit(3).all
  end # => Returns Array of results

Cannot specify error is raised between associations

Normally, activerecord-turntable detects shard keys on associated models, but auto-detection will fail with following conditions:


foreign key column != shard key column
Using different shard key names


For example:
class User
  # shard key:
  # foreign key: main_user_item_id
  belongs_to :main_user_item, class_name: "UserItem", required: false
end

class UserItem
shard key name: :user_id
  turntable :user_cluster, :user_id
end

This example raises CannotSpecifyShardError
> user.main_user_item
  User Load [Shard: master] (0.4ms)  SELECT  `users`.* FROM `users` ORDER BY `users`.`id` ASC LIMIT 1
[ActiveRecord::Turntable] Error on Building Fader: SELECT  `user_items`.* FROM `user_items` WHERE `user_items`.`id` = 2198059200000 LIMIT 1, on_method: select_all, err: cannot specifyshard for query: SELECT "user_items".* FROM `user_items` WHERE (`user_items`.`id` = 2198059200000) LIMIT 1
ActiveRecord::Turntable::CannotSpecifyShardError: cannot specify shard for query: SELECT "user_items".* FROM `user_items` WHERE (`user_items`.`id` = 2198059200000) LIMIT 1

Use foreignshardkey option to pass a shard key condition:
-belongs_to :main_user_item, class_name: "UserItem"
+belongs_to :main_user_item, class_name: "UserItem", foreign_shard_key: :id

> user.main_user_item
  User Load [Shard: master] (0.2ms)  SELECT  `users`.* FROM `users` ORDER BY `users`.`id` ASC LIMIT 1
[ActiveRecord::Turntable] Sending method: select_all, sql: #<:selectmanager:0x007f8080bd0670>, shards: ["user_shard_1"]
Changing UserItem's shard to user_shard_1
  UserItem Load [Shard: user_shard_1] (0.2ms)  SELECT  `user_items`.* FROM `user_items` WHERE `user_items`.`user_id` = 1 AND `user_items`.`id` = 2198059200000 LIMIT 1
Changing UserItem's shard to master
=> #

Performance Exception

To notice queries causing performance problem, Turntable has follow options.


raise_on_not_specified_shard_query - raises on queries execute on all shards
raise_on_not_specified_shard_update - raises on updates executed on all shards


Add to turntable.yml or turntable.rb:
development:
   ....
   raise_on_not_specified_shard_query: true
   raise_on_not_specified_shard_update: true

# Write on top level
raise_on_not_specified_shard_query true
raise_on_not_specified_shard_update true

Thanks

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

License

activerecord-turntable is released under the MIT 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
without limitation the rights to use, copy, modify, merge, publish,
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,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.