# ActiveRecord::Turntable

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

## Dependencies

activerecord(>=5.0.0, <6.0)

* Current latest stable branch is [stable branch](https://github.com/drecom/activerecord-turntable/tree/stable).

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

* activerecord 4.x - use activerecord-turntable version 2.x.([stable-2-x branch](https://github.com/drecom/activerecord-turntable/tree/stable-2-x))
* activerecord 3.x - use activerecord-turntable version 1.x.([stable-1-x branch](https://github.com/drecom/activerecord-turntable/tree/stable-1-x))

## Supported DBMS

MySQL only.

## Installation

Add to Gemfile:

```ruby
gem 'activerecord-turntable', '~> 4.3.0'
```

Run a bundle install:

```ruby
bundle install
```

Run install generator:

```bash
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

```ruby
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

```yaml
    development:
      clusters:
        user_cluster: # <-- cluster name
          algorithm: range_bsearch # <-- `range`, `range_bsearch` or `modulo`
          seq:
            user_seq: # <-- sequencer name
              seq_type: mysql # <-- sequencer type
              connection: user_seq_1 # <-- sequencer database connection setting
          shards:
            - connection: user_shard_1 # <-- shard name
              less_than: 100           # <-- shard range(like mysql partitioning) If you are using a modulo algorithm, it doesn't need it.
            - connection: user_shard_2
              less_than: 200
            - connection: user_shard_3
              less_than: 2000000000

```

* database.yml

```yaml
    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 database 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
```

#### About algorithms

* range, range_bsearch - distribute records by key range

```ruby
  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

```ruby
  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)`

```ruby
  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

```ruby
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

```yaml
    development:
      clusters:
        user_cluster: # <-- cluster name
          ...
          shards:
            - connection: user_shard_1
              less_than: 100
              slaves:
                - user_shard_1_1
            - connection: user_shard_2
              less_than: 200
              slaves:
                - user_shard_2_1
            - connection: user_shard_3
              less_than: 2000000000
              slaves:
                - user_shard_3_1
```

* config/database.yml

Add slave connection settings under `shards`.

```yaml
...
  shards:
    user_shard_1:
      <<: *default
      database: turntable_user_shard_1_test
    user_shard_1_1:
      <<: *default
      database: turntable_user_shard_1_1_test
    user_shard_2:
      <<: *default
      database: turntable_user_shard_2_test
    user_shard_2_1:
      <<: *default
      database: turntable_user_shard_2_1_test
    user_shard_3:
      <<: *default
      database: turntable_user_shard_3_test
    user_shard_3_1:
      <<: *default
      database: turntable_user_shard_3_1_test
```

Slave usage:

```ruby
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:

```bash
bundle exec rails g model user name:string
```

And Edit migration file:

```ruby
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) # <-- create sequence table
  end
end
```

Then please execute rake tasks:

```bash
bundle exec rake db:create
bundle exec rake db:migrate
```

Those rake tasks would be executed to shards too.

### Example Model

Add turntable [shard_key_name] to the model class:

```ruby
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: #<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">
```

### Retrieving

```
    > 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">
```

### 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: #<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">
```

### Counting

```
    > 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.

Turntable has follow 3 sequencers currently:

* :mysql - Use database table to generate ids.
* :barrage - Use [barrage](https://github.com/drecom/barrage) gem to generate ids
* :katsubushi - [katsubushi](https://github.com/kayac/go-katsubushi) sequencer backend

### Mysql example

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

* database.yml

```yaml
    development:
      ...
      seq: # <-- sequence database definition
        user_seq_1:
          <<: *spec
          database: sample_app_user_seq_development
```

* turntable.yml

```yaml
    development:
      clusters:
        user_cluster: # <-- cluster name
          ....
          seq:
            user_seq: # <-- sequencer name
              seq_type: mysql # <-- sequencer type
              connection: user_seq_1 # <-- sequencer database connection
```

Add below to the migration:

```ruby
create_sequence_for(:users) # <-- this line creates sequence table named `users_id_seq`
```

Next, add sequencer definition to the model:

```ruby
  class User < ApplicationRecord
    turntable :id
    sequencer :user_seq # <-- this line enables sequencer module
    has_one :status
  end
```

### Barrage example

First, add barrage gem to your Gemfile:

```ruby
gem 'barrage'
```

Then, add configuration to turntable.yml:

* turntable.yml

```yaml
    development:
      clusters:
        user_cluster: # <-- cluster name
          ....
          seq:
            barrage_seq: # <-- sequencer name
              seq_type: barrage # <-- sequencer type
              options: # <-- options passed to barrage
                generators:
                  - name: msec
                    length: 39 # MAX 17.4 years from start_at
                    start_at: 1396278000000 # 2014/04/01 00:00:00 JST
                  - name: redis_worker_id
                    length: 16
                    ttl: 300
                    redis:
                      host: '127.0.0.1'
                  - name: sequence
                    length: 9
```

Next, add sequencer definition to the model:

```ruby
  class User < ApplicationRecord
    turntable :id
    sequencer :barrage_seq # <-- this line enables sequencer module
    has_one :status
  end
```

### Katsubushi example

[katsubushi](https://github.com/kayac/go-katsubushi) is available as a sequence server.
ActiveRecord::Turntable accesses katsubushi via Dalli.

First, add dalli gem to your Gemfile:

```ruby
gem 'dalli'
```

Then, add configuration to turntable.yml:

* turntable.yml

```yaml
    development:
      clusters:
        user_cluster: # <-- cluster name
          ....
          seq:
            katsubushi_seq: # <-- sequencer name
              seq_type: katsubushi # <-- sequencer type
              options: # <-- options passed to dalli
                servers:
                  - host: localhost
                    port: 11212
                  - host: localhost
                    port: 11213
```

Next, add sequencer definition to the model:

```ruby
  class User < ApplicationRecord
    turntable :id
    sequencer :katsubushi_seq # <-- this line enables sequencer module
    has_one :status
  end
```

## Transactions
Turntable has some transaction support methods.

### shards_transaction

Pass AR::Base instances, `shards_transaction` method suitable shards

```ruby
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:

```ruby
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:

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

to specify shard:

```ruby
    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:

```ruby
    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 # \\{shard_name => shard_obj,....}
* AR::Base#turntable_shard # Returns current object's shard
* AR::Base.connection.shard_for(shard_key_value) #=> shard

### Send query to all shards

Use with_all method:

```ruby
  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:

```ruby
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

```ruby
> 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 foreign_shard_key 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
```

```ruby
> 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: #<Arel::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
=> #<UserItem id: 2198059200000, user_id: 1, item_id: 1, created_at: "2017-05-23 04:41:13", updated_at: "2017-05-23 04:41:13">
```

### 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:

```yaml
development:
   ....
   raise_on_not_specified_shard_query: true
   raise_on_not_specified_shard_update: true
```

```ruby
# 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.