Redirect ActiveRecord (Rails) reads to slave databases while ensuring all writes go to the master database.
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README.md

Active Record Slave

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Redirect ActiveRecord (Rails) reads to slave databases while ensuring all writes go to the master database.

Introduction

active_record_slave redirects all database reads to slave instances while ensuring that all writes go to the master database. active_record_slave ensures that any reads that are performed within a database transaction are by default directed to the master database to ensure data consistency.

Status

Production Ready. Actively used in large production environments.

Features

  • Redirecting reads to a single slave database.
  • Works with any database driver that works with ActiveRecord.
  • Supports all Rails 3, 4, or 5 read apis.
    • Including dynamic finders, AREL, and ActiveRecord::Base.select.
  • Transaction aware
    • Detects when a query is inside of a transaction and sends those reads to the master by default.
    • Can be configured to send reads in a transaction to slave databases.
  • Lightweight footprint.
  • No overhead whatsoever when a slave is not configured.
  • Negligible overhead when redirecting reads to the slave.
  • Connection Pools to both databases are retained and maintained independently by ActiveRecord.
  • The master and slave databases do not have to be of the same type.
    • For example Oracle could be the master with MySQL as the slave database.
  • Debug logs include a prefix of Slave: to indicate which SQL statements are going to the slave database.

Example showing Slave redirected read

# Read from the slave database
r = Role.where(name: 'manager').first
r.description = 'Manager'

# Save changes back to the master database
r.save!

Log file output:

03-13-12 05:56:05 pm,[2608],b[0],[0],  Slave: Role Load (3.0ms)  SELECT `roles`.* FROM `roles` WHERE `roles`.`name` = 'manager' LIMIT 1
03-13-12 05:56:22 pm,[2608],b[0],[0],  AREL (12.0ms)  UPDATE `roles` SET `description` = 'Manager' WHERE `roles`.`id` = 5

Example showing how reads within a transaction go to the master

Role.transaction do
  r = Role.where(name: 'manager').first
  r.description = 'Manager'
  r.save!
end

Log file output:

03-13-12 06:02:09 pm,[2608],b[0],[0],  Role Load (2.0ms)  SELECT `roles`.* FROM `roles` WHERE `roles`.`name` = 'manager' LIMIT 1
03-13-12 06:02:09 pm,[2608],b[0],[0],  AREL (2.0ms)  UPDATE `roles` SET `description` = 'Manager' WHERE `roles`.`id` = 4

Forcing a read against the master

Sometimes it is necessary to read from the master:

ActiveRecordSlave.read_from_master do
  r = Role.where(name: 'manager').first
end

Usage Notes

delete_all

Delete all executes against the master database since it is only a delete:

D, [2012-11-06T19:47:29.125932 #89772] DEBUG -- :   SQL (1.0ms)  DELETE FROM "users"

destroy_all

First performs a read against the slave database and then deletes the corresponding data from the master

D, [2012-11-06T19:43:26.890674 #89002] DEBUG -- :   Slave: User Load (0.1ms)  SELECT "users".* FROM "users"
D, [2012-11-06T19:43:26.890972 #89002] DEBUG -- :    (0.0ms)  begin transaction
D, [2012-11-06T19:43:26.891667 #89002] DEBUG -- :   SQL (0.4ms)  DELETE FROM "users" WHERE "users"."id" = ?  [["id", 3]]
D, [2012-11-06T19:43:26.892697 #89002] DEBUG -- :    (0.9ms)  commit transaction

Transactions

By default ActiveRecordSlave detects when a call is inside a transaction and will send all reads to the master when a transaction is active.

It is now possible to send reads to database slaves and ignore whether currently inside a transaction:

In file config/application.rb:

# Read from slave even when in an active transaction
config.active_record_slave.ignore_transactions = true

It is important to identify any code in the application that depends on being able to read any changes already part of the transaction, but not yet committed and wrap those reads with ActiveRecordSlave.read_from_master

Inquiry.transaction do
  # Create a new inquiry
  Inquiry.create
  
  # The above inquiry is not visible yet if already in a Rails transaction.
  # Use `read_from_master` to ensure it is included in the count below:
  ActiveRecordSlave.read_from_master do
    count = Inquiry.count
  end

end

Note

ActiveRecord::Base.execute is sometimes used to perform custom SQL calls against the database to bypass ActiveRecord. It is necessary to replace these calls with the standard ActiveRecord::Base.select call for them to be picked up by active_record_slave and redirected to the slave.

This is because ActiveRecord::Base.execute can also be used for database updates which we do not want redirected to the slave

Install

Add to Gemfile

gem 'active_record_slave'

Run bundler to install:

bundle

Or, without Bundler:

gem install active_record_slave

Configuration

To enable slave reads for any environment just add a slave: entry to database.yml along with all the usual ActiveRecord database configuration options.

For Example:

production:
  database: production
  username: username
  password: password
  encoding: utf8
  adapter:  mysql
  host:     master1
  pool:     50
  slave:
    database: production
    username: username
    password: password
    encoding: utf8
    adapter:  mysql
    host:     slave1
    pool:     50

Sometimes it is useful to turn on slave reads per host, for example to activate slave reads only on the linux host 'batch':

production:
  database: production
  username: username
  password: password
  encoding: utf8
  adapter:  mysql
  host:     master1
  pool:     50
<% if `hostname`.strip == 'batch' %>
  slave:
    database: production
    username: username
    password: password
    encoding: utf8
    adapter:  mysql
    host:     slave1
    pool:     50
<% end %>

If there are multiple slaves, it is possible to randomly select a slave on startup to balance the load across the slaves:

production:
  database: production
  username: username
  password: password
  encoding: utf8
  adapter:  mysql
  host:     master1
  pool:     50
  slave:
    database: production
    username: username
    password: password
    encoding: utf8
    adapter:  mysql
    host:     <%= %w(slave1 slave2 slave3).sample %>
    pool:     50

Slaves can also be assigned to specific hosts by using the hostname:

production:
  database: production
  username: username
  password: password
  encoding: utf8
  adapter:  mysql
  host:     master1
  pool:     50
  slave:
    database: production
    username: username
    password: password
    encoding: utf8
    adapter:  mysql
    host:     <%= `hostname`.strip == 'app1' ? 'slave1' : 'slave2' %>
    pool:     50

Dependencies

  • Tested on Rails 3 and Rails 4

See .travis.yml for the list of tested Ruby platforms

Possible Future Enhancements

  • Support for multiple named slaves (ask for it by submitting an issue)

Versioning

This project uses Semantic Versioning.

Author

Reid Morrison :: @reidmorrison