active_record_slave

ActiveRecord drop-in solution to efficiently redirect reads to slave databases

http://github.com/reidmorrison/active_record_slave

Introduction

active_record_slave allows all database reads to go to a slave while ensuring that all writes go to the master database. Also, active_record_slave ensures that any reads that are performed in a transaction will always go to the master database to ensure data consistency.

Features

Redirecting reads to a single slave database
Works with any database driver that works with ActiveRecord
Supports all Rails 3 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
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 one can be MySQL and the other Oracle if required.
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

Dependencies

Tested on Rails 3 and Rails 4

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

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

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:

development:
  database: myapp_development
  username: root
  password:
  encoding: utf8
  adapter:  mysql
  host:     127.0.0.1
  pool:     20
  slave:
    database: myapp_development_replica
    username: root
    password:
    encoding: utf8
    adapter:  mysql
    host:     127.0.0.1
    pool:     20

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

development:
  database: myapp_development
  username: root
  password:
  encoding: utf8
  adapter:  mysql
  host:     127.0.0.1
  pool:     20
<% if `hostname`.strip == 'batch' %>
  slave:
    database: myapp_development_replica
    username: root
    password:
    encoding: utf8
    adapter:  mysql
    host:     127.0.0.1
    pool:     20
<% end %>

Possible Future Enhancements

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

Authors

Reid Morrison :: [email protected] :: @reidmorrison

License

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.