active_record_slave
Redirect ActiveRecord (Rails) reads to slave databases while ensuring all writes go to the master database.
Introduction
Redirect database reads to slave instances while ensuring that all writes go to the master database. 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
- Redirect 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
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