Active Record Reader

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

• https://github.com/rocketjob/active_record_reader

Introduction

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

Status

Production Ready. Actively used in large production environments.

Features

• Redirecting reads to a single reader 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 primary by default.
  • Can be configured to send reads in a transaction to reader databases.
• Lightweight footprint.
• No overhead whatsoever when a reader is not configured.
• Negligible overhead when redirecting reads to the reader.
• Connection Pools to both databases are retained and maintained independently by ActiveRecord.
• The primary and reader databases do not have to be of the same type.
  • For example Oracle could be the primary with MySQL as the reader database.
• Debug logs include a prefix of Reader: to indicate which SQL statements are going to the reader database.

Example showing Reader redirected read

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

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

Log file output:

03-13-12 05:56:05 pm,[2608],b[0],[0],  Reader: 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 primary

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 primary

Sometimes it is necessary to read from the primary:

ActiveRecordReader.read_from_primary do
  r = Role.where(name: 'manager').first
end

Usage Notes

delete_all

Delete all executes against the primary 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 reader database and then deletes the corresponding data from the primary

D, [2012-11-06T19:43:26.890674 #89002] DEBUG -- :   Reader: 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 ActiveRecordReader detects when a call is inside a transaction and will send all reads to the primary when a transaction is active.

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

In file config/application.rb:

# Read from reader even when in an active transaction
config.active_record_reader.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 ActiveRecordReader.read_from_primary

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_primary` to ensure it is included in the count below:
  ActiveRecordReader.read_from_primary 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_reader and redirected to the reader.

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

Install

Add to Gemfile

gem 'active_record_reader'

Run bundler to install:

bundle

Or, without Bundler:

gem install active_record_reader

Configuration

To enable reader reads for any environment just add a reader: 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:     primary1
  pool:     50
  reader:
    database: production
    username: username
    password: password
    encoding: utf8
    adapter:  mysql
    host:     reader1
    pool:     50

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

production:
  database: production
  username: username
  password: password
  encoding: utf8
  adapter:  mysql
  host:     primary1
  pool:     50
<% if `hostname`.strip == 'batch' %>
  reader:
    database: production
    username: username
    password: password
    encoding: utf8
    adapter:  mysql
    host:     reader1
    pool:     50
<% end %>

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

production:
  database: production
  username: username
  password: password
  encoding: utf8
  adapter:  mysql
  host:     primary1
  pool:     50
  reader:
    database: production
    username: username
    password: password
    encoding: utf8
    adapter:  mysql
    host:     <%= %w(reader1 reader2 reader3).sample %>
    pool:     50

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

production:
  database: production
  username: username
  password: password
  encoding: utf8
  adapter:  mysql
  host:     primary1
  pool:     50
  reader:
    database: production
    username: username
    password: password
    encoding: utf8
    adapter:  mysql
    host:     <%= `hostname`.strip == 'app1' ? 'reader1' : 'reader2' %>
    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 readers (ask for it by submitting an issue)

Versioning

This project uses Semantic Versioning.

Author

Reid Morrison :: @reidmorrison