Postamt is a sane, solution for performing database reads against a hot standby server with Rails 4.1 and 4.2.
If you use Rails 3.2 or 4.0, use Postamt version 0.9.2.
Choose per model and/or controller&action whether a read-only query
should be sent to master or a hot standby.
Inside a transaction reads always happen against master.
Care has been taken to avoid common performance pitfalls. It's been battle tested in production at sauspiel.de.
Monkey-patching is kept to an absolute minimum, the hard work happens through officially-supported Rails APIs. That's why there's so little code compared to similar gems.
Postamt requires Rails 3.2+ and works with Rails 4.
Add this line to your application's Gemfile:
gem 'postamt'
# database.yml
development:
adapter: postgresql
database: app
username: app
password:
host: master.db.internal
encoding: utf8
slave:
host: slave.db.internal
username: app_readonlyclass UserController < ApplicationController
use_db_connection :slave, for: ['User'], only: [:search]
def search
# SELECTs here are sent to slave
# User#save and User.create would be sent to master anyways.
# Everything in a transaction block too.
@users = User.where(...) # sent to slave
@something_else = SomethingElse.first # sent to master
end
def create
@user = User.new(params[:user])
@user.save! # sent to master
end
def invoice
transaction do
@user = User.where(...) # sent to master
@invoices = Invoice.create(...) # sent to master
end
end
endclass ArchivedItem < ActiveRecord::Base
# default_connection can be overwritten with
# * Postamt.on(...) { ... },
# * ActiveRecord::Base.transaction { ... }, and
# * use_db_connection :other_connection, for: ['ArchivedItem'] in a controller.
self.default_connection = :slave
end
User.where(...) # sent to master
item = ArchivedItem.where(...) # sent to slave
item.title = "changed title"
item.save! # sent to master
item.reload # sent to slave, beware of replication lag here!
ActiveRecord::Base.transaction do
ArchivedItem.where(...) # sent to master, since we're in a transaction
User.where(...) # sent to master
end
Postamt.on(:master) do
ArchivedItem.where(...) # sent to master
User.where(...) # sent to master
end# If you don't want to test with a slave DB put this in config/environments/test.rb
Postamt.force_connection = :masterCreate the DB postamt_test and ensure the users master and
slave exist:
$ createdb postamt_test
$ createuser -s master # -s => superuser
$ createuser -s slave # better to restrict slave to be read-only
Migrate the DB in the Rails 4 app:
$ cd testapp
$ RAILS_ENV=test bundle exec rake db:migrate
$ bundle exec ruby -Itest test/integration/postamt_test.rb
You can't run the tests via a simple rake because Postamt deactivates
itself when it detects that a task starting with 'db' is run (like
db:test:prepare)
- Fork it
- Create your feature branch (
git checkout -b my-new-feature) - Commit your changes (
git commit -am 'Add some feature') - Push to the branch (
git push origin my-new-feature) - Create new Pull Request
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