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Postamt is a sane, Rails-4-ready solution for performing database reads against a hot standby server.
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branch: master


Gem Version

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

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'

Example usage

# database.yml
  adapter: postgresql
  database: app
  username: app
  host: master.db.internal
  encoding: utf8
    host: slave.db.internal
    username: app_readonly
class 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

  def create
    @user =[:user])! # sent to master

  def invoice
    transaction do
      @user = User.where(...) # sent to master
      @invoices = Invoice.create(...) # sent to master
class 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

User.where(...) # sent to master
item = ArchivedItem.where(...) # sent to slave
item.title = "changed title"! # 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

Postamt.on(:master) do
  ArchivedItem.where(...) # sent to master
  User.where(...) # sent to master
# If you don't want to test with a slave DB put this in config/environments/test.rb
Postamt.force_connection = :master


Create 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 testapp41 # Rails 4.1
$ 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)


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request
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