MongoSequence

MongoSequence provides light-weight, robust sequences in MongoDB. They are useful for auto-incrementing or counting. Compatible with any Mongo ODM.

MongoSequence creates named sequences in a "sequences" collection in your database and atomically increments and returns the counter on them using Mongo's findAndModify command. You won't have collisions--two processes trying to increment at the same time will alway get different numbers.

Usage

Install with Bundler:

gem "mongo_sequence"

Install without Bundler:

gem install mongo_sequence --no-ri --no-rdoc

If you're not using MongoMapper or Mongoid, you'll have to tell MongoSequence what database to use:

MongoSequence.database = Mongo::Connection.new.db('my_app_development')

Now, increment some sequences:

MongoSequence[:global].next    # => 1
MongoSequence[:global].next    # => 2

# get the current value...
# no guarantees of course on how long it's valid
# usually you use the return value of #next
MongoSequence[:global].current # => 2

# can also reset sequences if you need to
MongoSequence[:global] = 100
MongoSequence[:global].next    # => 101

# sequences with different names are independent
MongoSequence[:bluejay].next   # => 1
MongoSequence[:bluejay].next   # => 2

Here's how the sequences look in Mongo:

MongoSequence.collection.find_one(:_id => 'bluejay')
# =>
# {
#   "_id"     => "bluejay",
#   "current" => 2
# }

Why?

Why would anyone need atomically incrementing sequences with unique return values? Well, the most common case is for auto-incrementing id's in Mongo. Here's a MongoMapper example:

class Peregrine
  include MongoMapper::Document
  key :_id, Integer, :default => lambda { nil }

  before_create do
    self.id ||= MongoSequence[:peregrine_id].next # for id's unique among Peregrines
    # or
    self.id ||= MongoSequence[:mongo_id].next     # for id's unique across the database
  end
end

Help make it better!

Need something added? Please open an issue! Or, even better, code it yourself and send a pull request:

# fork it on github, then clone:
git clone git@github.com:your_username/mongo_sequence.git
bundle install
rspec
# hack away
git push
# then make a pull request

License

Authored by Brian Hempel. Public domain, no restrictions.