Moneta: A unified interface for key/value stores

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Moneta provides a standard interface for interacting with various kinds of key/value stores. A short overview of the features:

  • Supports a lot of backends (See below)
  • Allows a full configuration of the serialization -> compression -> adapter stack using proxies (Similar to Rack middlewares)
    • Configurable serialization via Moneta::Transformer proxy (Marshal/JSON/YAML and many more)
    • Configurable value compression via Moneta::Transformer proxy (Zlib, Snappy, LZMA, ...)
    • Configurable key transformation via Moneta::Transformer proxy
  • Expiration for all stores (Added via proxy Moneta::Expires if not supported natively)
  • Atomic incrementation and decrementation for most stores (Method #increment and #decrement)
  • Includes a very simple key/value server (Moneta::Server) and client (Moneta::Adapters::Client)
  • Integration with Rails, Rack as cookie and session store and Rack-Cache

Moneta is tested thoroughly using Travis-CI.


Supported backends

Out of the box, it supports the following backends:

  • Memory:
    • In-memory store (:Memory)
    • LRU hash - prefer this over :Memory! (:LRUHash)
    • LocalMemCache (:LocalMemCache)
    • Memcached store (:Memcached, :MemcachedNative and :MemcachedDalli)
  • Relational Databases:
    • DataMapper (:DataMapper)
    • ActiveRecord (:ActiveRecord)
    • Sequel (:Sequel)
    • Sqlite3 (:Sqlite)
  • Filesystem:
    • PStore (:PStore)
    • YAML store (:YAML)
    • Filesystem directory store (:File)
    • Filesystem directory store which spreads files in subdirectories using md5 hash (:HashFile)
  • Key/value databases:
    • Berkeley DB (:DBM)
    • Cassandra (:Cassandra)
    • GDBM (:GDBM)
    • HBase (:HBase)
    • LevelDB (:LevelDB)
    • Redis (:Redis)
    • Riak (:Riak)
    • SDBM (:SDBM)
    • TokyoCabinet (:TokyoCabinet)
  • Document databases:
    • CouchDB (:Couch)
    • MongoDB (:Mongo)
  • Other
    • Moneta key/value server client (:Client works with Moneta::Server)
    • Fog cloud storage which supports Amazon S3, Rackspace, etc. (:Fog)
    • Storage which doesn't store anything (:Null)

Some of the backends are not exactly based on key/value stores, e.g. the relational ones. These are useful if you already use the corresponding backend in your application. You get a key/value store for free then without installing any additional services and you still have the possibility to upgrade to a real key/value store.


In addition it supports proxies (Similar to Rack middlewares) which add additional features to storage backends:

  • Moneta::Proxy and Moneta::Wrapper proxy base classes
  • Moneta::Expires to add expiration support to stores which don't support it natively. Add it in the builder using use :Expires.
  • Moneta::Stack to stack multiple stores (Read returns result from first where the key is found, writes go to all stores). Add it in the builder using use :Stack.
  • Moneta::Transformer transforms keys and values (Marshal, YAML, JSON, Base64, MD5, ...). Add it in the builder using use :Transformer.
  • Moneta::Cache combine two stores, one as backend and one as cache (e.g. Moneta::Adapters::File + Moneta::Adapters::Memory). Add it in the builder using use :Cache.
  • Moneta::Lock to make store thread safe. Add it in the builder using use :Lock.
  • Moneta::Logger to log database accesses. Add it in the builder using use :Logger.
  • Moneta::Shared to share a store between multiple processes. Add it in the builder using use :Shared.

Supported serializers and compressors (Moneta::Transformer)

Supported serializers:

  • BEncode (:bencode)
  • BERT (:bert)
  • BSON (:bson)
  • JSON (:json)
  • Marshal (:marshal)
  • MessagePack (:msgpack)
  • Ox (:ox)
  • TNetStrings (:tnet)
  • YAML (:yaml)

Supported value compressors:

  • LZMA (:lzma)
  • LZO (:lzo)
  • Snappy (:snappy)
  • QuickLZ (:quicklz)
  • Zlib (:zlib)

Special transformers:

  • Digests (MD5, Shas, ...)
  • Add prefix to keys (:prefix)
  • HMAC to verify values (:hmac, useful for Rack::MonetaCookies)

Moneta API

The Moneta API is purposely extremely similar to the Hash API with a few minor additions. There are the additional methods #load, #increment, #decrement and #close. Every method takes also a optional option hash. In order so support an identical API across stores, Moneta does not support iteration or partial matches.

#initialize(options)                      options differs per-store, and is used to set up the store.

#[](key)                                  retrieve a key. If the key is not available, return nil.

#load(key, options = {})                  retrieve a key. If the key is not available, return nil.

#fetch(key, options = {}, &block)         retrieve a key. If the key is not available, execute the
                                          block and return its return value.

#fetch(key, value, options = {})          retrieve a key. If the key is not available, return the value,

#[]=(key, value)                          set a value for a key. If the key is already used, clobber it.
                                          keys set using []= will never expire.

#store(key, value, options = {})          same as []=, but you can supply options.

#delete(key, options = {})                delete the key from the store and return the current value.

#key?(key, options = {})                  true if the key exists, false if it does not.

#increment(key, amount = 1, options = {}) increment numeric value. This is a atomic operation
                                          which is not supported by all stores. Returns current value.

#decrement(key, amount = 1, options = {}) increment numeric value. This is a atomic operation
                                          which is not supported by all stores. Returns current value.
                                          This is just syntactic sugar for incrementing with a negative value.

#clear(options = {})                      clear all keys in this store.

#close                                    close database connection.

Creating a Store

There is a simple interface to create a store using

store =, :server => 'localhost:11211')

If you want to have control over the proxies, you have to use

store = do
  # Adds expires proxy
  use :Expires  # Transform key using Marshal and Base64 and value using Marshal

  use :Transformer, :key => [:marshal, :base64], :value => :marshal  # Memory backend

  adapter :Memory


The Cassandra, Memcached and Redis backends supports expires values directly:

cache =

# Or using the builder...
cache = do
  adapter :Memcached

# Expires in 60 seconds, value, :expires => 60)

# Update expires time if value is found
cache.load(key, :expires => 30)
cache.key?(key, :expires => 30)

You can add the expires feature to other backends using the Expires proxy:

# Using the :expires option
cache =, :dir => '...', :expires => true)

# or manually by using the proxy...
cache = => '...'))

# or using the builder...
cache = do
  use :Expires
  adapter :File, :dir => '...'

Incrementation and raw access

The stores support the #increment which allows atomic increments of unsigned integer values. If you increment a non existing value, it will be created. If you increment a non integer value an exception will be raised.

store.increment('counter')     # returns 1, counter created
store.increment('counter')     # returns 2
store.increment('counter', -1) # returns 1
store.increment('counter', 13) # returns 14
store.increment('counter', 0)  # returns 14
store.decrement('counter')     # returns 13
store['name'] = 'Moneta'
store.increment('name')        # raises an Exception

If you want to access the counter value you have to use raw access to the datastore. This is only important if you have a Moneta::Transformer somewhere in your proxy stack which transforms the values e.g. with Marshal.

store.increment('counter')          # returns 1, counter created
store.load('counter', :raw => true) # returns 1'counter', '10', :raw => true)
store.increment('counter') # returns 11

Fortunately there is a nicer way to do this using some syntactic sugar!

store.increment('counter') # returns 1, counter created
store.raw['counter']       # returns 1
store.raw.load('counter')  # returns 1

store.raw['counter'] = '10'
store.increment('counter') # returns 11

You can also keep the raw store in a variable and use it like this:

counters = store.raw

counters.increment('counter') # returns 1, counter created
counters['counter']           # returns 1
counters.load('counter')      # returns 1

counters['counter'] = '10'
counters.increment('counter') # returns 11

Stores which support incrementation (you have to use Moneta::Lock if you want to use the store in a multithreading environment.)

  • ActiveRecord
  • File
  • HBase
  • LRUHash
  • LevelDB
  • Memcached
  • Memory
  • Redis
  • Sequel
  • Sqlite
  • TokyoCabinet
  • YAML/PStore

Stores which don't support incrementation:

  • Cassandra
  • Couch
  • DataMapper
  • Fog
  • LocalMemCache
  • Mongo
  • Riak

Syntactic sugar and option merger

For raw data access as described before the class Moneta::OptionMerger is used. It works like this:

# All methods after 'with' get the options passed
store.with(:raw => true).load('key')

# You can also specify the methods
store.with(:raw => true, :only => :load).load('key')
store.with(:raw => true, :except => [:key?, :increment]).load('key')

# Syntactic sugar for raw access

# Access substore where all keys get a prefix
substore = store.prefix('sub')
substore['key'] = 'value'
store['key']    # returns nil
store['subkey'] # returns 'value'

# Set expiration time for all keys
short_lived_store = long_lived_store.expires(60)
short_lived_store['key'] = 'value'

Framework Integration

Inspired by redis-store there exist integration classes for Rails, Rack and Rack-Cache.

Rack session store

Use Moneta as a Rack session store:

require 'rack/session/moneta'

# Use only the adapter name
use Rack::Session::Moneta, :store => :Redis

# Use
use Rack::Session::Moneta, :store =>, :expires => true)

# Use the Moneta builder
use Rack::Session::Moneta do
  use :Expires
  adapter :Memory

Rack cache

Use Moneta as a Rack-Cache store:

require 'rack/cache/moneta'

use Rack::Cache,
      :metastore   => 'moneta://Memory?expires=true',
      :entitystore => 'moneta://Memory?expires=true'

# Or used named Moneta stores
Rack::Cache::Moneta['named_metastore'] = do
  use :Expires
  adapter :Memory
use Rack::Cache,
      :metastore => 'moneta://named_metastore',
      :entity_store => 'moneta://named_entitystore'

Rack cookies

Use Moneta to store cookies in Rack. It uses the Moneta::Adapters::Cookie. You might wonder what the purpose of this store or Rack middleware is: It makes it possible to use all the transformers on the cookies (e.g. :prefix, :marshal and :hmac for value verification).

require 'rack/moneta_cookies'

use Rack::MonetaCookies, :domain => '', :path => '/path'
run lambda do |env|
  req =
  req.cookies #=> is now a Moneta store!
  env['rack.request.cookie_hash'] #=> is now a Moneta store!
  req.cookies['key'] #=> retrieves 'key'
  req.cookies['key'] = 'value' #=> sets 'key'
  req.cookies.delete('key') #=> removes 'key'
  [200, {}, []]

Rails session store

Add the session store in your application configuration config/environments/*.rb.

require 'moneta'

# Only by adapter name
config.cache_store :moneta_store, :store => :Memory

# Use
config.cache_store :moneta_store, :store =>

# Use the Moneta builder
config.cache_store :moneta_store, :store => do
  use :Expires
  adapter :Memory

Rails cache store

Add the cache store in your application configuration config/environments/*.rb. Unfortunately the Moneta cache store doesn't support matchers. If you need these features use a different server-specific implementation.

require 'moneta'

# Only by adapter name
config.cache_store :moneta_store, :store => :Memory

# Use
config.cache_store :moneta_store, :store =>

# Use the Moneta builder
config.cache_store :moneta_store, :store => do
  use :Expires
  adapter :Memory


Build your own key value server

You can use Moneta to build your own key/value server which is shared between multiple processes. If you run the following code in two different processes, they will share the same data which will also be persistet in the database shared.db.

require 'moneta'

store = do
  use :Transformer, :key => :marshal, :value => :marshal
  use :Shared do
    use :Cache do
      cache do
        adapter :LRUHash
      backend do
        adapter :GDBM, :file => 'shared.db'

If you want to go further, you might want to take a look at Moneta::Server and Moneta::Adapters::Client which are used by Moneta::Shared and provide the networking communication. But be aware that they are experimental and subjected to change. They provide an acceptable performance (for being ruby only), but don't have a stable protocol yet.

You might wonder why I didn't use DRb to implement server and client - in fact my first versions used it, but with much worse performance and it was real fun to implement the networking directly :) There is still much room for improvement and experiments, try EventMachine, try Kgio, ...

ToyStore ORM

If you want something more advanced to handle your objects and relations, use John Nunemaker's ToyStore which works together with Moneta. Assuming that Person is a ToyStore::Object you can add persistence using Moneta as follows:

# Use the Moneta Redis backend
Person.adapter :memory,

Testing and Benchmarks

Testing is done using Travis-CI. Currently we support Ruby 1.8.7 and 1.9.3.

Benchmarks for each store are done on Travis-CI for each build. Take a look there to compare the speed of the different key value stores for different key/value sizes and size distributions. Feel free to add your own configurations! The impact of Moneta should be minimal since it is only a thin layer on top of the different stores.

More information


  • Horcrux: Used at github, supports batch operations but only Memcached backend
  • ToyStore: ORM mapper for key/value stores
  • ToyStore Adapter: Adapter to key/value stores used by ToyStore, Moneta can be used directly with the ToyStore Memory adapter


  • Daniel Mendler
  • Hannes Georg
  • Originally by Yehuda Katz and contributors