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. Moneta supports the well-known NoSQL and document based stores.

A short overview of the features:

If you are not yet convinced, you might ask why? What are the goals of the project?

Moneta is tested thoroughly using Travis-CI.

Getting started

Install Moneta via Rubygems

$ gem install moneta

or add it to your Gemfile

gem 'moneta'

Now you are ready to go:

require 'moneta'

# Create a simple file store
store =, dir: 'moneta')

# Store some entries
store['key'] = 'value'

# Read entry
store.key?('key') # returns true
store['key'] # returns 'value'



Supported backends

Out of the box, it supports the following backends. Use the backend name symbol in the Moneta constructor (e.g.

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.

Backend feature matrix

NOTE: The backend matrix is much more readable on than on github. Go there!

AdapterRequired gemsMulti-thread safe[1]Multi-process safe[2]Atomic increment[8]Atomic create[9]Native expires[3]PersistentDescription
Persistent stores
Mongomongo or mopedMongoDB database
MongoOfficialmongoMongoDB database
MongoMopedmopedMongoDB database
RedisredisRedis database
ActiveRecordactiverecordActiveRecord ORM
File-File store
LMDBlmdbSymas Lightning Memory-Mapped Database (LMDB)
SequelsequelSequel ORM
TokyoTyranttokyotyrant or ruby-tokyotyrantTokyoTyrant database
PStore-[10]PStore store
YAML-[10]YAML store
Sqlitesqlite3?[10]Sqlite3 database
Daybreakdaybreak(✓)[7]Incredibly fast pure-ruby key/value store Daybreak
DBM-Berkeley DB using DBM interface or NDBM (Depends on Ruby environment)
GDBM-GDBM database
LevelDBleveldbLevelDB database
SDBM-SDBM database
TDBtdbTDB database
KyotoCabinetkyotocabinetKyotoCabinet database
TokyoCabinettokyocabinetTokyoCabinet database
DataMapperdm-core, dm-migrationsDataMapper ORM
Couchfaraday, multi_jsonCouchDB database
HBasehbaserb?HBase database
Cassandracassandra?Cassandra distributed database
LocalMemCachelocalmemcacheLocalMemCache database
Fogfog?Fog cloud store
Riakriak-clientRiak database
Non persistent stores
MemcachedDallidalli[4]Memcached database with Dalli library
Memcacheddalli or memcached?[4]Memcached database
MemcachedNativememcached[4]Memcached database with native library
Cookie-(✓)[6]Cookie in memory store
LRUHash-(✓)[6]LRU memory store
Memory-(✓)[6]Memory store
Null-No database
Client-?[5]?[5]?[5]?[5]Moneta client adapter
RestClientfaraday?[5]Moneta REST client adapter


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

Serializers and compressors (Moneta::Transformer)

Supported serializers:

Supported value compressors:

Supported encoders:

Special transformers:

Moneta API

The Moneta API is purposely extremely similar to the Hash API with a few minor additions. 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 an atomic operation
                                          which is not supported by all stores. Returns current value.

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

#create(key, value, options = {})         create entry. This is an atomic operation which is not supported by all stores.
                                          Returns true if the value was created.

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

#close                                    close database connection.

#features                                 return array of features, e.g. [:create, :expires, :increment]

#supports?(feature)                       returns true if store supports a given feature

Creating a Store

There is a simple interface to create a store using You will get automatic key and value serialization which is provided by Moneta::Transformer. This allows you to store arbitrary Ruby objects. You can tune some options when you call However for very fine tuning use

store =, server: 'localhost:11211')
store['key'] = 'value'
store['hash_key'] = {a: 1, b: 2}
store['object_key'] =

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

  # IMPORTANT: adapter must be defined last for the builder to function properly.

  # Memory backend
  adapter :Memory

You can also directly access the underlying adapters if you don't want to use the Moneta stack.

db = 'directory')
db['key'] = {a: 1, b: 2} # This will fail since you can only store Strings

# However for Mongo and Couch this works
# The hash will be mapped directly to a Mongo/Couch document.
db =
db['key'] = {a: 1, b: 2}

db =
db['key'] = {a: 1, b: 2}


The Cassandra, Memcached, Redis and Mongo backends support expiration natively.

cache =

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

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

# Never expire, value, expires: 0), value, expires: false)

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

# Or remove the expiration if found
cache.load(key, expires: false)
cache.key?(key, expires: 0)

You can add the expires feature to other backends using the Moneta::Expires proxy. But be aware that expired values are not deleted automatically if they are not looked up.

# 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: '...'

Atomic operations

Atomic 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

Atomic create

The stores support the #create which allows atomic creation of entries. #create returns true if the value was created.

store.create('key', 'value') # returns true
store.create('key', 'other value') # returns false

Shared/distributed synchronization primitives

Moneta provides shared/distributed synchronization primitives which are shared database-wide between all clients.

Moneta::Mutex allows a single thread to enter a critical section.

mutex =, 'mutex_key')

mutex.synchronize do
   mutex.locked? # returns true

   # Synchronized access to counter
   store['counter'] += 1

  mutex.locked? # returns true
  # ...

Moneta::Semaphore allows max_concurrent threads to enter a critical section.

semaphore =, 'semaphore_counter', max_concurrent)

semaphore.synchronize do
   semaphore.locked? # returns true
   # ...

  semaphore.locked? # returns true
  # ...

Weak atomic operations

If an underlying adapter doesn't provide atomic #create or #increment and #decrement you can use the proxies Moneta::WeakIncrement and Moneta::WeakCreate to add support without atomicity.

But then you have to ensure that the store is not shared by multiple processes and thread-safety is provided by Moneta::Lock.

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 = store.expires(60)
short_lived_store['key'] = 'value'

Add proxies to existing store

You can add proxies to an existing store. This is useful if you want to compress only a few values for example.

compressed_store = store.with(prefix: 'compressed') do
  use :Transformer, value: :zlib

store['key'] = 'this value will not be compressed'
compressed_store['key'] = 'value will be compressed'

Framework Integration

Inspired by redis-store there exist integration classes for Rails and Rack/Rack-Cache. You can also use all the Rack middlewares together with Rails and the Sinatra framework. There exist the following integration classes:


Session store

You can use Moneta as a Rack session store. Use it in your like this:

require 'rack/session/moneta'

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

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

# Set rack options
use Rack::Session::Moneta, key: 'rack.session',
domain: '',
path: '/',
expire_after: 2592000,
store:, expires: true)

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

Moneta middleware

There is a simple middleware which places a Moneta store in the Rack environment at env['rack.moneta_store']. It supports per-request caching if you add the option cache: true. Use it in your like this:

# Add Rack::MonetaStore somewhere in your rack stack
use Rack::MonetaStore, :Memory, cache: true

run lambda { |env|
  env['rack.moneta_store'] # is a Moneta store with per-request caching

# Pass it a block like the one passed to
use Rack::MonetaStore do
  use :Transformer, value: :zlib
  adapter :Cookie

run lambda { |env|
  env['rack.moneta_store'] # is a Moneta store without caching

REST server

If you want to expose your Moneta key/value store via HTTP, you can use the Rack/Moneta REST service. Use it in your like this:

require 'rack/moneta_rest'

map '/moneta' do

# Or pass it a block like the one passed to
run do
  use :Transformer, value: :zlib
  adapter :Memory


You can use Moneta as a Rack-Cache store. Use it in your like this:

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'


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 { |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, {}, []]


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

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


Padrino adopted Moneta to replace their cache stores in padrino-cache. You use it like this

# Global Padrino caching
# Don't forget the expires: [true, Integer] if you want expiration support!
Padrino.cache =, expires: true)

# Application caching
# Don't forget the expires: [true, Integer] if you want expiration support!
set :cache,, expires: true)


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 >= 1.9.3.

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.

How to contribute?

Always feel free to open an issue on if something doesn't work as you expect it to work. Feedback is also very welcome!

My only request about patches is that you please try to test them before submitting.

Contribute an adapter

If you want support for another adapter you can at first at it to the list of missing adapters at

If you choose to implement an adapter please also add tests. Usually you only have to add a few lines to script/generate-specs to generate appropriate tests for your adapter. Please check also if travis.yml needs changes, for example if you need to start additional services.

Check if the default settings in Moneta#new are appropriate for your adapter. If not specify a better one.

Don't forget to edit the and the CHANGES.