Dalli

Dalli is a high performance pure Ruby client for accessing memcached servers. It works with memcached 1.4+ only as it uses the newer binary protocol. It should be considered a replacement for the memcache-client gem. The API tries to be mostly compatible with memcache-client with the goal being to make it a drop-in replacement for Rails.

The name is a variant of Salvador Dali for his famous painting The Persistence of Memory.

Dalli's development is sponsored by Membase. Many thanks to them!

I decided to write Dalli after maintaining memcache-client for the last two years for a few specific reasons:

0. The code is mostly old and gross. The bulk of the code is a single 1000 line .rb file.
1. It has a lot of options that are infrequently used which complicate the codebase.
2. The implementation has no single point to attach monitoring hooks.
3. Uses the old text protocol, which hurts raw performance.

So a few notes. Dalli:

0. uses the exact same algorithm to choose a server so existing memcached clusters with TBs of data will work identically to memcache-client.
1. is approximately 20% faster than memcache-client (which itself was heavily optimized) in Ruby 1.9.2.
2. contains explicit "chokepoint" methods which handle all requests; these can be hooked into by monitoring tools (NewRelic, Rack::Bug, etc) to track memcached usage.
3. comes with hooks to replace memcache-client in Rails.
4. supports SASL for use in managed environments, e.g. Heroku.
5. provides proper failover with recovery and adjustable timeouts
6. has a backwards-compatibility mode for people migrating from memcache-client (see Upgrade.md).

Dalli is geared towards 1.9.2+ but known to work on

• Ruby 1.9.2
• Ruby 1.9.3
• Ruby 1.8.7
• Rubinius 2.0 Preview

Remember, Dalli requires memcached 1.4+. You can check the version with memcached -h. Please note that memcached that Mac OS X Snow Leopard ships with is 1.2.8 and won't work. Install 1.4.x using Homebrew with

brew install memcached

You can verify your installation using this piece of code:

gem install dalli

require 'dalli'
dc = Dalli::Client.new('localhost:11211')
dc.set('abc', 123)
value = dc.get('abc')

The test suite requires memcached 1.4.3+ with SASL enabled (brew install memcached --enable-sasl ; mv /usr/bin/memcached /usr/bin/memcached.old). Currently only supports the PLAIN mechanism.

Dalli has no runtime dependencies and never will. You can optionally install the 'kgio' gem to give Dalli a 10-20% performance boost.

In your Gemfile:

gem 'dalli'

In config/environments/production.rb:

config.cache_store = :dalli_store

A more comprehensive example (note that we are setting a reasonable default for maximum cache entry lifetime (one day), enabling compression for large values, and namespacing all entries for this rails app. Remove the namespace if you have multiple apps which share cached values):

config.cache_store = :dalli_store, 'cache-1.example.com', 'cache-2.example.com',
    { :namespace => NAME_OF_RAILS_APP, :expires_in => 1.day, :compression => true }

To use Dalli for Rails session storage, in config/initializers/session_store.rb:

require 'action_dispatch/middleware/session/dalli_store'
Rails.application.config.session_store :dalli_store, :memcache_server => ['host1', 'host2'], :namespace => 'sessions', :key => '_foundation_session', :expire_after => 30.minutes

Dalli v1.1+ does not support Rails 2.3. Please use an earlier version: gem install dalli -v "~> 1.0.4"

Put this at the bottom of config/environment.rb:

if defined?(PhusionPassenger)
  PhusionPassenger.on_event(:starting_worker_process) do |forked|
    # Reset Rails's object cache
    # Only works with DalliStore
    Rails.cache.reset if forked

    # Reset Rails's session store
    # If you know a cleaner way to find the session store instance, please let me know
    ObjectSpace.each_object(ActionDispatch::Session::DalliStore) { |obj| obj.reset }
  end
end

Dalli::Client accepts the following options. All times are in seconds.

expires_in: Global default for key TTL. No default.

failover: Boolean, if true Dalli will failover to another server if the main server for a key is down.

nonascii: Boolean, if true Dalli will allow the use of non-ascii keys.

compression: Boolean, if true Dalli will gzip-compress values larger than 1K.

socket_timeout: Timeout for all socket operations (connect, read, write). Default is 0.5.

socket_max_failures: When a socket operation fails after socket_timeout, the same operation is retried. This is to not immediately mark a server down when there's a very slight network problem. Default is 2.

socket_failure_delay: Before retrying a socket operation, the process sleeps for this amount of time. Default is 0.01. Set to nil for no delay.

down_retry_delay: When a server has been marked down due to many failures, the server will be checked again for being alive only after this amount of time. Don't set this value to low, otherwise each request which tries the failed server might hang for the maximum socket_timeout. Default is 1 second.

value_max_bytes: The maximum size of a value in memcached. Defaults to 1MB, this can be increased with memcached's -I parameter. You must also configure Dalli to allow the larger size here.

username: The username to use for authenticating this client instance against a SASL-enabled memcached server. Heroku users should not need to use this normally.

password: The password to use for authenticating this client instance against a SASL-enabled memcached server. Heroku users should not need to use this normally.

async: Boolean, if true Dalli will assume its running inside the EventMachine reactor and use EM through the em-synchrony gem. Currently disables the socket_timeout option. Default is false.

Dalli is NOT 100% API compatible with memcache-client. If you have code which uses the MemCache API directly, it will likely need small tweaks. Method parameters and return values changed slightly. See Upgrade.md for more detail.

By default, Dalli is thread-safe. Disable thread-safety at your own peril.

Multi-threaded use will fail if used with EventMachine.

Note that Dalli does not require ActiveSupport or Rails. You can safely use it in your own Ruby projects.

If you have a fix you wish to provide, please fork the code, fix in your local project and then send a pull request on github. Please ensure that you include a test which verifies your fix and update History.md with a one sentence description of your fix so you get credit as a contributor.

Eric Wong - for help using his kgio library.

Brian Mitchell - for his remix-stash project which was helpful when implementing and testing the binary protocol support.

Membase - for their project sponsorship

Bootspring is my Ruby and Rails consulting company. We specialize in Ruby infrastructure, performance and scalability tuning for Rails applications. If you need help, please contact us today.

Mike Perham, mperham@gmail.com, mikeperham.com, @mperham If you like and use this project, please give me a recommendation at WWR. Happy caching!

Copyright (c) 2010 Mike Perham. See LICENSE for details.