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

Dalli's initial development was sponsored by CouchBase. Many thanks to them!

Design

I decided to write Dalli after maintaining memcache-client for 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. supports SASL for use in managed environments, e.g. Heroku.
4. provides proper failover with recovery and adjustable timeouts

XCompatible Version

The regular version of Dalli sets flags to identify whether a piece of data is a) marshalled and b) compressed. This causes a problem when this data is accessed by applications written in other languages (for e.g. PHP Memcached does not set a flag for serialized data. This library will croak if the serialized flag is set). This can be easily verified using telnet - set 0 0 33 \r\n hello world\r\n is what is read correctly by PHP, however set 1 0 33 \r\n hello world \r\n is not read correctly by PHP.

To fix this, I have removed the setting of flags by Dalli and I instead fall to the user set configuration flags to determine compression and marshalling.

For example, my Rails configuration looks like

config.cache_store = :dalli_store, '127.0.0.1',{ :expires_in => 1.day, :compress => false, :serializer => JSON }

Supported Ruby versions and implementations

Dalli should work identically on:

• JRuby 1.6+
• Ruby 1.9.2+
• Ruby 1.8.7+
• Rubinius 2.0

If you have problems, please enter an issue.

Installation and Usage

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

On Ubuntu you can install it by running:

apt-get 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 20-30% performance boost.

Usage with Rails 3.x

In your Gemfile:

gem 'dalli'

In config/environments/production.rb:

config.cache_store = :dalli_store

Here's a more comprehensive example that sets a reasonable default for maximum cache entry lifetime (one day), enables compression for large values and namespaces 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, :compress => true }

To use Dalli for Rails session storage that times out after 20 minutes, in config/initializers/session_store.rb:

For Rails >= 3.2.4:

Rails.application.config.session_store ActionDispatch::Session::CacheStore, :expire_after => 20.minutes

For Rails 3.x:

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 => 20.minutes

Dalli does not support Rails 2.x.

Configuration

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

expires_in: Global default for key TTL. Default is 0, which means no expiry.

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

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

compression_min_size: Minimum value byte size for which to attempt compression. Default is 1K.

compression_max_size: Maximum value byte size for which to attempt compression. Default is unlimited.

serializer: The serializer to use for objects being stored (ex. JSON). Default is Marshal.

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.

keepalive: Boolean, if true Dalli will enable keep-alives on the socket so inactivity

compressor: The compressor to use for objects being stored. Default is zlib, implemented under Dalli::Compressor. If serving compressed data using nginx's HttpMemcachedModule, set memcached_gzip_flag 2 and use Dalli::GzipCompressor

Features and Changes

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

Dalli does not need anything special in Unicorn/Passenger since 2.0.4. It will detect sockets shared with child processes and gracefully reopen the socket.

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

Helping Out

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.

Thanks

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.

CouchBase - for their project sponsorship

Author

Mike Perham, mperham@gmail.com, mikeperham.com, @mperham If you like and use this project, please give me a recommendation at WWR or send a few bucks my way via my Pledgie page below. Happy caching!

Copyright

Copyright (c) 2012 Mike Perham. See LICENSE for details.