In Part I I showed how easy it is to install memcached and use it for simple queries or manually stored objects.

Today I was experimenting with running against multiple memcached servers. I was preparing myself for a gruelling process of learning the memcache server configuration format, headaches while trying to get the two servers to talk to each other, etc. Then I read this:

...the API hashes your key to a unique server. If a host goes down, the API re-maps that dead host’s requests onto the servers that are available.

So the servers run without any knowledge of each other. All you have to do is start up a few instances of memcached, then tell your Rails app about them. The Ruby API is responsible for finding out about the servers and distributing the keys between them. It even recovers if one dies.

So easy!

Configuration

I tried this out using a laptop and a desktop machine. The only quirk was that I initially had a firewall on my desktop machine so the memcached port wasn’t open.

# In production.rb
require 'cached_model'

memcache_options = {
  :c_threshold => 10_000,
  :compression => true,
  :debug => false,
  :namespace => 'rails_production',
  :readonly => false,
  :urlencode => false
}

CACHE = MemCache.new memcache_options

# These are the IP addresses and ports of the memcached servers
CACHE.servers = ['192.168.0.3:11211', '192.168.0.2:11211']

Sessions

UPDATE: Argh…my current implementation of this isn’t any faster than ActiveRecord sessions. Back to the drawing board…

UPDATE 2: See the updated plugin for a Memcache session class that uses memcache-client instead of ruby-memcache.

Memcached can sometimes be faster than sessions stored in the database (see RailsExpress). The problem is that you lose all your sessions if you restart the memcached servers.

I have heard legends of an epic session library used at the Robot Coop. Sessions are stored primarily in the database but are also kept in memcached for speedy lookup. I found a nice SqlBypass session template in the Rails source and modified it to work similarly (packaged as a plugin). I’ll be benchmarking it later this week, but it works in the situations I’ve tried it in.

./script/plugin install http://topfunky.net/svn/plugins/db_memcache_store/

# In the Initializer section of environment.rb.
# Explicitly load the library since plugins haven't been loaded at this point.
require "#{RAILS_ROOT}/vendor/plugins/db_memcache_store/lib/db_memcache_store" 
config.action_controller.session_store = CGI::Session::DBMemcacheStore

Startup a few memcached servers.

#           Verbose, Port 11211, 128 MB RAM, IP to listen on
$ memcached -vv      -p 11211    -m 128      -l 192.168.0.2

You’ll see sessions being stored and retrieved. Kill one or more servers and it will keep working.

# Memcached server output
<3 server listening
<6 new client connection
<6 get rails_development:session:bd8b2ba714c059002af6a19283072997
>6 sending key rails_development:session:bd8b2ba714c059002af6a19283072997
>6 END
<6 set rails_development:session:bd8b2ba714c059002af6a19283072997 0 0 216
>6 STORED
<6 get rails_development:session:bd8b2ba714c059002af6a19283072997
>6 sending key rails_development:session:bd8b2ba714c059002af6a19283072997
>6 END

TODO

Actually, it’s back to the drawing board for this one…I found other details about the system used at the Robot Coop and other uses of memcached elsewhere.