= memcache

This is the Geni memcached client. It started out as a fork of fiveruns/memcache-client,
which was a fork of seattle.rb's memcache-client, but over time, our client has diverged,
and I've rewritten most of the code. Of course, a lot of credit is due to those whose code
served as a starting point for this code.

== Usage

 cache = Memcache.new(:server => "localhost:11211")
 cache.set('stuff', [:symbol, 'String', 1, {:bar => 5}])
 cache.get('stuff')
 => [:symbol, "String", 1, {:bar => 5}]

 cache['things'] = {:foo => '1', :bar => [1,2,3]}
 cache['things']
 => {:foo => "1", :bar => [1,2,3]}

== How is this different from memcache-client?

Like memcache-client, _memcache_ (shown in italics when I am referring to this
library) is a memcached client, but it differs significantly from memcache-client in
several important ways.

=== Interface

I tried to keep the basic interface as similar as I could to memcache-client. In some
cases, _memcache_ can be a near drop-in replacement for memcache-client. However, I did
rename the main class from +MemCache+ to +Memcache+ to prevent confusion and to force
those switching to _memcache_ to update their code. Here are the notable interface
changes:

- +expiry+ and +raw+ are specified as options in a hash now, instead of as unnamed parameters.

   cache.set('foo', :a,  :expiry => 10.minutes)
   cache.set('bar', :b,  :expiry => Time.parse('5:51pm Nov 24, 2018'))
   cache.set('baz', 'c', :expiry => 30.minutes, :raw => true)

- +get_multi+ has been replaced by a more versatile +get+ interface. If the first argument is
  an array, then a hash of key/value pairs is returned. If the first argument is not an
  array, then the value alone is returned.

   cache.get('foo')          # => :a
   cache.get(['foo', 'bar']) # => {"foo"=>:a, "bar"=>:b}
   cache.get(['foo'])        # => {"foo"=>:a}

- +get+ also supports updating the expiry for a single key. this can be used to keep
  frequently accessed data in cache longer than less accessed data, though usually the
  memcached LRU algorithm will be sufficient.

   cache.get('foo', :expiry => 1.day)

- Support for flags has been added to all methods. So you can store additional metadata on
  each value. Depending on which server version you are using, flags can be 16 bit or 32
  bit unsigned integers (though it seems that memcache 1.4.1 returns signed values if the
  upper bit is set).

   cache.set('foo', :aquatic, :flags => 0b11101111)
   value = cache.get('foo')
   => :aquatic
   value.memcache_flags.to_s(2)
   => "11101111"

   cache.set('foo', 'aquatic', :raw => true, :flags => 0xff08)
   cache.get('foo', :raw => true).memcache_flags.to_s(2)
   => "1111111100001000"

- +incr+ and +decr+ automatically initialize the value to 0 if the key doesn't
  exist. The +count+ method returns the integer count associated with a given key.

   cache.count('hits')    # => 0
   cache.incr('hits', 52) # => 52
   cache.decr('hits', 9)  # => 43
   cache.count('hits')    # => 43

- In addition to +add+, which was already supported, support has been added for +replace+,
  +append+ and +prepend+ from the memcached protocol.

   cache.add('foo', 1)
   cache.add('foo', 0)
   cache.get('foo')
   => 1
  
   cache.replace('foo', 2) 
   cache.get('foo')
   => 2

   cache.write('foo', 'bar')     ## shortcut for cache.set('foo', 'bar', :raw => true)
   cache.append('foo', 'none')   ## append and prepend only works on raw values
   cache.prepend('foo', 'foo')   ##
   cache.read('foo')             ## shortcut for cache.get('foo', :raw => true)
   => "foobarnone"

- Support has also been added for +cas+ (compare-and-set).

   value = cache.get('foo', :cas => true)
   cache.cas('foo', value.upcase, :cas => value.memcache_cas)
   cache.get('foo')
   => "FOOBARNONE"

   value = cache.get('foo', :cas => true)
   cache.set('foo', 'modified')
   cache.cas('foo', value.downcase, :cas => value.memcache_cas)
   cache.get('foo')
   => "modified"

- Several additional convenience methods have been added including +get_or_add+,
  +get_or_set+, +update+, +get_some+, +lock+, +unlock+, and +with_lock+.

=== Implementation

The underlying architechture of _memcache_ is more modular than memcache-client.
A given +Memcache+ instance has a group of servers, just like before, but much more of the
functionality in encapsulated inside the <tt>Memcache::Server</tt> object. Really, a +Server+
object is a thin wrapper around an remote memcached server that takes care of the socket
and protocol details along with basic error handling. The +Memcache+ class handles the
partitioning algorithm, marshaling of ruby objects and various higher-level methods.

By encapsulating the protocol inside the +Server+ object, it becomes very easy to plug-in
alternate server implementations. Right now, there are two basic, alternate servers:

[+LocalServer+] This is an in-process server for storing keys and values in local
                memory. It is good for testing, when you don't want to spin up an instance
                of memcached, and also as a second level of caching. For example, in a web
                application, you can use this as a quick cache which lasts for the
                duration of a request.

[+PGServer+] This is an implementation of memcached functionality using SQL. It stores all
             data in a single postgres table and uses +PGConn+ to select and update this
             table. This works well as a permanent cache or in the case when your objects
             are very large. It can also be used in a multi-level cache setup with
             <tt>Memcache::Server</tt> to provide persistence without sacrificing speed.

=== Very Large Values

Memcached limits the size of values to 1MB. This is done to reduce memory usage, but it
means that large data structures, which are also often costly to compute, cannot be stored
easily. We solve this problem by providing an additional server called
<tt>Memcache::SegmentedServer</tt>. It inherits from <tt>Memcache::Server</tt>, but
includes code to segment and reassemble large values. Mike Stangel at Geni originally
wrote this code as an extension to memcache-client and I adapted it for the new
architecture.

You can use segmented values either by passing +SegmentedServer+ objects to +Memcache+, or
you can use the +segment_large_values+ option.

 server = Memcache::SegmentedServer.new(:host => 'localhost', :port => 11211)
 cache = Memcache.new(:server => server)

 cache = Memcache.new(:server => 'localhost:11211', :segment_large_values => true)

=== Error Handling and Recovery

We handle errors differently in _memcache_ than memcache-client does. Whenever there is a
connection error or other fatal error, memcache-client marks the offending server as dead
for 30 seconds, and all calls that require that server fail for the next 30 seconds. This
was unacceptable for us in a production environment. We tried changing the retry timeout
to 1 second, but still found our exception logs filling up with failed web requests
whenever a network connection was broken.

So, the default behavior in _memcache_ is for reads to be stable even if the underlying
server is unavailable. This means, that instead of raising an exception, a read will just
return nil if the server is down. Of course, you need to monitor your memcached servers to
make sure they aren't down for long, but this allows your site to be resilient to minor
network blips. Any error that occurs while unmarshalling a stored object will also return nil.

Writes, on the other hand, cannot just be ignored when the server is down. For this reason,
every write operation is retried once by closing and reopening the connection before
finally marking a server as dead and raising an exception. We will not attempt to read
from a dead server for 5 seconds, but a write will always attempt to revive a dead server
by attempting to connect.

== Installation

  $ sudo gem install memcache --source http://gemcutter.org

== License:

Copyright (c) 2009 Justin Balthrop, Geni.com; Published under The MIT License, see LICENSE