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Caching with Memcached
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Caching, the act of storing the result of a computation for faster access later, can dramatically decrease a service s latency and increase throughput while keeping the underlying code maintainable and clean Broadly, services can cache two ways They can implement caching internally by storing results in the Ruby processes memory or in Memcached They can also leverage the caching support in the HTTP specification to cache data externally In a service-oriented design under high scalability demands, it s likely that the services would leverage both approaches
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8 Load Balancing and Caching
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Memcached is a scalable, open source system for caching objects in memory It s one of the simplest and fastest forms of a broader group of data storage implementations known as key/value stores At its core, the Memcached daemon process provides a welldefined interface for reading and writing to memory from other processes, with a bit of caching-oriented convenience such as time-based expiration and least-recently used (LRU) eviction layered in (covered in the following sections) It s completely temporal no data will survive a server failure but that s just fine in most caching use cases where the source data is stored in a more durable data store For these reasons, it s a good first choice for implementing internal caching in a Ruby-based service
The Memcached Client and ActiveRecord
A handful of Memcached clients are implemented for Ruby The memcache-client library maintained by Mike Perham is the easiest to get started with and also the most popular, in part because it s the default Memcached client used by ActiveSupport in Rails It can be installed with RubyGems, using the following command:
$ gem install memcache-client
Caching with Memcached
Once the memcache-client library is installed, you can create a connection to a local or remote Memcached process by instantiating the MemCache class By default, Memcached runs on port 11211:
require "memcache" $memcache = MemCachenew("localhost:11211")
With a connection established and stored in a global variable, the cache can be used from anywhere in the service The general pattern followed when implementing Memcached is to first check the cache for the desired value If the value is present, return it; otherwise, compute the result and store it in Memcached so it s available the next time it s needed Here s how this might look in an ActiveRecord class:
class Employee < ActiveRecord::Base def year_to_date_pay pay = $memcacheget("User:#{id}:ytd_pay") if paynil pay = calculate_year_to_date_pay # Expensive call $memcacheset("User:#{id}:ytd_pay") end pay end private def calculate_year_to_date_pay # end end
Memcached servers are aware of only strings, so a little bit of magic is necessary behind the scenes for them to store other types The memcache-client library uses Ruby s Marshal module to encode the integers returned from the calculate_year_to_date_pay
8 Load Balancing and Caching
method before sending them over the wire It decodes the strings it receives into their native Ruby types when they are returned from the server This allows Memcached to store most objects a Ruby process might use, from Booleans all the way up to arrays of ActiveRecord objects The caveat is that when values are read from Memcached, the classes must already have been loaded into the Ruby process for Marshalload to succeed If the objects stored are core Ruby types, this isn t a concern, but if they are application or library classes, they should be eagerly required when the process boots The previous example includes boilerplate code for the sake of illustration but can be improved with a simple abstraction The pattern of computing a result and storing it only if it s not available from the cache can be simplified by using Ruby blocks The memcache-client library implements a fetch method that does just that Here is how it could be applied to slim down the Employee class:
class Employee < ActiveRecord::Base def year_to_date_pay $memcachefetch("User:#{id}:ytd_pay") do calculate_year_to_date_pay # Expensive call end end # end
Adding a cache to an object is relatively easy, but cache invalidation can be a much trickier proposition Fortunately, Memcached provides some tools to help The simplest of these tools is time-based expiration