eZ components - Cache
~~~~~~~~~~~~~~~~~~~~~

.. contents:: Table of Contents

Introduction
============

The Cache package provides a collection of lightweight classes to cache
different kinds of data. Beside that, it provides a manager class, which takes
care of instantiating and reusing caches.

Class overview
==============

This section gives you an overview on all classes, that are intended to be
used directly.

ezcCacheManager
  This is the manager, which will take care of your caches. It is optional to
  use the manager, but recommended if your application needs to cache different
  data for different purposes. It allows you to configure all needed caches in
  a central place and to retrieve them through the ezcCacheManager. The cache
  manager will store only the configurations at first and only instantiate the
  cache object itself when you request it for use.

ezcCacheStorage
  This is the base class for all cache storages (the cache classes themselves).
  All cache classes inherit from this base class.

ezcCacheStorageFilePlain
  Cache objects of this class are capable to store plain text data on the file
  system. It utilizes the file_get_contents() and file_put_contents() functions
  of PHP.

ezcCacheStorageFileArray
  In contrast to ezcCacheStorageFilePlain, objects of this class can store
  array structures and will keep PHP data types intact.
  The ezcCacheStorageFileArray class generates PHP code, which will be stored
  on the file system. Restoring data from the cache uses the require()
  construct.

ezcCacheStorageFileEvalArray
  Objects of this storage class follow a similar approach like
  ezcCacheStorageFileArray and is also capable of storing array
  structures. The major difference between both classes is, that
  ezcCacheStorageFileEvalArray will use PHPs eval() method to restore the
  cached data, instead of using required() to restore the cache data. This has the effect,
  that the stored data will not be cached again in PHP accelerators like APC_.
  This might be desirable, if you store large amounts of data at once.

.. _APC: http://pecl.php.net/package/APC


Usage
=====

Terminology
-----------

_`Cache`
  A location that stores `cache item`_. A cache is created with the
  ezcCacheManager::createCache(), preferrably at the beginning of your script.
  A cache with it's `cache identifier`_ - the first parameter to
  ezcCacheManager::createCache() - can only be created once. After it has been
  created at the beginning of your request you can obtain a reference to it
  throughout the whole request by calling ezcCacheManager::getCache() with as
  first parameter the $id with which it was created.

_`Cache identifier`
  An identifier that uniquely identifies a `cache`_ which contains a number of
  `cache item`_ s.

_`Cache item`
  One single entry stored in the `cache`_. Cache items can be stored and
  retrieved by using the ezcCacheStorage object that is returned by 
  calling ezcCacheManager::getCache(). Cache items use a `cache key`_ to
  identify specific entries in the cache.

_`Cache key`
  A string representing a single `cache item`_ in a `cache`_.

_`Attribute`
  Additional information to complement the `cache key`_ for doing operations
  on `cache item`_ s.

A simple example
----------------

This example shows how to create and use a simple cache with the
ezcCacheManager:

.. include:: tutorial_example_01.php
   :literal:

In the options for the cache to create, the time-to-life is defined. If left
out, the cache has a lifetime of 24 hours. In this place, a lifetime of 30
seconds is defined. On line 9 the cache configuration is stored in the cache
manager. The `cache`_ created uses the `cache identifier` "simple" and will reside in the directory
/tmp/cache/plain (Note: This directory must exists and must be writable to
the user running the code, else you will get an ezcBaseFileNotFoundException or
an ezcBaseFilePermissionException!). To store a `cache item`_, the storage
class ezcConsoleStorageFilePlain will be used and as defined before, the stored
data will expire after 30 seconds.

Line 11 defines a `cache key`_ for the data to cache. The next line defines a
second unique `cache key`_ for a second cache item. After that (line 14), the
`cache`_ object is retrieved from the ezcCacheManager (the object itself
is created right now, since we access it for the first time). The lines 16 and
22 show how to check for cached data: The method ezcCacheStorage::restore()
will return bool false, if no valid cache data is found for a given ID.

In case the cache storage did not find valid data, the data will be generated
and stored in the cache afterwards (lines 17-18 and 24-25). The last line shows the
data, so you can follow how it's cached for 30 seconds, by simple running the
example multiple times in a short time frame. After 30 seconds the cache data
will become invalid and will be regenerated.

Using multiple caches
---------------------

The following example shows how the cache manager deals with multiple caches:

.. include:: tutorial_example_02.php
   :literal:

In the lines 12 and 13 you see how 2 caches are created. Each `cache`_ must
reside in its own location and must have a different `cache identifier`_. We
use 2 different options for the lifetime of the caches to show how they act
independently later.

Since the first `cache`_ reuses the location already used in example 1, 
we use a different `cache key`_ now.
Lines 15 to 25 are almost identical to the code from example 1, 
except that the program will sleep for 2 seconds, when it generated new data for 
the plain cache, to show different generation times in the 2 caches.

On line 30 the second `cache`_ object is retrieved, which is capable of storing
arrays.  Therefore, we store the data from the plain cache here and
additionally generate some more data, all stored in an array. Running this
example multiple times will give you different results now after some time,
since the second cache has a longer lifetime and will therefore hold its data
longer than the first one.

Complex caching
---------------

As the next example shows, ezcCacheStorage classes are capable of more advanced
features. This example uses attributes to identify cached data, additionally to
their `cache key`_:

.. include:: tutorial_example_03.php
   :literal:

After the creation of an array `cache_`, some sample data is created (lines
11-16). Each data is keyed by its `cache key`_, which is associated with an
array.  This array will be used as the content and the attributes together
later on. `Attribute`_ s describes a `cache item`_ in further detail. We will see,
what to do with `attribute`_ s of a cache item later on, too.

In line 20 a foreach loop starts, which stores all example data in the cache.
After that the method ezcCacheStorageFile::countDataItems() is used to let the
storage object count its cache items. The first parameter here would be a
`cache key`_.
When this is set, the method should always return 1 or 0, because only one cache
item per `cache key`_ may exist. Instead, the cache items with the specified attribute are
counted. The attributes to match are supplied as the second parameter. The
first method call will return 3 (line 28), since we have 3 cache items which
have the attribute "section" set to "articles". The second call (line 32)
should return 2, because 2 data items have the attribute "language" set to the
value "de".

On line 36 the storage object is told to delete all cache items, which have
the `attribute`_ "language" set to "de". Therefore the next to calls to
ezcCacheStorageFile::countDataItems() will return 2 and 0.

More Information
================

For more information, see the ezcCacheManager and ezcCacheStorageFile API
documentation.


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