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+\chapter{Cache Models}\label{section:3.8}

+Some values in a database are know to change slower than others. To improve

+performance, many developers like to cache often-used data to avoid making

+unnecessary trips back to the database. SQLMap provides its own caching

+system, that you configure through a \tt{<cacheModel>} element.

+

+The results from a query Mapped Statement can be cached simply by specifying

+the \tt{cacheModel} parameter in the statement tag (seen above). A cache model

+is a configured cache that is defined within your DataMapper configuration

+file. Cache models are configured using the \tt{cacheModel} element as

+follows:

+

+\begin{example}\label{example:3.45}

+Configuring a cache using the Cache Model element

+\begin{verbatim}

+<cacheModel id="product-cache" implementation="LRU" >

+  <flushInterval hours="24"/>

+  <flushOnExecute  statement="insertProduct"/>

+  <flushOnExecute  statement="updateProduct"/>

+  <flushOnExecute  statement="deleteProduct"/>

+  <property name="CacheSize" value="100"/>

+</cacheModel>

+\end{verbatim}

+\end{example}

+

+The cache model above will create an instance of a cache named

+``product-cache'' that uses a Least Recently Used (LRU) implementation. The

+value of the \tt{type} attribute is either a class name, or an alias for one

+of the included implementations (see below). Based on the flush elements

+specified within the cache model, this cache will be flushed every 24 hours.

+There can be only one flush interval element and it can be set using hours,

+minutes, seconds or milliseconds. In addition the cache will be flushed

+whenever the \tt{insertProduct}, \tt{updateProduct}, or \tt{deleteProduct}

+mapped statements are executed. There can be any number of ``flush on

+execute'' elements specified for a cache. Some cache implementations may need

+additional properties, such as the ``cache-size'' property demonstrated above.

+In the case of the LRU cache, the size determines the number of entries to

+store in the cache. Once a cache model is configured, you can specify the

+cache model to be used by a mapped statement, for example:

+

+\begin{example}\label{example:3.46}

+Specifying a Cache Model from a Mapped Statement

+\begin{verbatim}

+<statement id="getProductList" cacheModel="product-cache">

+  select * from PRODUCT where PRD_CAT_ID = #value#

+</statement>

+\end{verbatim}

+\end{example}

+

+\section{Cache Implementation}

+The cache model uses a pluggable framework for supporting different types of

+caches. The choice of cache is specified in the ``implementation'' attribute

+of the \tt{cacheModel} element as discussed above. The class name specified

+must be an implementation of the \tt{ISqlMapCache} interface, or one of the

+two aliases discussed below. Further configuration parameters can be passed to

+the implementation via the property elements contained within the body of the

+\tt{cacheModel}. Currently there are 2 implementations included with the PHP

+distribution.

+

+\subsection{Least Recently Used [LRU] Cache} The LRU cache implementation uses

+an Least Recently Used algorithm to determines how objects are automatically

+removed from the cache. When the cache becomes over full, the object that was

+accessed least recently will be removed from the cache. This way, if there is

+a particular object that is often referred to, it will stay in the cache with

+the least chance of being removed. The LRU cache makes a good choice for

+applications that have patterns of usage where certain objects may be popular

+to one or more users over a longer period of time (e.g. navigating back and

+forth between paginated lists, popular search keys etc.).

+

+The LRU implementation is configured as follows:

+\begin{example}\label{example:3.48}

+Configuring a LRU type cache

+\begin{verbatim}

+<cacheModel id="product-cache"  implementation="LRU" >

+  <flushInterval hours="24"/>

+  <flushOnExecute  statement="insertProduct"/>

+  <flushOnExecute  statement="updateProduct"/>

+  <flushOnExecute  statement="deleteProduct"/>

+   <property name="CacheSize" value="100"/>

+</cacheModel>

+\end{verbatim}

+\end{example}

+

+Only a single property is recognized by the LRU cache implementation. This

+property, named \tt{CacheSize} must be set to an integer value representing

+the maximum number of objects to hold in the cache at once. An important thing

+to remember here is that an object can be anything from a single string

+instance to an array of object. So take care not to store too much in your

+cache and risk running out of memory and disk space.

+

+\subsection{FIFO Cache}

+The FIFO cache implementation uses an First In First Out algorithm to

+determines how objects are automatically removed from the cache. When the

+cache becomes over full, the oldest object will be removed from the cache. The

+FIFO cache is good for usage patterns where a particular query will be

+referenced a few times in quick succession, but then possibly not for some

+time later.

+

+The FIFO implementation is configured as follows:

+

+\begin{example}\label{example:3.49}

+Configuring a FIFO type cache

+\begin{verbatim}

+<cacheModel id="product-cache" implementation="FIFO" >

+  <flushInterval hours="24"/>

+  <flushOnExecute  statement="insertProduct"/>

+  <flushOnExecute  statement="updateProduct"/>

+  <flushOnExecute  statement="deleteProduct"/>

+  <property name="CacheSize" value="100"/>

+</cacheModel>

+\end{verbatim}

+\end{example}

+

+Only a single property is recognized by the FIFO cache implementation. This

+property, named \tt{CacheSize} must be set to an integer value representing

+the maximum number of objects to hold in the cache at once. An important thing

+to remember here is that an object can be anything from a single String

+instance to an array of object. So take care not to store too much in your

+cache and risk running out of memory and disk space.