Server
Caching in ColdFusion
By: Brett Cortese
Senior Consultant
Allaire Corporation
Configured in the ColdFusion administrator under the "Caching"
section, server caching controls how much and when data
is stored in memory. An effective server caching strategy
can relieve stress on resources such as databases, CPUs,
and file systems while dramatically improving application
performance. This article explores server caching in ColdFusion,
introducing its different pieces and providing examples
of how server caching works and its benefits.
Template Cache
All ColdFusion templates are compiled into PCode before execution.
This compilation process can be resource intensive and slow
down an application. To avoid compiling a template on every
request, ColdFusion caches its PCode into memory the first
time that it is called. If the cache becomes full, the cache
is forced to purge templates on a first in first out basis
to accommodate new requests. As a result, the next time the
purged template is called, it must be recompiled. This purging
is referred to as a cache pop and can be seen when monitoring
CFSTAT.
Figure 1: Here three cache pops are visible when monitoring
CFSTAT.
If ever CP/Sec is greater than 0, Allaire recommends that
the "Template Cache Size" setting be increased. By default
"Template Cache Size" is set to 1024 kilobytes, but a good
rule of thumb is to set the template cache size two to five
times the total template size. Note that this setting is
a maximum limit and is not allocated until necessary. Each
template is cached only once, even if it is included in
several other templates.
Trusted Cache
Though a template's PCode may be stored in the template cache,
ColdFusion checks the actual file to see if it has been modified
after it was cached. This check may increase I/O wait and
can be avoided by turning on trusted cache, also in the ColdFusion
administrator.
With trusted cache enabled, ColdFusion will only access
the template cache—even if the template itself is modified.
This can be problematic if developers expect to see changes
when files are modified. To introduce modified templates
into the cache with out restarting the ColdFusion server,
disable trusted cache and make a request to each modified
template. Trusted cache can then be turned back on.
Database Connection Caching
To avoid the highly expensive task of opening and closing
a connection to the database for every request, ColdFusion
caches database connections by default. This means that the
connection to the database is only opened once for many requests,
thereby dramatically improving performance.
If you are connecting to a clustered database configuration,
it may be necessary to disable connection caching to allow
failover to function properly. This can be accomplished
by unchecking "Maintain Database Connections" in the attributes
of the data source but will strongly degrade performance.
To avoid unused connections to the database remaining
open for long periods of time, the "Limit cached database
connection inactive time" setting can be adjusted. It is
also possible to manually release all data source connections
from the "Verify Data Source" section of the ColdFusion
administrator.
Query Caching
Query caching greatly increases performance as result sets
are retrieved from memory rather than from the database. Developers
should consider caching queries whenever possible.
For example, the following query will be cached for two
hours:
<CFQUERY Name="MyQuery" DataSource="dsn" CachedWithin="#CreateTimeSpan(0,2,0,0)#">
Select * from Inventory where InventoryId =2
</CFQUERY>
While caching queries is controlled by code, the limit of
allowable cached queries is set in the ColdFusion administrator.
With the introduction of CF 4.5x, it became possible to cache
more that 100 queries at a time. The amount of queries that
can be cached is now limited only by the amount of memory
available on the server. As the size of result sets, amount
of available memory, and the use of cached queries in applications
vary, this setting should be tested under expected load for
optimal performance.
For more information about query caching, see the article
"Query Caching in ColdFusion" by Allaire's Daryl Banttari.
Final Words
When designing a server caching strategy, it is important
to take into account available sever memory and the need for
fully dynamic information. If poor application performance
is an issue, these settings may make a world of difference.
During implementation, testing should take place to ensure
that the application and server reacts as expected. Be sure
to monitor server memory and verify that application data
is correct.
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