MACROMEDIA COLDFUSION 4.5-CFML LANGUAGE Bedienungsanleitung PDF herunterladen (Seite 43)

43Using CFX Tags Safely: Locking and Thread Safety

Either line of code, used immediately before the CFX tag, will guard against any potential problems

if the CFX tag tries to add nested structures to an existing variable that is not itself a structure.

Whew, that was close!

Using CFX Tags Safely: Locking and Thread Safety

Because the ColdFusion server responds to page requests in a multithreaded fashion (meaning that

it can process more than one page request at the same time), it is possible that two instances of a CFX

tag may execute on the same server at the same time. If the CFX tag is not thread-safe in terms of

how it deals with memory or how it works logically, problems might arise unless you take steps to

avoid them. Like other concurrency issues, such problems are likely to show up only under load,

making it appear that the tag (or ColdFusion itself) does not scale well, when in fact the problem

could be avoided by making sure the tag does not execute in more than one page request at once.

Understanding Thread Safety

A full discussion of what it means for a compiled program to be thread-safe is well beyond the scope

of what can be explained in this chapter. For purposes of this discussion, it will have to sufﬁce that

thread-safe basically means that the tag has been coded in such a way that it can run in several threads

at the same time without any possibility of the various threads being able to create, change, edit—

or, in some instances, merely access-a shared resource (where shared resource means any shared vari-

able, memory, ﬁle, or the like).

Generally speaking, a CFX tag is probably thread-safe if it does not use any global variables (that is,

if none of the variables are declared outside of the

processRequest() function in Java or the

ProcessTagRequest() function in C++) and also does not access any external or third-party APIs

that themselves are not thread-safe.

NOTE

If you’re not familiar with what a thread is, don’t worry about it too much right now. In the context of ColdFusion, a thread is basically

the same thing conceptually as a page request from a user (because each simultaneous page request is processed by a different

worker thread within the server).

Locking CFX Tags with <cflock>

You can still use CFX tags even if you know they aren’t thread-safe. Just wrap the <cflock> tag around

every use of the tag, using

type=”Exclusive” and a name attribute equal to the name of the CFX tag

(or some other name, as long as it is always the same for every single use of the tag). For instance, if

you knew (or suspected) that the

<CFX_ComputeStatistics> tag was not thread-safe, you would use

code similar to the following:

<cflock type=”Exclusive”

name=”CFX_ComputeStatistics”

timeout=”10”>

<CFX_ComputeStatistics query=”GetFilms”

column=”RatingID”

variable=”FilmsStats”>

</cflock>

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