Adobe 38043740 Developing Applications - Page 327

DEVELOPING COLDFUSION 9 APPLICATIONS 322 Developing CFML Applications Controlling and minimizing lock time-outs Include a timeout attribute in your cflock tag. The timeout attribute specifies the maximum time, in seconds, to wait to obtain the lock if it is not available. By default, if the lock does not become available within the time-out period, ColdFusion generates a Lock type exception error, which you can handle using cftry and cfcatch tags. If you set the cflockthrowOnTimeout attribute to No, processing continues after the time-out at the line after the end tag. Code in the cflock tag body does not run if the time-out occurs before ColdFusion can acquire the lock. Therefore, never use the throwOnTimeout attribute for CFML that must run. Normally, it does not take more than a few seconds to obtain a lock. Very large time-outs can block request threads for long periods of time and radically decrease throughput. Always use the smallest time-out value that does not result in a significant number of time-outs. To prevent unnecessary time-outs, lock the minimum amount of code possible. Whenever possible, lock only code that sets or reads variables, not business logic or database queries. One useful technique is to do the following: 1 Perform a time-consuming activity outside a cflock tag 2 Assign the result to a Variables scope variable 3 Assign the Variables scope variable's value to a shared scope variable inside a cflock block. For example, if you want to assign the results of a query to a session variable, first get the query results using a Variables scope variable in unlocked code. Then, assign the query results to a session variable inside a locked code section. The following code shows this technique: SELECT FirstName, LastName FROM Users WHERE UserID = #request.UserID# Considering lock granularity When you design your locking strategy, consider whether you should have multiple locks containing small amounts of code or few locks with larger blocks of code. There is no simple rule for making such a decision, and you might do performance testing with different options to help make your decision. However, consider the following issues: • If the code block is larger, ColdFusion spends more time inside the block, which can increase the number of times an application waits for the lock to released. • Each lock requires processor time. The more locks you have, the more processor time is spent on locking code. Nesting locks and avoiding deadlocks Inconsistent nesting of cflock tags and inconsistent naming of locks can cause deadlocks (blocked code). If you are nesting locks, you must consistently nest cflock tags in the same order and use consistent lock scopes (or names). A deadlock is a state in which no request can execute the locked section of the page. All requests to the protected section of the page are blocked until there is a time-out. The following table shows one scenario that would cause a deadlock: Last updated 1/20/2012