Mark Kruger has a nice article (with mention from Sean Corfield) on APPLICATION variable locking.
Sean basically added that in the initial example (BAD code) if 2 users hit the <cfif> condition at the same time, only one will have a lock on the code while the other will wait.
The problem lies in the fact that the 2nd user is waiting for the lock to expire, then they will perform a write operation NOT checking the <cfif> condition again!
The article is here.
BAD code snippet:
<cfif NOT isDefined(’Application.curIps’)OR IsDefined(’url.forceAp’)>
<cflock scope=”APPLICATION” timeout=”1″>
<cfscript>
curIps = queryNew(”IP”);
queryAddRow(curIps);
querySetCell(curIps,”IP”,cgi.remote_addr);
Application.curIps = curIps;
</cfscript>
</cflock>
GOOD code snippet:
<!— check condition —>
<cfif NOT isDefined(’Application.curIps’) OR IsDefined(’url.forceAp’)>
>!— Use a named lock —>
<cflock name=”ipInitialize” timeout=”1″ type=”exclusive”>
<!— check to make sure condition is still true —>
<cfif NOT IsDefined(’Application.curIps’) OR IsDefined(’url.forceAp’)>
<cfscript>
curIps = queryNew(”IP”);
queryAddRow(curIps);
querySetCell(curIps,”IP”,cgi.remote_addr);
Application.curIps = curIps;
</cfscript>
</cfif>
</cflock>
</cfif>
Another useful mention is that this seems redundant when using CFMX7 due to applicationOnStart in application.cfc unless you want to restart the application manually
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