Here is a case study in the use of locks around session and application
variables. Read on, at the end I have a few questions to the great minds and
gurus of this list (especially Raymond Camden).
The project I am working on right now is the rewrite of an application which
never should have gone into production in the first place. The application
was written three years ago and errors abound (common messages shown below):
-----
Error Diagnostic Information
unknown error condition
unknown error while executing a tag
-----
Or:
-----
Request cancelled or ignored by serverServer busy or unable to fulfill
request. The server is unable to fulfill your request due to extremely high
traffic or an unexpected internal error.
-----
We see at least 50 of these in the application log daily, much more around
election time when traffic skyrockets. Every once in a while I also see a
PCode error, but I don't have the syntax in front of me.
The app runs on NT 4 with CF4.5 and makes extensive use of application and
session variables. By extensive, I mean it has large structures contained in
the application scope and calls data from them 10 - 80 times per page. I
looked at the CPU usage for Cold Fusion to determine the total amount of
data in the application scope, and saw it go up from about 30000k to 89000k
just when the application loads. So there is a lot of data, but that should
not be a big issue - the app is running on a pretty powerful server.
The challenge, it would seem, is to have the code access the data properly.
The code possesses no locks anywhere (!). What I am doing is exclusively
locking all of the application and session variables when they are set, and
putting readonly locks around these variables whenever they are read. My
code looks like this:
<cflock scope="session" type="readOnly" timeout="10">
<cfif NOT isdefined("application.regional_development_orgs")>
<cfset needs_init = 1>
</cfif>
</cflock>
<cfif needs_init eq 1>
<cflock name="data_detailed_smobe_data" throwontimeout="Yes" timeout="10"
type="EXCLUSIVE">
<cfset temp = structNew()>
<cfloop from="1" to="#listlen(application.state_names_list)#"
index="dex">
<cfset temp2 = structInsert(temp,
listgetat(application.state_names_list, dex), "")>
</cfloop>
... do more stuff ...
<cfset application.regional_development_orgs = structCopy(temp)>
<cfset needs_init = 0>
</cflock>
</cfif>
I am taking the entire session scope and setting it to a request variable at
the top of every page, and calling these request variables when needed
(doing it this way...):
<cflock scope="session" type="readonly" timeout="20">
<cfset request.session = structCopy(session)>
</cflock>
<cfoutput>#request.session.state_name#</cfoutput>
.. Calling variables from the request scope instead of session
I am doing something similar with application variables by putting specific
structures in the request scope on pages where they are used, a la:
<cflock scope="SESSION" type="READONLY" timeout="20">
<cfset request.senate_demographic_data =
structCopy(application.senate_demographic_data[request.session.state])>
</cflock>
<cfoutput>#request.senate_demographic_data[request.session.state_name]["min_
bus"]#</cfoutput>
.. Calling variables from the request scope instead of application
My question is, does it sound like I am addressing the core problem or am I
just enhancing the code? Could the real problem be with the amount of data
in the application scope, or is there something else I am not thinking of?
Does anyone have any advice on how to pinpoint the exact cause of the
problems I am running into (or have any experience with applications using
this much data in shared memory)? Am I writing these locks as efficiently as
possible? Should I be thinking about working with my sysadmin to fine-tune
the server, and, if so, what kinds of things should I be looking for?
I am about halfway done with locking down the code, and haven't run into the
same problems with the rewritten code. The server has frozen several times
and required me to restart Cold Fusion server, but this could be attributed
to the old code.
While I am confident I am going to end up with a superior product in the
end, I don't want to end up with egg on my face when the new version goes
live because I got fixated on locks ...
Any ideas would be appreciated.
Thanks,
M
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