Wrestling with expired chart images in Adobe ColdFusion

TL;DR

Change the timeouts specified in webcharts3d.xml

Long Version

We are in the midst of migrating several existing Web applications to Adobe ColdFusion 10 (ACF10) as part of a server consolidation effort at work. One of these apps has a page that takes a Web-eternity to generate (over 30 seconds, in some situations based on load and other what the user has opted to include) based on the sheer volume of data that has to be digested, and includes several charts created by invoking cfchart. For years, we've struggled with situations where the chart images in this particular report don't show up and instead are replaced with placeholder images indicating the image has "expired". In spite of playing with the limited configuration options related to chart caching in the ACF administrator UI, we've never really found a way to ensure the chart images don't expire. (And, yes, we've invested a significant amount of time to improving the efficiency of the logic behind the report and improving performance of the underlying queries used to retrieve the data.) I think we finally made a bit of a breakthrough on this front yesterday.

I was wrestling with this problem on my development server, which although reasonably fast, is significantly slower than our staging server where we are testing this particular app. Because my dev server is slower, the problem with these expired chart images was exacerbated and I was consistently getting this particular report back with all of the images expired: good for testing and troubleshooting, but useless in terms of the report itself. The ACF admin UI provides only two configuration options that seemed relevant to solving the problem: the size of the cache (how many chart images will be cached) and whether the chart images are cached to disk or to memory. The cache size on my dev server was configured at 200 charts, but given that I am the only one who can connect to my dev server, it made no sense that the half-dozen charts on this report could be expiring based on the cache being filled quickly enough to cause them to be expired. They had to be expiring based on something else... age, perhaps?

I verified this by switching the cache to be on disk and watching the content of the folder being used for the cache. In spite of the specified cache size in the ACF admin UI, there were never more than a handful of image files present in the folder. Something other than the number of chart images was at play here.

A quick search of the ACF bug list yielded nothing relevant in terms of a known problem. Searching the ACF forums brought up a couple other topics related to expired images but nothing that on the surface seemed really relevant to what I was seeing. The entry that seemed most relevant was in a clustered environment and seemed to be related to how requests got moved among the various servers in the cluster and how that seemed to be leading to expired chart images. But... in that post there was a reference to an XML configuration file for the underlying charting engine... and a timeout parameter in that file.

The ACF forum post referred to minTimeout and maxTimeout parameters in a configuration file named webcharts3d.xml. At least on my system, that file resides at {cf_home}/cfusion/lib/webcharts3d.xml. Locate the <server ...> element near the top of the file, and adjust the minTimeout and maxTimeout attributes. On my system, they were set to 5000 and 30000, respectively, which I believe to be 5 and 30 seconds. When I saw those values, I was convinced I had found my solution, as it seemed like when the report in question took more than 30 seconds to generate, I was consistently getting the expired images. Recognizing that this long-running report can take as long as a couple minutes to generate under load, I changed the minTimeout attribute to 120000 (120 seconds) and the maxTimeout attribute to 300000 (5 minutes). I saved the file and bounced ACF on my dev server, and... no more expired images. Watching the cache folder, I started seeing images pile up (as I would have expected). Problem solved.

Having said that, I still have a couple questions:

1. What exactly do those configuration parameters in the ACF admin UI for cache size and number of threads actually control (if anything)?
2. Is there documentation somewhere for the webcharts3d.xml file, explaining what the cache, minTimeout, maxTimeout, cacheSize, and maxThreads attributes do? Several of those have similarly-named settings in the CF admin UI but updating the CF admin UI values does not touch this file. So far, I've not been able to find any documentation on-line.

As is often the case, now that I have some idea of how to solve the problem, I can find at least half a dozen posts or articles via Google describing the solution. With this post, there is now one more.

And for the record, this just might be the first time I've found something of value in the ACF forums, but that's a topic for another post and another day.