2.3 Setting Cache Sizes

The OPeNDAP Catalog/Aggregation Server uses two independent caches. One is for serving local netCDF files, and the other is used in the process of aggregating remote files.

If your Aggregation Server serves a small number of files, it will work fine with the default settings. If you plan on serving large number of files, you must deal with this issue.

There are two independent caches, one for local netCDF files, and one for remote OPeNDAP files that you are aggregating. Generally, OPeNDAP files take up memory, but no system resources. The default is to set this size to zero, which means there is no limit to the cache. A netCDF file uses a system file handle, so you are limited by maximum number of open files possible on your system. The default is 100.

Set the cache size by adjusting parameters in Tomcat's web.xml configuration file (see the web.xml example). The file contains a couple of parameter initializations like this:

<init-param>
  <param-name>maxDODSDatasetsCached</param-name>
  <param-value>0</param-value>
</init-param>
<init-param>
  <param-name>maxNetcdfFilesCached</param-name>
  <param-value>100</param-value>
</init-param>

Here are some tips for selecting an appropriate size for your cache:

• The larger the cache the better for performance. Resources are only used if needed.
• Cache size must be at least as large as the largest number of datasets in a JoinExisting aggregation.
• Setting the cache size at least as large as the largest number of datasets in a JoinNew or Union aggregation helps performance, but is not necessary.