Using ECCU, it is recommended that you limit the number of matches to 250 or fewer—see ECCU Matches and the purge.data File for a definition of ECCU matches. Submitting more than 250 invalidation requests at one time can result in a “global invalidation” –revalidating all content for that host header. The way to avoid this is to invalidate multiple objects or paths in one request, as described in Best Practices in Requests: Use Space-Separated Values.
You should avoid matching on content that is unique to a single or few users, and submit no more than 100 requests per day. This recommendation is not an absolute maximum, and more matches or requests on a particular day may be processed without incident, but consistently exceeding the recommended limits may result in degraded performance as measured by timely refreshing of the objects.
The purge.data file consumes server memory to read, parse, and hold the resulting parse tree in memory. Thus, a large number of ECCU matches—a large purge.data file—may lead to decreased performance showing as longer service times for your content. It is best to keep purge.data files small. You can occasionally prune your purge.data file with a match of “/*.*” and a timestamp of, for example, two weeks ago to keep the file size down. You’d need to use ECCU file upload or the SOAP API to specify a timestamp other than “now.” The pruning will cause a higher forward request rate for your objects matched by “/*.*” that were not otherwise revalidated, but in practice, this small increase has not been significant enough to cause problems.
Requests are automatically pruned using a match of /*.* (all directories, all extensions) with a timestamp as distant in the past as possible to slightly reduce file size, when the purge.data file grows beyond a configurable size (for example, 250KB)
Note that you cannot otherwise delete matches from the purge.data file. The “Delete” instruction in the ECCU API does not delete matches from the purge.data file, it removes ECCU requests from view in Control Center.
Finally, note that the ECCU invalidates content; it does not remove it. If you need to actively remove objects from cache you must use Fast Purge.