The image illustrates the expires caching technique for the second request. The details are as follows:
mod_plsql detects that it has a cached copy of the content that is expires-based.
mod_plsql checks the content's validity by taking the difference between the current time and the time this cache file was created.
If this difference is within the validity period, the cached copy is still fresh and will be used without any database interaction. The cached content is directly streamed back to the browser.
If the difference is not within the validity period, the cached copy is stale. mod_plsql invokes the PL/SQL procedure and generates new content. The procedure then decides whether to use expires-based caching again. If so, it also determines the validating period for this new content. The newly generated content is streamed back to the browser.