The circulation manager builds an Authentication For OPDS document when it needs one, and holds on to it until the cache of those documents is invalidated. Currently the cache is invalidated whenever an admin makes a change to the site configuration using the admin interface. If there are no changes the document will be cached in definitely.

The addition of announcements to the Authentication For OPDS document means that these documents will go stale with the passage of time. Even if nothing about the site configuration changes, announcements will enter and leave the A4OPDS document over time. So we can no longer cache the document indefinitely.

These documents are cheap to make (just not so cheap that we want to make them for every patron who logs in), so expiring the cache once an hour should be plenty.