* isolation can largely be maintained by doing the following:
* - a) Calling delete() on entity change *and* creation, before DB commit
* - b) Keeping transaction duration shorter than delete() hold-off TTL
+ *
* However, pre-snapshot values might still be seen if an update was made
* in a remote datacenter but the purge from delete() didn't relay yet.
*
* - b) Thus, dependent keys will be known to be invalid, but not
* for how long (they are treated as "just" purged), which
* effects any lockTSE logic in getWithSetCallback()
+ *
* The advantage is that this does not place high TTL keys on every cache
* server, making it better for code that will cache many different keys
* and either does not use lockTSE or uses a low enough TTL anyway.
* the 'lockTSE' option in $opts. If cache purges are needed, also:
* - a) Pass $key into $checkKeys
* - b) Use touchCheckKey( $key ) instead of delete( $key )
- * Following this pattern lets the old cache be used until a
- * single thread updates it as needed. Also consider tweaking
- * the 'lowTTL' parameter.
*
* Example usage (typical key):
* @code