[danenania]

It definitely does depend on various factors, but if your query is indexed, both the SQL DB request and the Redis/Memcache lookup of the full object are likely to be dominated by internal network latency. If your object is large, the DB single-field lookup could easily be faster since you're sending less back over the wire.

In other words, a single-field indexed DB lookup can be treated more like a cache request. Though for heavier/un-indexed queries, your "avoid round trips to the database" advice certainly applies.

With this architecture, the internal invalidation request is just an optimization. It isn't necessary and it doesn't matter if it fails, since you always check the timestamp with a strongly consistent DB read before returning a cached object.