The problem I always run into with eTags is that the point of an eTag is that it either: one, has to be cheap to re-calculate, or two, you need some pubsub syst

The problem I always run into with eTags is that the point of an eTag is that it either: one, has to be cheap to re-calculate, or two, you need some pubsub system to mark all cached content as stale and invalidate them. The former is much less likely to break (less maintenance overhead) but harder to set up and cash in on, because essentially you need to move all of the deterministic work to the end of the request, do all of the nondeterministic parts first, calculate the eTag from that data, and then if it matches don't do the rest. And the problem is that pure functional part at the end often just isn't that expensive compared to all the fanout that comes before. It's data transforms and the cost of writing to cache. So you have to do that pattern in your services too in order to get some oomph out of this solution, except perhaps in some narrow problem domains.