I am moving discussion from https://github.com/apple/foundationdb/pull/1213 over to the forums.
A common pattern in layers is that there is a rarely changing schema that needs to be validated within each transaction. These reads of a small amount of data can be a burden on clusters, because the reads are concentrated on a very small section of key space.
The PR adds a key to the system keyspace which has its value sent to the client with every read version. This means that clients can see the value of this key without communicating with storage servers.
If the read version is not known yet, it will block until the read version comes back from the proxy. If you already have a read version, it is guaranteed to be synchronous.
This should not be too big of a concern. Each change will cause every client to invalidate their cache, so if you have 1000 clients, changing it will cause 1000 reads to the location where metadata is stored.
This is a plausible use case. @ajbeamon might know more about how easy it would be to implement.