Zone-aware snapshot selection

[ecordell]

From the Zanzibar paper:

When a zookie is not provided, the server uses a default staleness chosen to ensure that all transactions are evaluated at a timestamp that is as recent as possible without impacting latency. On each read request it makes to Spanner, Zanzibar receives a hint about whether or not the data at that timestamp required an out-of-zone read and thus incurred additional latency. Each server tracks the frequency of such out-of-zone reads for data at a default staleness as well as for fresher and staler data, and uses these frequencies to compute a binomial proportion confidence interval of the probability that any given piece of data is available locally at each staleness. Upon collecting enough data, the server checks to see if each staleness value has a sufficiently low probability of incurring an out-of-zone read, and thus will be low-latency. If so, it updates the default staleness bound to the lowest “safe” value. If no known staleness values are safe, we use a twoproportion z-test to see if increasing the default will be a statistically significant amount safer. In that case, we increase the default value in the hopes of improving latency. This default staleness mechanism is purely a performance optimization. It does not violate consistency semantics because Zanzibar always respects zookies when provided.

CockroachDB supports regions and zones. There isn't a way (that I can see) to get statistics on zone information during a single read, but locality data can be fetched within the same transaction with a show range for row query.