A regional pool is a single cluster that holds a large object few people read, in place of a copy in every cluster.
The group Powai chess players may hold ten thousand people, and few people in the US, China or Australia look at it. So the object is kept in one cluster, close to India. The other caches do not have it, and the pool is not replicated across regions.
A read from the US is not answered by the US cache. Mcrouter looks at the key and routes the read to the Indian server. The cross continent call is rare, and the single copy saves a lot of memory. A cache failure impacts a small set of users.
Three factors, measured against thresholds, decide that an object goes to a regional pool:
- It is accessed infrequently.
- It is large.
- It does not affect many users, so the blast radius is small.
The object is then assigned to a single cluster, and mcrouter knows where to send a key by looking at a routing table.