Sessions are currently stored in Redis, a highly-optimized in-memory store with request latency reportedly on the order of ~1 ms. Given the nature of the proposed session storage service, latency can only be higher than what we're accustomed to; It seems implied that we are striking a bargain to trade away some latency in return for multi-master replication, but have not yet agreed upon a price.
We should to establish an SLA for the proposed session storage service.
(WARNING) In the interest of starting a discussion, [[ https://meta.wikimedia.org/wiki/Cunningham%27s_Law | here are some proposed values ]]:
| SET,GET | mean latency | 75ms |
| SET,GET |90p latency | 125ms |
| DELETE | mean latency | 100ms |
| DELETE | 90p latency | 150ms |
| SET | errors | 1% |
| GET | errors | 1% |
| DELETE | errors | 1% |
NOTE: [[ https://www.mediawiki.org/wiki/Requests_for_comment/SessionStorageAPI#delete | `DELETE` ]] may need it's own latency SLA since its proposed consistency semantics requires a quorum of nodes spanning data-centers.