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Powered by Redis: StackOverflow Caches

Posting about this Redis-based StackOverflow demo/clone, reminded me that StackOverflow is actually using Redis for site-level and global caches:

  1. A “local cache,” which can only be accessed from 1 server/site pair
    • Contains things like user sessions, and pending view count updates
    • This resides purely in memory, no network or DB access
  2. A “site cache,” which can be accessed by any instance (on any server) of a single site
    • Most cached values go here, things like hot question id lists and user acceptance rates are good examples
    • This resides in Redis (in a distinct DB, purely for easier debugging)
  3. A “global cache,” which is shared amongst all sites and servers
    • Inboxes, API usage quotas, and a few other truly global things live here
    • This resides in Redis (in DB 0, likewise for easier debugging)

It is kind of impressive to read engineers expressing feelings like (nb: when they know what they are talking about):

Redis is so fast that the slowest part of a cache lookup is the time spent reading and writing bytes to the network. This is not surprising, really, if you think about it.

Original title and link: Powered by Redis: StackOverflow Caches (NoSQL databases © myNoSQL)