Review cache TTLs (and global default TTL)
Much (most?) of our rails cached data in Redis has a TTL of 2 weeks: production#2263 (comment 359345686). Incidents that cause cache inconsistency (e.g. open issue sidebar counters not matching length of open issue list) have to be resolved by dropping the whole cache (production#2263 (closed)).
The higher the cache TTL, the higher the cache hit rate. However, there is an argument that the TTL for a type of data should not be set significantly higher than the maximum acceptable time for that cached data to be inconsistent. We can't expect to never have an inconsistent cache unless it is updated transactionally with the main database - which would involve holding open a database transaction while performing external work, which has its own well-known problems.