## Problem
A user can perform many database connections at the same instant of time
- these will all cache miss and materialise as requests to the control
plane. #5705
## Summary of changes
I am using a `DashMap` (a sharded `RwLock<HashMap>`) of endpoints ->
semaphores to apply a limiter. If the limiter is enabled (permits > 0),
the semaphore will be retrieved per endpoint and a permit will be
awaited before continuing to call the wake_compute endpoint.
### Important details
This dashmap would grow uncontrollably without maintenance. It's not a
cache so I don't think an LRU-based reclamation makes sense. Instead,
I've made use of the sharding functionality of DashMap to lock a single
shard and clear out unused semaphores periodically.
I ran a test in release, using 128 tokio tasks among 12 threads each
pushing 1000 entries into the map per second, clearing a shard every 2
seconds (64 second epoch with 32 shards). The endpoint names were
sampled from a gamma distribution to make sure some overlap would occur,
and each permit was held for 1ms. The histogram for time to clear each
shard settled between 256-512us without any variance in my testing.
Holding a lock for under a millisecond for 1 of the shards does not
concern me as blocking
This patch adds a timed LRU cache implementation and a compute node info cache on top of that.
Cache entries might expire on their own (default ttl=5mins) or become invalid due to real-world events,
e.g. compute node scale-to-zero event, so we add a connection retry loop with a wake-up call.
Solved problems:
- [x] Find a decent LRU implementation.
- [x] Implement timed LRU on top of that.
- [x] Cache results of `proxy_wake_compute` API call.
- [x] Don't invalidate newer cache entries for the same key.
- [x] Add cmdline configuration knobs (requires some refactoring).
- [x] Add failed connection estab metric.
- [x] Refactor auth backends to make things simpler (retries, cache
placement, etc).
- [x] Address review comments (add code comments + cleanup).
- [x] Retry `/proxy_wake_compute` if we couldn't connect to a compute
(e.g. stalled cache entry).
- [x] Add high-level description for `TimedLru`.
TODOs (will be addressed later):
- [ ] Add cache metrics (hit, spurious hit, miss).
- [ ] Synchronize http requests across concurrent per-client tasks
(https://github.com/neondatabase/neon/pull/3331#issuecomment-1399216069).
- [ ] Cache results of `proxy_get_role_secret` API call.
