This continues the work done in #14908 where a crude solution to prevent a
goroutine leak was implemented. The former code would launch a perpetual
goroutine family every iteration (+1 +1) and the fixed code simply caused a
new goroutine family to first cancel the prior one to prevent the
leak (-1 +1 == 0).
This PR refactors this code completely to:
- make it more understandable
- remove the recursion-via-goroutine strangeness
- prevent unnecessary RPC fetches when the prior one has errored.
The core issue arose from a conflation of the entry.Fetching field to mean:
- there is an RPC (blocking query) in flight right now
- there is a goroutine running to manage the RPC fetch retry loop
The problem is that the goroutine-leak-avoidance check would treat
Fetching like (2), but within the body of a goroutine it would flip that
boolean back to false before the retry sleep. This would cause a new
chain of goroutines to launch which #14908 would correct crudely.
The refactored code uses a plain for-loop and changes the semantics
to track state for "is there a goroutine associated with this cache entry"
instead of the former.
We use a uint64 unique identity per goroutine instead of a boolean so
that any orphaned goroutines can tell when they've been replaced when
the expiry loop deletes a cache entry while the goroutine is still running
and is later replaced.
This implements a solution for #7863
It does:
Add a new config cache.entry_fetch_rate to limit the number of calls/s for a given cache entry, default value = rate.Inf
Add cache.entry_fetch_max_burst size of rate limit (default value = 2)
The new configuration now supports the following syntax for instance to allow 1 query every 3s:
command line HCL: -hcl 'cache = { entry_fetch_rate = 0.333}'
in JSON
{
"cache": {
"entry_fetch_rate": 0.333
}
}
This should very slightly reduce the amount of memory required to store each item in
the cache.
It will also enable setting different TTLs based on the type of result. For example
we may want to use a shorter TTL when the result indicates the resource does not exist,
as storing these types of records could easily lead to a DOS caused by
OOM.
* Add State storage and LastResult argument into Cache so that cache.Types can safely store additional data that is eventually expired.
* New Leaf cache type working and basic tests passing. TODO: more extensive testing for the Root change jitter across blocking requests, test concurrent fetches for different leaves interact nicely with rootsWatcher.
* Add multi-client and delayed rotation tests.
* Typos and cleanup error handling in roots watch
* Add comment about how the FetchResult can be used and change ca leaf state to use a non-pointer state.
* Plumb test override of root CA jitter through TestAgent so that tests are deterministic again!
* Fix failing config test
* Add cache types for catalog/services and health/services and basic test that caching works
* Support non-blocking cache types with Cache-Control semantics.
* Update API docs to include caching info for every endpoint.
* Comment updates per PR feedback.
* Add note on caching to the 10,000 foot view on the architecture page to make the new data path more clear.
* Document prepared query staleness quirk and force all background requests to AllowStale so we can spread service discovery load across servers.
R.B. Boyer