promquery: add BulkQueryCache #100
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This implementation originates in Limes (imported from there at `internal/util/promcache.go`), but I want to reuse it in Castellum where the idea of bulk requesting and caching Prometheus metrics originates. This implementation differs slightly from the one in Limes: - The `Prometheus` prefix was removed from the type names. - The `refreshRate` field was renamed to `refreshInterval`. - The constructor method takes a Client instance instead of its Config. - A full-featured documentation example was added to better demonstrate how all the pieces fit together.
| // Get returns the entry for this key, or a zero-initialized entry if this key | ||
| // does not exist in the dataset. | ||
| func (c *BulkQueryCache[K, V]) Get(key K) (entry V, err error) { | ||
| err = c.fillCacheIfNecessary() |
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Wouldn't this cause spikes in the execution time of Get() whenever the interval is past? So we shouldn't use this in any time critical functions like http requests which are supposed to return fast.
All the alternative ideas I have cause constant background refreshes even if things are idle or the first request would always be cashed even if the interval is past.
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Yes, but we are initially only using this for background tasks (in limes-collector and castellum-observer), so this performance characteristic is not a problem.
If we want to use this for frontend tasks later on, the design affords adding explicit refreshes ahead of schedule that bypass the existing "if necessary" check. This would be similar to what we do in swift-s3-credential-prewarmer, except at a different scale (within a single process instead of within an entire service).
This implementation originates in Limes (imported from there at
internal/util/promcache.go), but I want to reuse it in Castellum where the idea of bulk requesting and caching Prometheus metrics originates.This implementation differs slightly from the one in Limes:
Prometheusprefix was removed from the type names to fit the package name.refreshRatefield was renamed torefreshInterval("rate" would be the inverse of an interval).Note: Because of the year-end change freeze, please do not merge this yet. Once approved, I will merge after the end of the change freeze.