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plugin/cache: Fix prefetching issues (#1363)

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* Improve plugin/cache metrics

* Add coredns_cache_prefetch_total metric to track number of prefetches.
* Remove unnecessary Cache.get() call which would incorrectly increment
  cache counters.
* Initialize all counters and gauges at zero.

* Allow prefetching of a single request per ttl

The original implementation didn't allow prefetching queries which are
only requested once during the duration of a TTL. The minimum amount of
queries which had to be seen was therefore capped at 2.

This change also implements a real prefetch test. The existing test was
a noop and always passed regardless of any prefetch implementation.

* Fix prefetching for items with a short TTL

The default prefetch threshold (percentage) is 10% of the lifetime of a
cache item. With the previous implementation, this disabled prefetching
for all items with a TTL < 10s (the resulting percentage would be 0, at
which point a cached item is already discarded).

This change uses a time based threshold calculation and ensures that
a prefetch is triggered at a TTL of 1 at the latest.

* Fix wrong duration reporting of cached responses

The logging and metrics plugins (among others) included the duration of
a cache prefetch in the request latency of client request. This change
fixes this wrong reporting and executes the prefetch request in a
goroutine in the background.
This commit is contained in:
Tobias Schmidt
2018-01-17 08:35:22 +01:00
committed by Miek Gieben
parent fe0767987e
commit dd9fc8962c
8 changed files with 212 additions and 74 deletions
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4
plugin/cache/README.md vendored
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@@ -44,8 +44,8 @@ cache [TTL] [ZONES...] {
* `prefetch` will prefetch popular items when they are about to be expunged from the cache.
Popular means **AMOUNT** queries have been seen with no gaps of **DURATION** or more between them.
**DURATION** defaults to 1m. Prefetching will happen when the TTL drops below **PERCENTAGE**,
which defaults to `10%`. Values should be in the range `[10%, 90%]`. Note the percent sign is
mandatory. **PERCENTAGE** is treated as an `int`.
which defaults to `10%`, or latest 1 second before TTL expiration. Values should be in the range `[10%, 90%]`.
Note the percent sign is mandatory. **PERCENTAGE** is treated as an `int`.
## Metrics