* 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.
The cache plugin always returned a minimum TTL of 5 seconds, regardless
of the actual TTL of the records. A cache is not authoritative for the
record TTL and should not extend it.
Cache would let the first response through and would then cap subsequent
ones to whatever the cache duration was. This would lead to huge drops
in TTL values: 3600 -> 20 for instance, which is not only bad, but can
mess up your careful TTL planning business.
This PR fixes that and applies the cache duration to all replies. As a
bonus I could remove a time.Sleep() from the cache test and just check
for the cache duration as the TTL on the reply.
Fixes#1038
* Rename middleware to plugin
first pass; mostly used 'sed', few spots where I manually changed
text.
This still builds a coredns binary.
* fmt error
* Rename AddMiddleware to AddPlugin
* Readd AddMiddleware to remain backwards compat
