With \fIcache\fR enabled, all records except zone transfers and metadata records will be cached for up to 3600s\. Caching is mostly useful in a scenario when fetching data from the backend (upstream, database, etc\.) is expensive\.
\fBTTL\fR max TTL in seconds\. If not specified, the maximum TTL will be used, which is 3600 for noerror responses and 1800 for denial of existence ones\. Setting a TTL of 300: \fBcache 300\fR would cache records up to 300 seconds\.
Each element in the cache is cached according to its TTL (with \fBTTL\fR as the max)\. For the negative cache, the SOA\'s MinTTL value is used\. A TTL of zero is not allowed\. A cache is divided into 256 shards, each holding up to 512 items by default \- for a total size of 256 * 512 = 131,072 items\.
\fBsuccess\fR, override the settings for caching successful responses\.\fBCAPACITY\fR indicates the maximum number of packets we cache before we start evicting (\fIrandomly\fR)\.\fBTTL\fR overrides the cache maximum TTL\.
\fBdenial\fR, override the settings for caching denial of existence responses\.\fBCAPACITY\fR indicates the maximum number of packets we cache before we start evicting (LRU)\.\fBTTL\fR overrides the cache maximum TTL\. There is a third category (\fBerror\fR) but those responses are never cached\.
\fBprefetch\fR will prefetch popular items when they are about to be expunged from the cache\. Popular means \fBAMOUNT\fR queries have been seen with no gaps of \fBDURATION\fR or more between them\.\fBDURATION\fR defaults to 1m\. Prefetching will happen when the TTL drops below \fBPERCENTAGE\fR, which defaults to \fB10%\fR, or latest 1 second before TTL expiration\. Values should be in the range \fB[10%, 90%]\fR\. Note the percent sign is mandatory\.\fBPERCENTAGE\fR is treated as an \fBint\fR\.
When specifying \fBCAPACITY\fR, the minimum cache capacity is 131,072\. Specifying a lower value will be ignored\. Specifying a \fBCAPACITY\fR of zero does not disable the cache\.
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Eviction is done per shard \- i\.e\. when a shard reaches capacity, items are evicted from that shard\. Since shards don\'t fill up perfectly evenly, evictions will occur before the entire cache reaches full capacity\. Each shard capacity is equal to the total cache size / number of shards (256)\.
