+++
title = "redisc"
description = "*redisc* - enables a networked cache using Redis."
weight = 10
tags = [  "plugin" , "redisc" ]
categories = [ "plugin", "external" ]
date = "2018-02-17T19:36:00+00:00"
repo = "https://github.com/miekg/redis"
home = "https://github.com/miekg/redis/blob/master/README.md"
+++

## Description

With *redisc* responses can be cached for up to 3600s. Caching in Redis is mostly usefull in
a setup where multiple CoreDNS instances share a VIP. E.g. multiple CoreDNS pods in a Kubernetes
cluster.

If Redis is not reacheable this plugin will be a noop. The *cache* and *redisc* plugin can be used
together, where *cache* is the L1 and *redisc* is the L2 level cache.
If multiple CoreDNS instances get a cache miss for the same item, they will all be fetching the same
information from an upstream and updating the cache, i.e. there is no (extra) coordination between
those instances.

If Redis is not available CoreDNS will simply not cache anything if metrics are enabled this will be
visible in the `set_errors_total` metric.

## Syntax

~~~ txt
redisc [TTL] [ZONES...]
~~~

* **TTL** 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: `redisc 300` would cache records up to 300 seconds.
* **ZONES** zones it should cache for. If empty, the zones from the configuration block are used.

Each element in the Redis cache is cached according to its TTL (with **TTL** as the max). For the negative
cache, the SOA's MinTTL value is used. When no endpoint is specfied the default of `127.0.0.1:6379` will
be used.

If you want more control:

~~~ txt
redisc [TTL] [ZONES...] {
    endpoint ENDPOINT
}
~~~

* **TTL**  and **ZONES** as above.
* `endpoint` specifies which **ENDPOINT** to use for Redis, this default to `127.0.0.1:6379`.

## Metrics

If monitoring is enabled (via the *prometheus* directive) then the following metrics are exported:

* `coredns_redisc_hits_total{server}` - Counter of cache hits.
* `coredns_redisc_misses_total{server}` - Counter of cache misses.
* `coredns_redisc_set_errors_total{server}` - Counter of errors when connecting to Redis.
* `coredns_redisc_drops_total{server}` - Counter of dropped messages.

The `server` label indicates which server handled the request, see the *metrics* plugin for details.

## Examples

Enable caching for all zones, cache locally and also cache for up to 40s in the cluster wide Redis.

~~~ corefile
. {
    cache 30
    redisc 40 {
        endpoint 10.0.240.1:69
    }
    whoami
}
~~~

Proxy to Google Public DNS and only cache responses for example.org (and below).

~~~ corefile
. {
    proxy . 8.8.8.8:53
    redisc example.org
}
~~~

## See Also

See [the Redis site for more information](https://redis.io) on Redis. An external plugin called
[redis](https://coredns.io/explugins/redis) already exists, hence this is named *redisc*, for
"redis cache".

## Bugs

There is little unit testing.