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Merge pull request #3884 from rberlind/master 

Updated Stale Reads section of DNS Caching Guide
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Jack Pearkes 2018-03-13 16:56:58 -07:00 committed by GitHub
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@ -24,30 +24,30 @@ customize how DNS queries are handled.
## Stale Reads
Stale reads can be used to reduce latency and increase the throughput
of DNS queries. By default, all reads are serviced by a
[single leader node](/docs/internals/consensus.html).
These reads are strongly consistent but are limited by the throughput
of a single node. Doing a stale read allows any Consul server to
of DNS queries. The [settings](/docs/agent/options.html) used to control stale reads
are [`dns_config.allow_stale`](/docs/agent/options.html#allow_stale),
which must be set to enable stale reads, and [`dns_config.max_stale`](/docs/agent/options.html#max_stale)
which limits how stale results are allowed to be.
Since Consul 0.7.1, [`allow_stale`](/docs/agent/options.html#allow_stale)
is enabled by default, using a [`max_stale`](/docs/agent/options.html#max_stale)
value that defaults to a near-indefinite threshold (10 years) to allow DNS queries to continue to be served in the event
of a long outage with no leader. A new telemetry counter has also been added at
`consul.dns.stale_queries` to track when agents serve DNS queries that are stale
by more than 5 seconds.
Doing a stale read allows any Consul server to
service a query, but non-leader nodes may return data that is
out-of-date. By allowing data to be slightly stale, we get horizontal
read scalability. Now any Consul server can service the request, so we
increase throughput by the number of servers in a cluster.
The [settings](/docs/agent/options.html) used to control stale reads
are [`dns_config.allow_stale`](/docs/agent/options.html#allow_stale),
which must be set to enable stale reads, and
[`dns_config.max_stale`](/docs/agent/options.html#max_stale)
which limits how stale results are allowed to be.
Starting from Consul 0.7, [`allow_stale`](/docs/agent/options.html#allow_stale)
is enabled by default, using a [`max_stale`](/docs/agent/options.html#max_stale)
value that defaults to 5 seconds, meaning that we will use data from
any Consul server that is within 5 seconds of the leader. In Consul 0.7.1, the
default for `max_stale` was been increased from 5 seconds to a near-indefinite
threshold (10 years) to allow DNS queries to continue to be served in the event
of a long outage with no leader. A new telemetry counter has also been added at
`consul.dns.stale_queries` to track when agents serve DNS queries that are stale
by more than 5 seconds.
If you want to prevent
stale reads or limit how stale they can be, you can set `allow_stale`
to false or use a lower value for `max_stale`. Doing the first will ensure that
all reads are serviced by a [single leader node](/docs/internals/consensus.html).
The reads will then be strongly consistent but will be limited by the throughput
of a single node.
## Negative Response Caching