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Merge pull request #3884 from rberlind/master

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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
Stale reads can be used to reduce latency and increase the throughput Stale reads can be used to reduce latency and increase the throughput
of DNS queries. By default, all reads are serviced by a of DNS queries. The [settings](/docs/agent/options.html) used to control stale reads
[single leader node](/docs/internals/consensus.html). are [`dns_config.allow_stale`](/docs/agent/options.html#allow_stale),
These reads are strongly consistent but are limited by the throughput which must be set to enable stale reads, and [`dns_config.max_stale`](/docs/agent/options.html#max_stale)
of a single node. Doing a stale read allows any Consul server to 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 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out-of-date. By allowing data to be slightly stale, we get horizontal out-of-date.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 read scalability. Now any Consul server can service the request, so we
increase throughput by the number of servers in a cluster. increase throughput by the number of servers in a cluster.
The [settings](/docs/agent/options.html) used to control stale reads If you want to prevent
are [`dns_config.allow_stale`](/docs/agent/options.html#allow_stale), stale reads or limit how stale they can be, you can set `allow_stale`
which must be set to enable stale reads, and to false or use a lower value for `max_stale`. Doing the first will ensure that
[`dns_config.max_stale`](/docs/agent/options.html#max_stale) all reads are serviced by a [single leader node](/docs/internals/consensus.html).
which limits how stale results are allowed to be. The reads will then be strongly consistent but will be limited by the throughput
of a single node.
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.
## Negative Response Caching ## Negative Response Caching