mirror of https://github.com/hashicorp/consul
109 lines
4.9 KiB
Markdown
109 lines
4.9 KiB
Markdown
---
|
|
layout: "docs"
|
|
page_title: "DNS Caching"
|
|
sidebar_current: "docs-guides-dns-cache"
|
|
description: |-
|
|
One of the main interfaces to Consul is DNS. Using DNS is a simple way to integrate Consul into an existing infrastructure without any high-touch integration.
|
|
---
|
|
|
|
# DNS Caching
|
|
|
|
One of the main interfaces to Consul is DNS. Using DNS is a simple way to
|
|
integrate Consul into an existing infrastructure without any high-touch
|
|
integration.
|
|
|
|
By default, Consul serves all DNS results with a 0 TTL value. This prevents
|
|
any caching. The advantage is that each DNS lookup is always re-evaluated,
|
|
so the most timely information is served. However, this adds a latency hit
|
|
for each lookup and can potentially exhaust the query throughput of a cluster.
|
|
|
|
For this reason, Consul provides a number of tuning parameters that can
|
|
customize how DNS queries are handled.
|
|
|
|
<a name="stale"></a>
|
|
## Stale Reads
|
|
|
|
Stale reads can be used to reduce latency and increase the throughput
|
|
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out-of-date.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.
|
|
|
|
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
|
|
|
|
Some DNS clients cache negative responses - that is, Consul returning a "not
|
|
found" style response because a service exists but there are no healthy
|
|
endpoints. What this means in practice is that cached negative responses may
|
|
mean that services appear "down" for longer than they are actually unavailable
|
|
when using DNS for service discovery.
|
|
|
|
One common example is that Windows will default to caching negative responses
|
|
for 15 minutes. DNS forwarders may also cache negative responses, with the same
|
|
effect. To avoid this problem, check the negative response cache defaults for
|
|
your client operating system and any DNS forwarder on the path between the
|
|
client and Consul and set the cache values appropriately. In many cases
|
|
"appropriately" simply is turning negative response caching off to get the best
|
|
recovery time when a service becomes available again.
|
|
|
|
<a name="ttl"></a>
|
|
## TTL Values
|
|
|
|
TTL values can be set to allow DNS results to be cached downstream of Consul. Higher
|
|
TTL values reduce the number of lookups on the Consul servers and speed lookups for
|
|
clients, at the cost of increasingly stale results. By default, all TTLs are zero,
|
|
preventing any caching.
|
|
|
|
To enable caching of node lookups (e.g. "foo.node.consul"), we can set the
|
|
[`dns_config.node_ttl`](/docs/agent/options.html#node_ttl) value. This can be set to
|
|
"10s" for example, and all node lookups will serve results with a 10 second TTL.
|
|
|
|
Service TTLs can be specified in a more granular fashion. You can set TTLs
|
|
per-service, with a wildcard TTL as the default. This is specified using the
|
|
[`dns_config.service_ttl`](/docs/agent/options.html#service_ttl) map. The "*"
|
|
service is the wildcard service.
|
|
|
|
For example, a [`dns_config`](/docs/agent/options.html#dns_config) that provides
|
|
a wildcard TTL and a specific TTL for a service might look like this:
|
|
|
|
```javascript
|
|
{
|
|
"dns_config": {
|
|
"service_ttl": {
|
|
"*": "5s",
|
|
"web": "30s"
|
|
}
|
|
}
|
|
}
|
|
```
|
|
|
|
This sets all lookups to "web.service.consul" to use a 30 second TTL
|
|
while lookups to "db.service.consul" or "api.service.consul" will use the
|
|
5 second TTL from the wildcard.
|
|
|
|
[Prepared Queries](/api/query.html) provide an additional
|
|
level of control over TTL. They allow for the TTL to be defined along with
|
|
the query, and they can be changed on the fly by updating the query definition.
|
|
If a TTL is not configured for a prepared query, then it will fall back to the
|
|
service-specific configuration defined in the Consul agent as described above,
|
|
and ultimately to 0 if no TTL is configured for the service in the Consul agent.
|