Updates to DNS Caching Guide (#5001)

* Updates to DNS Caching Guide

* Spelling and grammar

website/source/docs/guides/dns-cache.html.md

@@ -16,47 +16,89 @@ 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.
 
+In this guide, we will review important parameters for tuning
+stale reads, negative response caching, and TTL. All of the DNS config
+parameters must be set in set in the agent's configuration file.
+
 <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.
+are:
 
-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
+* [`dns_config.allow_stale`](/docs/agent/options.html#allow_stale) must be
+set to true to enable stale reads.
+* [`dns_config.max_stale`](/docs/agent/options.html#max_stale) limits how stale results
+are allowed to be when querying DNS.
+
+With these two settings you can allow or prevent stale reads. Below we will discuss
+the advanatages and disadvatages of both.
+
+### Allow Stale Reads
+
+Since Consul 0.7.1, `allow_stale` is enabled by default and uses a `max_stale`
+value that defaults to a near-indefinite threshold (10 years).
+This allows 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.
 
+```javascript
+"dns_config" {
+  "allow_stale" = true
+  "max_stale" = "87600h"
+}
+```
+
+~> NOTE: The above example is the default setting. You do not need to set it explicitly.
+
 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`
+### Prevent Stale Reads
+
+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.
 
+```javascript
+"dns_config" {
+  "allow_stale" = false
+}
+```
+
 ## 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
+endpoints. In practice, this could mean that the cached negative responses may
+cause that service to appear "down" for longer than they are actually unavailable
 when using DNS for service discovery.
 
+### Configure SOA
+
+In Consul 1.3.0 and newer, it is now possible to tune SOA
+responses and modify the negative TTL cache for some resolvers. It can
+be achieved using the [`soa.min_ttl`](/docs/agent/options.html#soa_min_ttl)
+configuration within the [`soa`](/docs/agent/options.html#soa) configuration.
+
+```javascript
+"dns_config" {
+  "soa" {
+   "min_ttl" = "60s"
+  }
+}
+```
+
 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
@@ -65,11 +107,6 @@ 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.
 
-With versions of Consul greater than 1.3.0, it is now possible to tune SOA
-responses and modify the negative TTL cache for some resolvers. It can
-be achieved using the [`soa.min_ttl`](/docs/agent/options.html#soa_min_ttl)
-configuration within the [`soa`](/docs/agent/options.html#soa) configuration.
-
 <a name="ttl"></a>
 ## TTL Values
 
@@ -78,6 +115,17 @@ TTL values reduce the number of lookups on the Consul servers and speed lookups
 clients, at the cost of increasingly stale results. By default, all TTLs are zero,
 preventing any caching.
 
+```javascript
+{
+  "dns_config": {
+    "service_ttl" = "0s"
+    "node_ttl" = "0s"
+  }
+}
+```
+
+### Enable 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.
@@ -108,15 +156,23 @@ a wildcard TTL and a specific TTL for a service might look like this:
 ```
 
 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.
-
+while lookups to "api.service.consul" will use the 5 second TTL from the wildcard.
 All lookups matching "db*" would get a 10 seconds TTL except "db-master"
 that would have a 3 seconds TTL.
 
+### Prepared Queries
+
 [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.
+
+## Summary
+
+In this guide we covered several of the parameters for tuning DNS queries. We reviewed
+how to enable or disable stale reads and how to configure the amount of time when stale
+reads are allowed. We also looked at the minimum TTL configuration options
+for negative responses from services. Finally, we reviewed how to setup TTLs
+for service lookups.