Merge pull request prometheus#1971 from bwplotka/disable-ha-explicit

docs/flags: Make it explicit that HA is enabled by default and how to disable it.

README.md

@@ -327,15 +327,15 @@ amtool config routes --alertmanager.url=http://localhost:9090
 
 ## High Availability
 
-AlertManager's high availability is in production use at many companies.
+AlertManager's high availability is in production use at many companies and is enabled by default.
 
 > Important: Both UDP and TCP are needed in alertmanager 0.15 and higher for the cluster to work.
 
 To create a highly available cluster of the Alertmanager the instances need to
 be configured to communicate with each other. This is configured using the
 `--cluster.*` flags.
 
-- `--cluster.listen-address` string: cluster listen address (default "0.0.0.0:9094")
+- `--cluster.listen-address` string: cluster listen address (default "0.0.0.0:9094"; empty string disables HA mode)
 - `--cluster.advertise-address` string: cluster advertise address
 - `--cluster.peer` value: initial peers (repeat flag for each additional peer)
 - `--cluster.peer-timeout` value: peer timeout period (default "15s")
@@ -381,7 +381,6 @@ alerting:
 
 If running Alertmanager in high availability mode is not desired, setting `--cluster.listen-address=` will prevent Alertmanager from listening to incoming peer requests.
 
-
 ## Contributing to the Front-End
 
 Refer to [ui/app/CONTRIBUTING.md](ui/app/CONTRIBUTING.md).

cmd/alertmanager/main.go

@@ -172,7 +172,7 @@ func run() int {
 		getConcurrency = kingpin.Flag("web.get-concurrency", "Maximum number of GET requests processed concurrently. If negative or zero, the limit is GOMAXPROC or 8, whichever is larger.").Default("0").Int()
 		httpTimeout    = kingpin.Flag("web.timeout", "Timeout for HTTP requests. If negative or zero, no timeout is set.").Default("0").Duration()
 
-		clusterBindAddr = kingpin.Flag("cluster.listen-address", "Listen address for cluster.").
+		clusterBindAddr = kingpin.Flag("cluster.listen-address", "Listen address for cluster. Set to empty string to disable HA mode.").
 				Default(defaultClusterAddr).String()
 		clusterAdvertiseAddr = kingpin.Flag("cluster.advertise-address", "Explicit address to advertise in cluster.").String()
 		peers                = kingpin.Flag("cluster.peer", "Initial peers (may be repeated).").Strings()