Update documentation

doc/06-distributed-monitoring.md

@@ -2793,3 +2793,35 @@ Add the global zone `global-templates` in case it did not exist.
       global = true
     }
     EOF
+
+## Using Multiple Environments <a id="distributed-monitoring-environments"></a>
+
+In some cases it might be useful to run multiple Icinga instance on the same host. Two potential scenarios include:
+
+* running different versions of the same monitoring configuration (e.g. production and testing)
+* running disparate sets of checks for entirely unrelated monitoring environments (e.g. infrastructure and applications)
+
+Icinga supports these scenarios by providing a global variable called `Environment`. It can be set like any other
+global variable, e.g. in the `constants.conf` file or in a file in a global template zone.
+
+The `Environment` variable can also be set using the `--env` command-line option. Setting the variable this way
+has the added effect that Icinga tries to determine if the `Icinga Environments` add-on is available and
+if so uses its CLI commands to determine the configuration and data paths. This command-line option also has the
+effect of changing the listener port for cluster feature to a random port which is made available to the
+`Icinga Environments` add-on through its `config.json` file.
+
+When Icinga establishes a TLS connection to another cluster instance it automatically uses the SNI extension
+to signal which endpoint it is attempting to connect to. On its own this can already be used to position multiple
+Icinga instances behind a load balancer.
+
+SNI example: `icinga2-client1.localdomain`
+
+However, if the `Environment` variable is non-empty Icinga will append the environment name to the SNI hostname
+like this:
+
+SNI example with environment: `icinga2-client1.localdomain:production`
+
+The Icinga Environments add-on provides a reverse proxy which does SNI-based port multiplexing. I.e., it uses a
+single externally-visible TCP port (usually 5665) and forwards connections to one or more Icinga instances
+which are bound to a local TCP port. It does so by inspecting the environment name that is sent as part of the
+SNI extension.