website: document alias check

website/source/api/agent/check.html.md

@@ -116,6 +116,16 @@ The table below shows this endpoint's support for
     continue to be accepted in future versions of Consul), and `Args` in Consul
     1.0.1 and later.
 
+- `AliasNode` `(string: "")` - Specifies the ID of the node for an alias check.
+  If no service is specified, the check will alias the health of the node.
+  If a service is specified, the check will alias the specified service on
+  this particular node.
+
+- `AliasService` `(string: "")` - Specifies the ID of a service for an
+  alias check. If the service is not registered with the same agent,
+  `AliasNode` must also be specified. Note this is the service _ID_ and
+  not the service _name_ (though they are very often the same).
+
 - `DockerContainerID` `(string: "")` - Specifies that the check is a Docker
   check, and Consul will evaluate the script every `Interval` in the given
   container using the specified `Shell`. Note that `Shell` is currently only

website/source/docs/agent/checks.html.md

@@ -101,6 +101,17 @@ There are several different kinds of checks:
   TLS certificate is expected. Certificate verification can be turned off by setting the
   `tls_skip_verify` field to `true` in the check definition.
 
+* <a name="alias"></a>Alias - These checks alias the health state another Consul
+  node or service. The state of the check will be updated asynchronously,
+  but is nearly instant. For aliased services on the same agent, the local
+  state is monitored and no additional network resources are consumed. For
+  other services and nodes, the check maintains a blocking query over the
+  agent's connection with a current server and allows stale requests. If there
+  are any errors in watching the aliased node or service, the check will be
+  unhealthy. For the blocking query, the check will use the ACL token set
+  on the service definition, otherwise falling back to the default ACL
+  token set with the agent (`acl_token`).
+
 ## Check Definition
 
 A script check:
@@ -165,7 +176,7 @@ A Docker check:
 
 ```javascript
 {
-"check": {
+  "check": {
     "id": "mem-util",
     "name": "Memory utilization",
     "docker_container_id": "f972c95ebf0e",
@@ -180,7 +191,7 @@ A gRPC check:
 
 ```javascript
 {
-"check": {
+  "check": {
     "id": "mem-util",
     "name": "Service health status",
     "grpc": "127.0.0.1:12345",
@@ -190,6 +201,17 @@ A gRPC check:
 }
 ```
 
+An alias check for a local service:
+
+```javascript
+{
+  "check": {
+    "id": "web-alias",
+    "alias_service": "web"
+  }
+}
+```
+
 Each type of definition must include a `name` and may optionally provide an
 `id` and `notes` field. The `id` must be unique per _agent_ otherwise only the
 last defined check with that `id` will be registered. If the `id` is not set
@@ -205,6 +227,8 @@ a TTL check via the HTTP interface can set the `notes` value.
 Checks may also contain a `token` field to provide an ACL token. This token is
 used for any interaction with the catalog for the check, including
 [anti-entropy syncs](/docs/internals/anti-entropy.html) and deregistration.
+For Alias checks, this token is used if a remote blocking query is necessary
+to watch the state of the aliased node or service.
 
 Script, TCP, HTTP, Docker, and gRPC checks must include an `interval` field. This
 field is parsed by Go's `time` package, and has the following