add an api endpoint for getting the current leader (#741)

* add an api endpoint for getting the current leader

This adds a /scheduler/leader endpoint for getting the current leader.
It returns JSON in the structure of {'leader': leader}.

* add documentation for the /leader endpoint

docs/docs/api.md

@@ -31,6 +31,13 @@ All examples in this section assume that you've found a running leader at `chron
 When you have multiple Chronos nodes running, only one of them will be elected as the leader.
 The leader is the only node that responds to API requests, but if you attempt to talk to a non-leader your request will automatically be redirected to a leader.
 
+To get the current leader you can hit the following endpoint.
+
+* Endpoint: __/leader__
+* Method: __GET__
+* Example: `curl -L chronos-node:8080/leader`
+* Response: A JSON dict containing a single `leader` key.
+
 ## Listing Jobs
 
 * Endpoint: __/scheduler/jobs__

src/main/scala/org/apache/mesos/chronos/scheduler/api/ChronosRestModule.scala

@@ -52,6 +52,7 @@ class ChronosRestModule extends ServletModule {
     bind(classOf[MetricsServlet]).in(Scopes.SINGLETON)
     bind(classOf[LogConfigServlet]).in(Scopes.SINGLETON)
     bind(classOf[ConstraintViolationExceptionMapper]).in(Scopes.SINGLETON)
+    bind(classOf[LeaderResource]).in(Scopes.SINGLETON)
 
     serve(pingUrl).`with`(classOf[PingServlet])
     serve(metricsUrl).`with`(classOf[MetricsServlet])

src/main/scala/org/apache/mesos/chronos/scheduler/api/LeaderRestModule.scala

@@ -0,0 +1,18 @@
+package org.apache.mesos.chronos.scheduler.api
+
+import org.apache.mesos.chronos.scheduler.jobs.JobScheduler
+import javax.ws.rs.core.{MediaType, Response}
+import javax.ws.rs.{GET, Path, Produces}
+import com.google.inject.Inject
+
+
+@Path(PathConstants.getLeaderPattern)
+@Produces(Array(MediaType.APPLICATION_JSON))
+class LeaderResource @Inject()(
+  val jobScheduler: JobScheduler
+) {
+  @GET
+  def getLeader(): Response = {
+    Response.ok(Map("leader" -> jobScheduler.getLeader)).build
+  }
+}

src/main/scala/org/apache/mesos/chronos/scheduler/api/PathConstants.scala

@@ -20,6 +20,7 @@ object PathConstants {
   final val jobGraphDotPath = "dot"
   final val jobGraphCsvPath = "csv"
   final val killTaskPattern = "kill/{jobName}"
+  final val getLeaderPattern = "/leader"
 
   final val isMasterPath = "isMaster"
   final val taskBasePath = "/task"