twitter-server: Add a readiness endpoint to query for accepting traffic.

Problem
We need an endpoint where we can rely on the HTTP status code to tell us if the server is ready to accept traffic.

Solution
Add a new endpoint called /ready that returns a 503 when the server is warming up and a 200 when the server
 finishes warming up and is ready for traffic.

Differential Revision: https://phabricator.twitter.biz/D1240266

Author: Benjamin Erichsen

server/src/main/scala/com/twitter/server/Lifecycle.scala

@@ -84,10 +84,12 @@ object Lifecycle {
   private[Lifecycle] object Warmup {
 
     /**
-     * Initialize warmup code. Ensures that the /health endpoint will not return on "OK" response.
+     * Initialize warmup code. Ensures that the /health endpoint will not return an "OK" response.
      */
-    def initializeWarmup(): Unit =
+    def initializeWarmup(): Unit = {
       HttpMuxer.addHandler(Route("/health", new ReplyHandler("warming up\n")))
+      HttpMuxer.addHandler(Route("/ready", new ReadinessHandler(isReady = false)))
+    }
 
     /**
      * Prebind warmup code. Used for warmup tasks that we want to run before we
@@ -98,8 +100,10 @@ object Lifecycle {
     /**
      * The service is bound to a port and warmed up, announce health.
      */
-    def warmupComplete(): Unit =
+    def warmupComplete(): Unit = {
       HttpMuxer.addHandler(Route("/health", new ReplyHandler("OK\n")))
+      HttpMuxer.addHandler(Route("/ready", new ReadinessHandler(isReady = true)))
+    }
 
   }
 

server/src/main/scala/com/twitter/server/handler/ReadinessHandler.scala

@@ -0,0 +1,20 @@
+package com.twitter.server.handler
+
+import com.twitter.finagle.Service
+import com.twitter.finagle.http.Request
+import com.twitter.finagle.http.Response
+import com.twitter.finagle.http.Status
+import com.twitter.io.Buf
+import com.twitter.server.util.HttpUtils.newOk
+import com.twitter.server.util.HttpUtils.newResponse
+import com.twitter.util.Future
+
+class ReadinessHandler(isReady: Boolean) extends Service[Request, Response] {
+  def apply(req: Request): Future[Response] =
+    if (isReady) newOk("Ready")
+    else
+      newResponse(
+        status = Status.ServiceUnavailable,
+        contentType = "text/plain;charset=UTF-8",
+        content = Buf.Utf8("Not ready. Warming up."))
+}