Add a ping-cronjob to keep instance alive.

Use a cron-job to prevent the AppEngine instance from going to sleep.

This should keep requests quick since the JVM is not being restarted often
on new requests and all caches stay intact.

This addresses issue #1.

appengine/src/main/java/org/retrostore/MainServlet.java

@@ -43,6 +43,7 @@
 import org.retrostore.request.FaviconRequest;
 import org.retrostore.request.ImportRpkRequest;
 import org.retrostore.request.LoginRequest;
+import org.retrostore.request.PingRequest;
 import org.retrostore.request.PolymerRequest;
 import org.retrostore.request.PublicSiteRequest;
 import org.retrostore.request.ReportAppRequest;
@@ -114,6 +115,7 @@ static class Modules {
 
   private static List<Request> createRequests(Modules m) {
     return ImmutableList.of(new FaviconRequest(m.defaultResourceLoader),
+        new PingRequest(),
         new PublicSiteRequest(m.defaultResourceLoader),
         new ReportAppRequest(m.defaultResourceLoader, m.appManagement, m.imgServWrapper,
             m.mailService),

appengine/src/main/java/org/retrostore/request/PingRequest.java

@@ -0,0 +1,36 @@
+/*
+ *  Copyright 2018, Sascha Häberling
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *         http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+package org.retrostore.request;
+
+import org.retrostore.data.user.UserService;
+
+/**
+ * We use ping requests to check that the AppEngine instance is alive. This is the class that
+ * handles these requests.
+ */
+public class PingRequest implements Request {
+  private static final String PATH_SERVE = "/ping";
+
+  @Override
+  public boolean serveUrl(RequestData requestData, Responder responder, UserService userService) {
+    if (!PATH_SERVE.equals(requestData.getUrl())) {
+      return false;
+    }
+    responder.respond("OK", Responder.ContentType.PLAIN);
+    return true;
+  }
+}

appengine/src/main/webapp/WEB-INF/cron.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~  Copyright 2018, Sascha Häberling
+  ~
+  ~  Licensed under the Apache License, Version 2.0 (the "License");
+  ~  you may not use this file except in compliance with the License.
+  ~  You may obtain a copy of the License at
+  ~
+  ~         http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~  Unless required by applicable law or agreed to in writing, software
+  ~  distributed under the License is distributed on an "AS IS" BASIS,
+  ~  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  ~  See the License for the specific language governing permissions and
+  ~  limitations under the License.
+  -->
+
+<cronentries>
+    <cron>
+        <url>/ping</url>
+        <description>Keep-alive for the AppEngine instance.</description>
+        <schedule>every 1 minutes</schedule>
+    </cron>
+</cronentries>