Add simple healthcheck (#24)

Fixes #23

Author: slominskir

src/main/java/org/jlab/epics2web/controller/Healthcheck.java

@@ -0,0 +1,101 @@
+package org.jlab.epics2web.controller;
+
+import jakarta.json.Json;
+import jakarta.json.JsonArrayBuilder;
+import jakarta.json.JsonObjectBuilder;
+import jakarta.servlet.ServletException;
+import jakarta.servlet.annotation.WebServlet;
+import jakarta.servlet.http.HttpServlet;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.Map;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+import org.jlab.epics2web.Application;
+import org.jlab.epics2web.epics.ChannelManager;
+import org.jlab.epics2web.epics.ChannelMonitor;
+
+/**
+ * Controller for Healthcheck page. Return 200 OK, for healthy Return 503 Service Unavailable for
+ * unhealthy (or ANYTHING non 200-299).
+ *
+ * @author slominskir
+ */
+@WebServlet(
+    name = "Healthcheck",
+    urlPatterns = {"/healthcheck"})
+public class Healthcheck extends HttpServlet {
+
+  private final ChannelManager channelManager = Application.channelManager;
+  private static final Logger LOGGER = Logger.getLogger(Healthcheck.class.getName());
+
+  /**
+   * Handles the HTTP <code>GET</code> method.
+   *
+   * @param request servlet request
+   * @param response servlet response
+   * @throws ServletException if a servlet-specific error occurs
+   * @throws IOException if an I/O error occurs
+   */
+  @Override
+  protected void doGet(HttpServletRequest request, HttpServletResponse response)
+      throws ServletException, IOException {
+
+    boolean healthy = true;
+
+    Map<String, ChannelMonitor> monitorMap = channelManager.getMonitorMap();
+
+    Instant now = Instant.now();
+
+    JsonArrayBuilder unhealthyChannelArray = Json.createArrayBuilder();
+
+    for (Map.Entry<String, ChannelMonitor> entry : monitorMap.entrySet()) {
+      String pv = entry.getKey();
+      ChannelMonitor monitor = entry.getValue();
+
+      // If never an update, then we assume PV doesn't exist.  Might miss some cases.  Better than
+      // nothing health check!
+      if (monitor.getLastTimestamp() != null) {
+        Instant lastTimestamp = monitor.getLastTimestamp().toInstant();
+        Duration duration = Duration.between(now, lastTimestamp);
+        long differenceInSeconds = Math.abs(duration.toSeconds());
+
+        if (monitor.getState() != ChannelMonitor.MonitorState.CONNECTED
+            && (differenceInSeconds > 30)) {
+          healthy = false;
+          JsonObjectBuilder unhealthyChannel = Json.createObjectBuilder();
+          unhealthyChannel.add("name", pv);
+          unhealthyChannel.add(
+              "disconnected_minutes", String.format("%.1f", differenceInSeconds / 60.0));
+          unhealthyChannelArray.add(unhealthyChannel);
+        }
+      }
+    }
+
+    response.setContentType("application/json");
+
+    PrintWriter pw = response.getWriter();
+
+    response.setStatus(HttpServletResponse.SC_OK);
+
+    if (!healthy) {
+      response.setStatus(HttpServletResponse.SC_SERVICE_UNAVAILABLE);
+    }
+
+    String jsonStr = unhealthyChannelArray.build().toString();
+
+    pw.write(jsonStr);
+
+    pw.flush();
+
+    boolean error = pw.checkError();
+
+    if (error) {
+      LOGGER.log(Level.SEVERE, "PrintWriter Error");
+    }
+  }
+}

src/main/java/org/jlab/epics2web/epics/ChannelMonitor.java

@@ -62,7 +62,7 @@ public class ChannelMonitor implements Closeable {
   private final ExecutorService callbackExecutor;
   private final String pv;
 
-  enum MonitorState {
+  public enum MonitorState {
     CONNECTING,
     CONNECTED,
     DISCONNECTED;