Add Pub/Sub ack deadline example (#315)

Add Pub/Sub ack deadline example with 1 worker.

samples/subscriptions.js

@@ -340,94 +340,85 @@ function synchronousPull(projectName, subscriptionName) {
   );
   // The maximum number of messages returned for this request.
   // Pub/Sub may return fewer than the number specified.
-  const maxMessages = 3;
-  const ackDeadlineSeconds = 30;
+  const maxMessages = 1;
+  const newAckDeadlineSeconds = 30;
   const request = {
     subscription: formattedSubscription,
     maxMessages: maxMessages,
   };
-  // `messages` is a dict that stores message ack ids as keys, and message
-  // data and the processing states (true if done, false if not) as values.
-  const messages = {};
 
-  // The worker function takes a message and starts a long-running process.
+  let isProcessed = false;
+
+  // The worker function is meant to be non-blocking. It starts a long-
+  // running process, such as writing the message to a table, which may
+  // take longer than the default 10-sec acknowledge deadline.
   function worker(message) {
-    const target = Math.floor(Math.random() * 1e5);
-    console.log(`Processing "${message.message.data}" for ${target / 1e3}s...`);
+    console.log(`Processing "${message.message.data}"...`);
 
     setTimeout(() => {
       console.log(`Finished procesing "${message.message.data}".`);
-      // After the message has been processed, set its processing state to true.
-      messages[message.ackId][1] = true;
-    }, target);
+      isProcessed = true;
+    }, 30000);
   }
 
-  // The subscriber pulls a specific number of messages.
+  // The subscriber pulls a specified number of messages.
   client
     .pull(request)
     .then(responses => {
       // The first element of `responses` is a PullResponse object.
       const response = responses[0];
+      // Obtain the first message.
+      const message = response.receivedMessages[0];
 
-      // Initialize `messages` with message ackId, message data and `false` as
-      // processing state. Then, start each message in a worker function.
-      response.receivedMessages.forEach(message => {
-        messages[message.ackId] = [message.message.data, false];
-        worker(message);
-      });
-
-      let numProcessed = 0;
+      // Send the message to the worker function.
+      worker(message);
 
-      // setInterval() gets run every 10s.
+      // setInterval() checks the worker process every 5 sec.
+      // If the pre-set ack deadline is n sec, it is best to
+      // set the interval to be every (n/2) sec.
       const interval = setInterval(function() {
-        // Every 10s, we do a check on the processing states of the messages.
-        Object.keys(messages).forEach(ackId => {
-          if (messages[ackId][1]) {
-            // If the processing state for a particular message is true,
-            // We will ack the message.
-            const ackRequest = {
-              subscription: formattedSubscription,
-              ackIds: [ackId],
-            };
-
-            client.acknowledge(ackRequest).catch(err => {
+        // If the message has been processed..
+        if (isProcessed) {
+          const ackRequest = {
+            subscription: formattedSubscription,
+            ackIds: [message.ackId],
+          };
+
+          //..acknowledges the message.
+          client
+            .acknowledge(ackRequest)
+            .then(() => {
+              console.log(`Acknowledged: "${message.message.data}".`);
+              // Exit after the message is acknowledged.
+              clearInterval(interval);
+              console.log(`Done.`);
+            })
+            .catch(err => {
               console.error(err);
             });
-
-            console.log(`Acknowledged: "${messages[ackId][0]}".`);
-
-            // Increment numProcessed by 1.
-            numProcessed += 1;
-
-            // Remove this message from `messages`.
-            delete messages[ackId];
-          } else {
-            // If the processing state of a particular message remains false,
-            // we will modify its ack deadline.
-            const modifyAckRequest = {
-              subscription: formattedSubscription,
-              ackIds: [ackId],
-              ackDeadlineSeconds: ackDeadlineSeconds,
-            };
-
-            client.modifyAckDeadline(modifyAckRequest).catch(err => {
+        } else {
+          // If the message is not yet processed..
+          const modifyAckRequest = {
+            subscription: formattedSubscription,
+            ackIds: [message.ackId],
+            ackDeadlineSeconds: newAckDeadlineSeconds,
+          };
+
+          //..reset its ack deadline.
+          client
+            .modifyAckDeadline(modifyAckRequest)
+            .then(() => {
+              console.log(
+                `Reset ack deadline for "${
+                  message.message.data
+                }" for ${newAckDeadlineSeconds}s.`
+              );
+            })
+            .catch(err => {
               console.error(err);
             });
-
-            console.log(
-              `Reset ack deadline for "${
-                messages[ackId][0]
-              }" for ${ackDeadlineSeconds}s.`
-            );
-          }
-
-          // If all messages have been processed, we clear out of the interval.
-          if (numProcessed === response.receivedMessages.length) {
-            clearInterval(interval);
-            console.log(`Done.`);
-          }
-        });
-      }, 10000);
+        }
+      }, 5000);
     })
     .catch(err => {
       console.error(err);