Region ID
The REGION_ID
is an abbreviated code that Google assigns
based on the region you select when you create your app. The code does not
correspond to a country or province, even though some region IDs may appear
similar to commonly used country and province codes. For apps created after
February 2020, REGION_ID.r
is included in
App Engine URLs. For existing apps created before this date, the
region ID is optional in the URL.
Learn more about region IDs.
Pub/Sub provides reliable, many-to-many, asynchronous messaging between applications. Publisher applications can send messages to a topic, and other applications can subscribe to that topic to receive the messages.
This document describes how to use the Cloud Client Library to send and receive Pub/Sub messages in a Java 8 app.
Prerequisites
- Follow the instructions in "Hello, World!" for Java 8 on App Engine to set up your environment and project, and to understand how App Engine Java 8 apps are structured.
- Write down and save your project ID, because you will need it to run the
sample application described in this document.
Cloning the sample app
Copy the sample apps to your local machine, and navigate to the
pubsub
directory:git clone https://github.com/GoogleCloudPlatform/java-docs-samples cd java-docs-samples/appengine-java8/pubsub
Creating a topic and subscription
Create a topic and subscription, which includes specifying the endpoint to which the Pub/Sub server should send requests:
gcloud pubsub topics create YOUR_TOPIC_NAME gcloud pubsub subscriptions create YOUR_SUBSCRIPTION_NAME \ --topic YOUR_TOPIC_NAME \ --push-endpoint \ https://YOUR_PROJECT_ID.REGION_ID.r.appspot.com/pubsub/push?token=YOUR_TOKEN \ --ack-deadline 10
Replace
YOUR_TOKEN
with a secret random token. The push endpoint uses this to verify requests.Setting environment variables
Edit the
appengine-web.xml
file to set the environment variables for your topic and verification token:Code review
The sample app uses the Cloud Client Library.The sample app uses the values you set in the
appengine-web.xml
file to configure environment variables. The push request handler uses these values to confirm that the request came from Pub/Sub and originated from a trusted source:String pubsubVerificationToken = System.getenv("PUBSUB_VERIFICATION_TOKEN");
The sample app maintains a Cloud Datastore database instance to store messages. The
PubSubPush
servlet receives pushed messages and adds them to themessageRepository
database instance:The
PubSubPublish
servlet interacts with the App Engine web app to publish new messages and display received messages:Running the sample locally
When running locally, you can use the Google Cloud CLI to provide authentication to use Google Cloud APIs. Assuming you set up your environment as described in Prerequisites, you have already run the
gcloud init
command, which provides this authentication.mvn clean package
Then set environment variables before starting your application:
export PUBSUB_VERIFICATION_TOKEN=[your-verification-token] export PUBSUB_TOPIC=[your-topic] mvn appengine:run
Simulating push notifications
The application can send messages locally, but it is not able to receive push messages locally. You can, however, simulate a push message by making an HTTP request to the local push notification endpoint. The sample includes the file
sample_message.json
.You can use
curl
or ahttpie
client to send an HTTPPOST
request:curl -H "Content-Type: application/json" -i --data @sample_message.json "localhost:8080/pubsub/push?token=[your-token]"
Or
http POST ":8080/pubsub/push?token=[your-token]" < sample_message.json
Response:
HTTP/1.1 200 OK Date: Wed, 26 Apr 2017 00:03:28 GMT Content-Length: 0 Server: Jetty(9.3.8.v20160314)
After the request completes, you can refresh
localhost:8080
and see the message in the list of received messages.Running on App Engine
To deploy the demo app to App Engine by using the
gcloud
command-line tool, you run the following command from the directory where yourpom.xml
is located:mvn package appengine:deploy -Dapp.deploy.projectId=PROJECT_ID
Replace PROJECT_ID with the ID of your Cloud project. If your
pom.xml
file already specifies your project ID, you don't need to include the-Dapp.deploy.projectId
property in the command you run.You can now access the application at
https://PROJECT_ID.REGION_ID.r.appspot.com
. You can use the form to submit messages, but there's no guarantee of which instance of your application will receive the notification. You can send multiple messages and refresh the page to see the received message.