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How to subscribe to a Google Cloud Pub/Sub topic and process messages?

This guide details how to subscribe to a Google Cloud Pub/Sub topic (specifically the assumed 'cloud and google' topic) and process the messages published to it. We'll cover various subscription types, setup steps, and best practices for reliable and efficient message handling. Please adapt the 'cloud and google' topic to your actual topic name where required.

Understanding Google Cloud Pub/Sub

Google Cloud Pub/Sub is a fully managed, real-time messaging service that allows you to send and receive messages between independent applications. It's commonly used for asynchronous communication, event notification, and streaming data ingestion. Key concepts include:

• Topic: A named resource to which publishers send messages. Think of it as a channel.
• Message: The data sent by a publisher.
• Subscription: A named resource representing a stream of messages from a single topic, to be delivered to the subscribing application.
• Publisher: An application that sends messages to a topic.
• Subscriber: An application that receives messages from a subscription.

Prerequisites

Before subscribing to a Pub/Sub topic, ensure you have the following:

• Google Cloud Project: A Google Cloud project with Pub/Sub enabled.
• Service Account: A service account with appropriate permissions (roles/pubsub.subscriber and roles/pubsub.viewer are typically required). For testing locally, using your Google Cloud SDK configured account works, but service accounts are recommended for production.
• Google Cloud SDK (gcloud) or Client Libraries: The Google Cloud SDK or relevant client libraries installed for your chosen programming language (e.g., Python, Java, Go, Node.js). We will demonstrate examples using Python.

Step-by-Step Guide: Subscribing and Processing Messages

1. Authentication

Authenticating is crucial. Here's how you authenticate based on where your application runs:

• Running on Google Cloud (Compute Engine, Cloud Functions, App Engine, GKE): These environments often handle authentication automatically via the metadata server. The service account associated with the environment should already be configured.
• Running Locally (Development):
  1. Install the Google Cloud SDK (gcloud).
  2. Authenticate with gcloud: gcloud auth application-default login. This uses your Google account credentials and saves them for local development purposes. Ensure the selected account has sufficient permissions.
  3. Alternatively, you can explicitly set the GOOGLE_APPLICATION_CREDENTIALS environment variable to the path of your service account key file (JSON). This is the more secure and recommended option for production deployments when not on a Google Cloud resource.
• Running Outside of Google Cloud (e.g., on-premise servers): You *must* use a service account key file (JSON) and set the GOOGLE_APPLICATION_CREDENTIALS environment variable. Never embed credentials directly in your code.

2. Choose a Subscription Type: Pull vs. Push

Pub/Sub offers two main types of subscriptions:

• Pull Subscription: The subscriber application actively pulls messages from Pub/Sub. This offers more control over message processing and throttling. You control when and how often to pull messages.
• Push Subscription: Pub/Sub pushes messages to a designated endpoint (typically an HTTP webhook) that your application exposes. This can simplify message delivery and be more efficient when the application is continuously listening for messages. The responsibility is on the subscriber to quickly acknowledge (ACK) or negatively acknowledge (NACK) messages.

For 'cloud and google' topic and the examples below, we'll start with a Pull Subscription example.

3. Creating a Subscription

You can create a subscription using the Google Cloud Console, the gcloud command-line tool, or the Pub/Sub API directly. Let's use gcloud:

 gcloud pubsub subscriptions create my-subscription --topic=cloud-and-google-topic --ack-deadline=20 #--topic=YOUR_TOPIC_NAME_HERE
 

Replace cloud-and-google-topic with the actual name of your Pub/Sub topic and my-subscription with the desired name for your subscription. ack-deadline defines the time window within which a subscriber must acknowledge a message. If it doesn't, Pub/Sub will re-deliver the message.

For push subscriptions, use the following:

 gcloud pubsub subscriptions create --topic=cloud-and-google-topic \
 --push-endpoint=https://your-application.example.com/messages \
 --ack-deadline=20
 

The --push-endpoint flag specifies the HTTPS endpoint where Pub/Sub should push messages.

4. Implementing the Subscriber Application (Pull Subscription - Python Example)

Here's a Python example using the Google Cloud Pub/Sub client library to pull messages from the "cloud and google" topic.

 from google.cloud import pubsub_v1

 # Replace with your project ID and subscription ID
 project_id = "your-project-id" # Or retrieve it from env vars: os.environ["GOOGLE_CLOUD_PROJECT"]
 subscription_id = "my-subscription"

 subscriber = pubsub_v1.SubscriberClient()
 subscription_path = subscriber.subscription_path(project_id, subscription_id)

 def callback(message: pubsub_v1.subscriber.message.Message):
 """Receives messages from a Pub/Sub subscription."""
 print(f"Received message: {message.data.decode()}")
 # Add your message processing logic here.
 # For example, parse JSON, update a database, etc.

 message.ack() # Acknowledge the message to prevent re-delivery


 streaming_pull_future = subscriber.subscribe(subscription_path, callback=callback)
 print(f"Listening for messages on {subscription_path}...\n")

 # Keep the script running to receive messages. This could use threading
 # to run as a background task in a more complex application.

 try:
 streaming_pull_future.result() # Blocks until the subscription is terminated.
 except TimeoutError:
 streaming_pull_future.cancel() # Trigger the shutdown.
 streaming_pull_future.result() # Block until the shutdown is complete.
 

Explanation:

• Import Libraries: Import the necessary pubsub_v1 library.
• Configuration: Set your project_id and subscription_id. Replace "your-project-id" with your actual Google Cloud Project ID and "my-subscription" with your susbcription ID from the gcloud pubsub subscriptions create command.
• Create Subscriber Client: Create a SubscriberClient instance.
• Subscription Path: Construct the full subscription path using the project and subscription IDs.
• Callback Function: Define a callback function that will be executed for each received message. Inside the callback:
  
  • message.data contains the message payload (as bytes). Decode it (e.g., message.data.decode() for UTF-8 encoded messages).
  • Add your message processing logic. This could involve parsing JSON, updating a database, triggering other actions, etc.
  • Crucially, call message.ack() to acknowledge that the message has been processed successfully. If you *don't* acknowledge the message within the ack_deadline, Pub/Sub will re-deliver it.
  
  
• Subscribe: Call the subscriber.subscribe() method, passing the subscription path and the callback function. This initiates the asynchronous message pulling.
• Keep Application Running: The streaming_pull_future.result() call blocks the main thread, allowing the application to continue receiving messages until the subscription is cancelled. For long-running services, use asyncio to prevent blocking operations or consider using other threading strategies.. Proper exception handling using try...except blocks and gracefully shutting down is essential.

Before running the script, ensure you have installed the Google Cloud Pub/Sub client library:

 pip install google-cloud-pubsub
 

5. Implementing the Subscriber Application (Push Subscription)

For push subscriptions, your application must expose an HTTPS endpoint that can receive messages from Pub/Sub. Here's a basic Python example using Flask:

 from flask import Flask, request, abort
 import json

 app = Flask(__name__)

 @app.route('/messages', methods=['POST'])
 def receive_message():
 if request.method == 'POST':
 envelope = request.get_json()
 if not envelope:
 abort(400, 'No message received')

 if not isinstance(envelope, dict) or 'message' not in envelope:
 abort(400, 'Invalid message format')

 message = envelope['message']
 name = message['data'] if isinstance(message, dict) and 'data' in message else None

 if not name:
 abort(400, 'Data missing from payload')

 print(f"Received message: {name}")
 # Acknowledge the message
 return ('', 204)

 else:
 abort(405)

 if __name__ == '__main__':
 app.run(debug=True, host='0.0.0.0', port=8080) #Production: consider gunicorn + https.

 

Explanation:

• Flask App: Sets up a Flask web application. For a robust production environment consider a production grade webserver, such as Gunicorn, rather than Flask's built-in development webserver.
• Message Route: Defines a /messages route that listens for POST requests. This is where Pub/Sub will push messages. HTTPS is mandatory for push subscriptions (TLS required) to prevent message interception.
• Message Processing: The function extracts the message data from the request and performs the necessary processing. Ensure input validation. The message['data'] portion represents the Base64-encoded message body; proper decoding will be necessary in the general case, adding more robustness by adding error checking as well as considering alternate payload formats to avoid crashes caused by corrupted payloads.
• Acknowledgement: Returns a 204 No Content status code to acknowledge the message. Returning anything other than 200 OK, 204 No Content or 500 Internal Server Error can lead to message redelivery attempts by Pub/Sub.Returning 500 Internal Server Error signifies processing failure and enables potential message retry behavior defined by the subscription settings.
  Ensure proper security by adding authorization layers on top of the endpoint handling in addition to request and input sanitation/validation in the same way an API endpoint is usually defended against security vulnerabilities and failures to meet the constraints that dictate correct behaviour and correct states for resources. The handling above should represent "happy case only" , and a real-world handling has far greater depth to manage both security and errors caused either by user malfunction and corrupted message format payloads and by external errors in connected dependency systems like remote databanks for information consistency or logging malfunctions/malware infection detection strategies, by providing layers on the above described code by extending or adding further blocks that provide for that functionality through custom additions tailored to security specifics on authorization, intrusion prevention and consistency checking, the handling gets better protection/prevention of failures that would normally cripple the above scenario or represent a breach against intrusion scenarios. Proper use of HTTPS protects communications for intercept tampering/read access scenarios , the addition to endpoint-targeted denial-of-service or API specific intrusions protection adds greater protection , coupled with correct handling to ensure the communication is secure.
  
  Consider all aspects related to Security: TLS transport with correctly updated Certificate Authorities certificates lists/validation rules applied for security ; implement API Key based /OAUTH/other token -based request validation and proper use.
  

5.5 Data Encoding and Message Schema Considerations

When working with Pub/Sub, especially across different systems and languages, think about how you serialize and deserialize data within the message body. The message.data in the Pull Subscription callback and Push Endpoint handler carries data that often needs conversion from its raw binary representation to human-readable formats, or from a machine friendly serialization form.

• Serialization Format: Choosing a standard serialization format (JSON, Protocol Buffers, Avro) makes your message easier to handle, especially in heterogeneous environments (different languages, systems). JSON is often the simplest choice.
  import json
try:
data = json.loads(message.data.decode('utf-8')) # From bytes to Python dict/list
except json.JSONDecodeError as e:
print(f"Error decoding JSON: {e}")
# Log and potentially nack the message or trigger error-handling workflow.

• Data Encoding: Pay attention to character encoding when decoding the data from bytes to string. UTF-8 is a widely used and reliable choice:
• Versioning: If the message format changes over time, include a version number in the message body. Subscribers can then adapt their processing logic based on the message version.

6. Testing Your Subscriber

You can publish test messages to your "cloud and google" topic using the Google Cloud Console, the gcloud tool, or the Pub/Sub API.

Using gcloud:

 gcloud pubsub topics publish cloud-and-google-topic --message="Hello, Pub/Sub from gcloud"
 

Check the output of your subscriber application to verify that it receives and processes the messages correctly.

Best Practices

• Error Handling: Implement robust error handling in your subscriber application. Log errors, handle exceptions gracefully, and consider negative acknowledgements (NACKs) or retries for failed message processing in scenarios where appropriate.
• Message Ordering: Pub/Sub provides best-effort message ordering. If strict message order is critical, design your application accordingly (e.g., include sequence numbers in the messages or utilize single publisher).
• Scalability and Throughput: Pub/Sub is highly scalable. Tune your subscriber application to handle the expected message volume. Consider using multiple subscribers or increasing the concurrency of your message processing threads/processes.
• Monitoring and Logging: Monitor your Pub/Sub usage and the performance of your subscriber application. Use Cloud Monitoring and Cloud Logging to track metrics and diagnose issues. Track acknowledgements, processing times and errors using your existing solutions or Cloud Trace for the cases when requests propagate.
• Acknowledgement Deadlines: Choose an appropriate ack_deadline. If your subscriber needs more time to process messages, increase the ack_deadline when creating the subscription. Shorter durations for delivery may suit best cases where transient outages/downtimes can trigger quicker processing, even using smaller acknowledgement timeout windows for less performance delays if/when delivery failures may occur due network failure in zones/across cloud providers, causing more efficient error correction and better system operation to ensure system operation resilience at high velocity and traffic intensity and speed of propagation, delivery as dictated from business specifics when failure cases matter. For data sensitive/availability/high demand business requirements may be of paramount importance. When high resilience matters but velocity propagation is not as important can enable delays instead which are acceptable while in exchange ensuring better processing of delivered units when processing completion is required without the constraint being for as high delivery speed , or if an acceptable tradeoff involves more propagation and message passing-across nodes which might be useful from system processing or reliability or even fault tollerance / availability , it dictates design of acknowledment timelines being either more agressively timed than high propagation ones - for the case being , design and timeline decisions involve design factors being key between resilience availability or tolerance , against throughput performance and network and node tolerance for errors to propagation completion and state coherence against communication interruption, either internal nodes of cloud provision against the end target services and all the infrastructure layers composing/transacting with these.
• Dead Letter Queues (DLQ): Consider configuring a Dead Letter Queue to handle messages that repeatedly fail to be processed. This helps prevent messages from being lost and provides a mechanism to investigate and resolve processing issues.
  

Troubleshooting

• Permission Issues: Verify that your service account has the necessary Pub/Sub permissions.
• Connectivity Issues: Ensure that your subscriber application can connect to the Pub/Sub API endpoint. Check network configuration and firewall rules.
• Acknowledgement Errors: If you see messages being re-delivered repeatedly, investigate why acknowledgements are failing. Check for exceptions during message processing or insufficient ack_deadline.
• Resource Exhaustion: Monitor the resource usage of your subscriber application. Increase resources (e.g., CPU, memory) if necessary. Check code with external dependency calls as if there would be errors calling those dependencies this translates to failure to properly operate. Handle failures early to trigger NACK , ensuring no deadlock may occur, due any service being used. Validate correctness on state transition calls.

Conclusion

Subscribing to a Google Cloud Pub/Sub topic and processing messages involves understanding different subscription types, setting up the necessary infrastructure, implementing a subscriber application, and following best practices. This guide provides a comprehensive overview to help you get started with Pub/Sub. By carefully choosing the right subscription type, handling errors gracefully, and optimizing your application, you can build reliable and scalable messaging solutions using Google Cloud Pub/Sub to support processing from subscribed events propagated for all intended audiences following Google's high availability rules. By properly protecting, verifying and maintaining both network access layers through correct firewalls and loadbalancers (with both TCP checks plus validation-access based ones), one gains higher operation of cloud assets by maintaining all components safe to operate, which would in turn trigger/propagate/operate those components reliably to maintain correctness/availability requirements by protecting against unwanted threats to high business demands and data retention operation security guidelines that must be correctly applied for security.

Pub/Sub Subscribe Messages Subscription 

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