💥 Test Pub/Sub with the Functions Framework and Pub/Sub Emulator 🔧

3 min readFeb 4, 2020

EDIT: This article may contain technical issues. Read with a grain of salt.

Google Cloud Functions come in 2 flavors: HTTP & background functions.

You use background functions when you want to have your Cloud Function invoked indirectly in response to an event, such as:

✉️ A message on a Pub/Sub topic
📦 A change in a Cloud Storage bucket
🔥 A Firebase event

In this blogpost, we’ll talk about Pub/Sub and teach you how you can test your code using the Node Function Framework and Pub/Sub Emulator.

But before we begin, let’s level set.

What is Cloud Pub/Sub and Pub/Sub triggers?

Cloud Pub/Sub is a fully-managed scalable event ingestion and delivery system for asynchronous independent serverless applications. It’s the backbone for communication between many event-driven programs.

The Cloud Pub/Sub hexagon with a little tool icon

With the Cloud Pub/Sub Emulator, you can emulate a production environment on your computer. Read more in the docs.

With Google Cloud Functions, you can easily trigger your code to run on a new event with the Cloud Pub/Sub trigger.

Together, you can more easily build decoupled systems. Let’s try using Cloud Pub/Sub and Cloud Functions together…

Create a Google Cloud Function with a Cloud Pub/Sub Trigger

Create a Cloud Function

Let’s create a Cloud Function with a Cloud Pub/Sub trigger. Create an index.js file like this:

A simple Pub/Sub event handler.

Create a helper npm script:

{
  "scripts": {
    "start": "npx @google-cloud/functions-framework --target=helloPubSub --signature-type=event"
  }
}

Now when you run npm run start, you’ll see your Cloud Function running locally. Even though the function is running at localhost, we’re listening to Events, not raw HTTP requests.

Pub/Sub Emulator

To develop and test your application locally, you can use the Cloud Pub/Sub emulator, which provides local emulation of the production Cloud Pub/Sub environment.

Prepare your Pub/Sub Emulator

Create a new file mockPubsub.json with the following contents:

A mock Pub/Sub message

Then in a terminal session, start the emulator:

gcloud beta emulators pubsub start

Test your Function

To send an individual Cloud Pub/Sub message, use cURL:

A cURL request to localhost.

In your terminal running the Functions Framework, you’ll see a log message from your Node app with the contents of your Cloud Pub/Sub event:

Sample output from your function.

There you have it! You’ve now tested your Cloud Pub/Sub Function locally.

When you’re ready to test in production, use gcloud with a script similar to below:

gcloud pubsub topics create MY_TOPIC # Create a Pub/Sub topic
gcloud functions deploy MY_FUNCTION \
  --trigger-topic MY_TOPIC \
  --runtime nodejs10 # Deploy a Function with a Pub/Sub trigger
gcloud pubsub topics publish MY_TOPIC --message '{ "greeting": "Hello" }'

Thanks for Reading!

Learn more about these technologies in the following pages: