Cloud Functions with Pub/Sub triggers in GCP - Step-by-Step Execution

Process Flow - Cloud Functions with Pub/Sub triggers

Pub/Sub Topic receives message

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Trigger Cloud Function

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Cloud Function executes code

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Process message data

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Function completes and acknowledges message

When a message is published to a Pub/Sub topic, it triggers the Cloud Function to run and process that message.

Execution Sample

GCP

def pubsub_trigger(event, context):
    message = event['data']
    print(f"Received message: {message}")

This Cloud Function runs when a Pub/Sub message arrives and prints the message data.

Process Table

Step	Event Received	Message Data	Action	Output
1	Message published to topic	SGVsbG8gd29ybGQ=	Trigger function	Function starts
2	Function triggered	SGVsbG8gd29ybGQ=	Decode message	Decoded to 'Hello world'
3	Processing message	Hello world	Print message	Output: Received message: Hello world
4	Function completes	Hello world	Acknowledge message	Message acknowledged
5	No new messages	N/A	Wait for next message	Idle

💡 No new messages, function waits for next trigger

Status Tracker

Variable	Start	After Step 2	After Step 3	Final
event['data']	SGVsbG8gd29ybGQ=	SGVsbG8gd29ybGQ=	Hello world	Hello world
message	undefined	SGVsbG8gd29ybGQ=	Hello world	Hello world

Key Moments - 3 Insights

Why does the message data look like random letters before decoding?

The message data is base64 encoded when received (see execution_table step 2). It must be decoded to get the original text.

What triggers the Cloud Function to run?

Publishing a message to the Pub/Sub topic triggers the function immediately (execution_table step 1).

What happens if no messages are published?

The function stays idle waiting for the next message (execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution table, what is the message data after decoding at step 2?

A SGVsbG8gd29ybGQ=

B Hello world

C Received message

D N/A

💡 Hint

Check the 'Message Data' column at step 2 in the execution_table

At which step does the function acknowledge the message?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column for message acknowledgement in the execution_table

If no messages are published, what is the function's state according to the execution table?

A Function starts

B Idle, waiting for next message

C Processing message

D Message acknowledged

💡 Hint

Refer to the last row in the execution_table under 'Output'

Concept Snapshot

Cloud Functions with Pub/Sub triggers:
- Pub/Sub topic receives message
- Message triggers Cloud Function
- Function decodes and processes message
- Function acknowledges message
- Waits idle if no messages

Full Transcript

When a message is published to a Pub/Sub topic, it triggers the associated Cloud Function to run. The function receives the event containing the message data, which is base64 encoded. The function decodes this data to get the original message text. It then processes the message, for example by printing it. After processing, the function acknowledges the message to Pub/Sub. If no messages are published, the function remains idle, waiting for the next trigger.