Event triggered functions in GCP - Step-by-Step Execution

Process Flow - Event triggered functions

Event Occurs

↓

Cloud Event Detected

↓

Trigger Function

↓

Function Executes

↓

Function Completes

↓

Optional: Emit Result or Log

When an event happens in the cloud, it triggers a function to run automatically, which then executes and finishes its task.

Execution Sample

GCP

def hello_gcs(event, context):
    print(f"File {event['name']} uploaded.")

This function runs when a file is uploaded to Cloud Storage and prints the file name.

Process Table

Step	Event	Function Triggered	Action	Output
1	File uploaded to bucket 'photos'	hello_gcs	Function starts	No output yet
2	N/A	hello_gcs	Reads event data: file name 'image1.png'	No output yet
3	N/A	hello_gcs	Prints message: 'File image1.png uploaded.'	Message logged
4	N/A	hello_gcs	Function completes	Execution ends

💡 Function completes after processing the event triggered by file upload.

Status Tracker

Variable	Start	After Step 2	After Step 3	Final
event	{}	{'name': 'image1.png', 'bucket': 'photos'}	{'name': 'image1.png', 'bucket': 'photos'}	{'name': 'image1.png', 'bucket': 'photos'}
context	{}	context object	context object	context object

Key Moments - 3 Insights

Why does the function run automatically without being called explicitly?

Because the cloud platform detects the event (like a file upload) and triggers the function automatically, as shown in execution_table step 1.

What information does the function receive about the event?

The function receives event data like the file name and bucket, visible in execution_table step 2 and variable_tracker for 'event'.

Does the function run before or after the event happens?

The function runs after the event happens; the event triggers the function, as shown in concept_flow and execution_table step 1.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed by the function at step 3?

A "Function starts"

B "File image1.png uploaded."

C "File uploaded to bucket 'photos'"

D "Execution ends"

💡 Hint

Check the 'Action' and 'Output' columns at step 3 in execution_table.

At which step does the function complete its execution?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Look at the 'Action' column for 'Function completes' in execution_table.

If the event was a file deletion instead of upload, what would change in the execution_table?

A The function would not trigger at all.

B The function would print the same upload message.

C The event description in step 1 would change to file deletion.

D The function would complete earlier.

💡 Hint

Focus on the 'Event' column in execution_table step 1.

Concept Snapshot

Event triggered functions run automatically when a cloud event happens.
They receive event data and context as input.
The function executes its code and then finishes.
Common events: file upload, database change, message arrival.
No manual call needed; cloud detects and triggers.
Useful for automation and reacting to changes.

Full Transcript

Event triggered functions in cloud platforms run automatically when specific events occur, such as a file upload to storage. The cloud detects the event and triggers the function, passing event details like file name and bucket. The function executes its code, for example printing a message about the uploaded file, and then completes. This process requires no manual invocation, enabling automatic responses to cloud events.

Practice

1. What is the main purpose of an event triggered function in Google Cloud Functions?

easy

A. To store data permanently in the cloud

B. To manually start a function from the console

C. To schedule functions to run at fixed times

D. To automatically run code when a specific event occurs

Solution

1. Step 1: Understand event triggered functions

   Event triggered functions run automatically when a specific event happens, like a file upload or database change.

2. Step 2: Compare options with the definition

   Only To automatically run code when a specific event occurs describes automatic running on events. Options B and C describe manual or scheduled runs, and D is unrelated.

3. Final Answer:

   To automatically run code when a specific event occurs -> Option D

4. Quick Check:

   Event triggered function = automatic run on event [OK]

Hint: Event triggered means automatic run on event [OK]

Common Mistakes:

• Confusing event triggered with scheduled functions
• Thinking manual start is event triggered
• Mixing storage with function triggers

2. Which of the following is the correct way to specify a Cloud Function triggered by a Cloud Storage event in deployment?

easy

A. gcloud functions deploy myFunc --trigger-resource my-bucket --trigger-event google.storage.object.finalize

B. gcloud functions deploy myFunc --trigger-topic myTopic

C. gcloud functions deploy myFunc --trigger-http

D. gcloud functions deploy myFunc --trigger-schedule "every 5 minutes"

Solution

1. Step 1: Identify Cloud Storage event trigger syntax

   Cloud Storage events use --trigger-resource for the bucket and --trigger-event for the event type like object finalize.

2. Step 2: Match options with correct syntax

   gcloud functions deploy myFunc --trigger-resource my-bucket --trigger-event google.storage.object.finalize correctly uses --trigger-resource and --trigger-event for Cloud Storage. Others are for HTTP, Pub/Sub, or scheduled triggers.

3. Final Answer:

   gcloud functions deploy myFunc --trigger-resource my-bucket --trigger-event google.storage.object.finalize -> Option A

4. Quick Check:

   Cloud Storage trigger = resource + event flags [OK]

Hint: Storage triggers need resource and event flags [OK]

Common Mistakes:

• Using --trigger-http for storage events
• Confusing Pub/Sub topic triggers with storage
• Using schedule flags for event triggers

3. Consider this Cloud Function triggered by a Pub/Sub topic named 'updates'. What will happen when a message is published to 'updates'?

exports.processUpdate = (event, context) => {
  const message = Buffer.from(event.data, 'base64').toString();
  console.log(`Received message: ${message}`);
};

medium

A. The function ignores the message and does nothing

B. The function throws an error due to missing parameters

C. The function logs the decoded message content

D. The function logs the raw base64 message without decoding

Solution

1. Step 1: Understand event data decoding

   The function decodes the base64 event.data to a string and logs it.

2. Step 2: Check function behavior on message publish

   When a message is published, the function runs and logs the decoded message content.

3. Final Answer:

   The function logs the decoded message content -> Option C

4. Quick Check:

   Pub/Sub event data decoded and logged [OK]

Hint: Pub/Sub data is base64 decoded before use [OK]

Common Mistakes:

• Forgetting to decode base64 data
• Assuming function errors without checking code
• Thinking function ignores messages

4. You deployed a Cloud Function triggered by Firestore document creation, but it never runs. Which of these is the most likely cause?

medium

A. The function is missing the correct Firestore trigger path

B. The function code has a syntax error preventing deployment

C. The Cloud Function is set to trigger on HTTP instead of Firestore

D. The Firestore database is empty

Solution

1. Step 1: Check trigger configuration

   If the trigger path is incorrect, the function won't run on document creation events.

2. Step 2: Eliminate other causes

   Syntax errors prevent deployment (not silent failure). HTTP trigger won't respond to Firestore events. Firestore being empty doesn't stop triggers on new documents.

3. Final Answer:

   The function is missing the correct Firestore trigger path -> Option A

4. Quick Check:

   Wrong trigger path = no function run [OK]

Hint: Check trigger path matches Firestore event exactly [OK]

Common Mistakes:

• Assuming empty Firestore blocks triggers
• Confusing HTTP triggers with event triggers
• Ignoring trigger path correctness

5. You want to create a Cloud Function that triggers when a new file is uploaded to any folder inside a Cloud Storage bucket named 'photos'. Which trigger configuration is correct?

hard

A. Use --trigger-resource photos and --trigger-event google.storage.object.metadataUpdate

B. Use --trigger-resource photos and --trigger-event google.storage.object.finalize

C. Use --trigger-resource photos and --trigger-event google.storage.object.archive

D. Use --trigger-resource photos/* and --trigger-event google.storage.object.delete

Solution

1. Step 1: Identify correct event for new file upload

   The event google.storage.object.finalize triggers when a new object is created or overwritten.

2. Step 2: Confirm resource and event usage

   The resource is the bucket name 'photos'. Wildcards like 'photos/*' are not valid for --trigger-resource.

3. Final Answer:

   Use --trigger-resource photos and --trigger-event google.storage.object.finalize -> Option B

4. Quick Check:

   New file upload = finalize event on bucket resource [OK]

Hint: Use bucket name and finalize event for new uploads [OK]

Common Mistakes:

• Using delete or archive events instead of finalize
• Trying to use wildcards in trigger-resource
• Using metadataUpdate event which triggers on metadata changes