Event triggers for Lambda in AWS - Step-by-Step Execution

Process Flow - Event triggers for Lambda

Event Occurs

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Event Source Detects

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Trigger Lambda Function

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Lambda Executes Code

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Lambda Returns Result

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Process Complete

When an event happens, the event source detects it and triggers the Lambda function to run its code and return a result.

Execution Sample

AWS

1. S3 bucket receives a file upload
2. S3 triggers Lambda
3. Lambda processes the file
4. Lambda returns success

This example shows how an S3 upload event triggers a Lambda function to process the file.

Process Table

Step	Event Source	Event Detected	Lambda Triggered	Lambda Action	Result
1	S3 Bucket	File uploaded	Yes	Start Lambda function	Lambda starts execution
2	Lambda	Running	N/A	Process file content	File processed
3	Lambda	Execution complete	N/A	Return success	Success response sent
4	System	No new event	No	No action	Waiting for next event

💡 No new event detected, so Lambda is not triggered again.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Event Detected	No	Yes	Yes	Yes	No
Lambda State	Idle	Triggered	Running	Completed	Idle
Result	None	None	Processing	Success	Success

Key Moments - 3 Insights

Why does Lambda only run after the event source detects an event?

Because Lambda functions are event-driven, they only start when the event source signals an event, as shown in execution_table step 1.

What happens if no event occurs?

Lambda stays idle and does not run, as shown in execution_table step 4 where no new event means no trigger.

Does Lambda run continuously or only on events?

Lambda runs only when triggered by events, not continuously, as seen by the state changes in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution table, at which step does Lambda start processing the file?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Lambda Action' column in execution_table row for Step 2.

According to variable_tracker, what is the Lambda State after Step 3?

A Idle

B Running

C Completed

D Triggered

💡 Hint

Look at the 'Lambda State' row under 'After Step 3' in variable_tracker.

If no file is uploaded, what will happen to the 'Event Detected' variable?

A It stays No

B It becomes Processing

C It becomes Yes

D It becomes Success

💡 Hint

Refer to the 'Event Detected' row in variable_tracker and the exit_note in execution_table.

Concept Snapshot

Event triggers start Lambda functions automatically when something happens.
Common triggers: file upload, HTTP request, database change.
Lambda runs code only after trigger.
No event means no Lambda run.
This makes Lambda efficient and cost-effective.

Full Transcript

This visual execution shows how AWS Lambda functions are triggered by events. When an event occurs, such as a file upload to an S3 bucket, the event source detects it and triggers the Lambda function. The Lambda function then runs its code to process the event and returns a result. If no event happens, Lambda remains idle and does not run. This event-driven model ensures Lambda runs only when needed, saving resources.