Lambda execution model in AWS - Step-by-Step Execution

Process Flow - Lambda execution model

Event triggers Lambda

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Lambda service receives event

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Check for existing container

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Reuse container

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Run handler code inside container

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Return response or error

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Container kept warm or terminated

This flow shows how AWS Lambda receives an event, checks for a container to run the code, executes the handler, and then either keeps the container ready or ends it.

Execution Sample

AWS

def lambda_handler(event, context):
    print('Start')
    result = event['value'] * 2
    print('End')
    return result

A simple Lambda function that doubles a value from the event and prints start and end messages.

Process Table

Step	Action	Container State	Handler Execution	Output
1	Event triggers Lambda	No container	Not started	None
2	Create new container	Container created	Not started	None
3	Run handler code	Container active	Print 'Start'	None
4	Process event data	Container active	Calculate result = value * 2	None
5	Run handler code	Container active	Print 'End'	None
6	Return result	Container active	Return doubled value	Result returned
7	Container kept warm	Container ready for next event	Idle	None
8	No new event arrives	Container terminated after timeout	Stopped	None

💡 Execution stops when no new events arrive and container is terminated after idle timeout.

Status Tracker

Variable	Start	After Step 4	After Step 6	Final
event['value']	5 (example)	5	5	5
result	undefined	10	10	10
container_state	No container	Container active	Container active	Container terminated

Key Moments - 3 Insights

Why does Lambda sometimes create a new container and sometimes reuse an existing one?

Lambda reuses containers to save time on startup. If a container is available (see Step 3), it runs the handler there. If not, it creates a new container (Step 2).

What happens to the container after the Lambda function finishes running?

After returning the result (Step 6), the container stays warm for a while (Step 7) to handle new events quickly, then terminates if idle (Step 8).

Does the variable 'result' keep its value between Lambda invocations?

No, each invocation runs fresh code. Variables like 'result' are reset each time the handler runs (Step 4 and 6).

Visual Quiz - 3 Questions

Look at the execution table, at which step is the handler code first executed?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Check the 'Handler Execution' column for when 'Print Start' happens.

According to the variable tracker, what is the value of 'result' after Step 4?

A undefined

B 5

C 10

D None

💡 Hint

Look at the 'result' row under 'After Step 4' column.

If no new event arrives, what happens to the container according to the execution table?

A It stays active indefinitely

B It is kept warm for a while then terminated

C It immediately terminates after Step 6

D It restarts automatically

💡 Hint

See Steps 7 and 8 in the 'Container State' column.

Concept Snapshot

AWS Lambda execution model:
- Event triggers Lambda service
- Lambda checks for existing container
- Reuses container or creates new one
- Runs handler code inside container
- Returns result
- Container kept warm for reuse or terminated after idle
- Variables reset each invocation

Full Transcript

AWS Lambda runs code in response to events. When an event arrives, Lambda checks if a container is ready. If not, it creates one. Then it runs your handler code inside that container. After running, it returns the result. The container stays ready for a short time to handle more events quickly. If no events come, the container shuts down. Variables inside your code reset each time the handler runs.