Lambda with API Gateway pattern in AWS - Step-by-Step Execution

Process Flow - Lambda with API Gateway pattern

Client sends HTTP request

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API Gateway receives request

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API Gateway triggers Lambda function

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Lambda function executes code

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Lambda returns response to API Gateway

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API Gateway sends HTTP response to client

This flow shows how a client request goes through API Gateway to trigger a Lambda function, which processes and returns a response back through API Gateway to the client.

Execution Sample

AWS

import json

def lambda_handler(event, context):
    name = event.get('queryStringParameters', {}).get('name', 'World')
    return {
        'statusCode': 200,
        'body': json.dumps({'message': f'Hello, {name}!'})
    }

A simple Lambda function triggered by API Gateway that reads a 'name' query parameter and returns a greeting message.

Process Table

Step	Action	Input	Lambda Processing	Output
1	Client sends HTTP GET request	GET /?name=Alice	N/A	Request received by API Gateway
2	API Gateway triggers Lambda	Event with queryStringParameters={'name':'Alice'}	Reads 'name' parameter	Prepares greeting message
3	Lambda executes	{'queryStringParameters': {'name': 'Alice'}}	Creates message 'Hello, Alice!'	{'statusCode': 200, 'body': '{"message": "Hello, Alice!"}'}
4	API Gateway sends response	Lambda output	N/A	HTTP 200 with body '{"message": "Hello, Alice!"}' sent to client
5	Client receives response	HTTP 200 with body	N/A	Displays message 'Hello, Alice!'
6	Client sends HTTP GET request	GET / (no name)	N/A	Request received by API Gateway
7	API Gateway triggers Lambda	Event with queryStringParameters=None	Uses default 'World'	Prepares greeting message
8	Lambda executes	{'queryStringParameters': None}	Creates message 'Hello, World!'	{'statusCode': 200, 'body': '{"message": "Hello, World!"}'}
9	API Gateway sends response	Lambda output	N/A	HTTP 200 with body '{"message": "Hello, World!"}' sent to client
10	Client receives response	HTTP 200 with body	N/A	Displays message 'Hello, World!'
11	No more requests	N/A	N/A	Execution ends

💡 Execution stops after client receives response and no further requests are made.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 7	After Step 8	Final
event	N/A	{'queryStringParameters': {'name': 'Alice'}}	{'queryStringParameters': {'name': 'Alice'}}	{'queryStringParameters': None}	{'queryStringParameters': None}	N/A
name	N/A	'Alice'	'Alice'	'World'	'World'	N/A
response	N/A	N/A	{'statusCode': 200, 'body': '{"message": "Hello, Alice!"}'}	N/A	{'statusCode': 200, 'body': '{"message": "Hello, World!"}'}	N/A

Key Moments - 3 Insights

Why does the Lambda function use 'World' when no 'name' parameter is provided?

When the event's 'queryStringParameters' is None or missing 'name', the code uses a default value 'World' as shown in steps 7 and 8 in the execution_table.

How does API Gateway pass the HTTP request data to Lambda?

API Gateway converts the HTTP request into an event object with details like 'queryStringParameters' and passes it to Lambda, as seen in steps 2 and 7.

What happens if Lambda returns a statusCode other than 200?

API Gateway will forward that status code in the HTTP response to the client, affecting how the client interprets the result. This is implied in step 4.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What is the value of 'name' inside Lambda?

A 'Alice'

B 'World'

C None

D 'Bob'

💡 Hint

Check the 'Lambda Processing' column at step 3 in the execution_table.

At which step does the Lambda function use the default name 'World'?

A Step 3

B Step 8

C Step 2

D Step 5

💡 Hint

Look for where 'Uses default "World"' appears in the execution_table.

If the client sends a request with '?name=Bob', what will the Lambda's response message be?

A 'Hello, Alice!'

B 'Hello, World!'

C 'Hello, Bob!'

D An error message

💡 Hint

Refer to how the Lambda reads the 'name' parameter in the variable_tracker and execution_table.

Concept Snapshot

Lambda with API Gateway pattern:
- API Gateway receives HTTP request
- Passes event to Lambda function
- Lambda reads event data (e.g., query parameters)
- Lambda returns response with statusCode and body
- API Gateway sends HTTP response to client
- Default values used if parameters missing

Full Transcript

This visual execution shows how a client sends an HTTP request to API Gateway, which triggers a Lambda function. The Lambda function reads the query parameter 'name' from the event. If 'name' is missing, it defaults to 'World'. The Lambda returns a JSON message greeting the name. API Gateway sends this response back to the client. The execution table traces each step from request to response, showing variable values and actions. Key moments clarify default values and data passing. The quiz tests understanding of variable values and flow.