Lambda integration in AWS - Step-by-Step Execution

Process Flow - Lambda integration

Client Request

↓

API Gateway receives request

↓

API Gateway triggers Lambda function

↓

Lambda function executes code

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Lambda returns response

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API Gateway sends 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 to the client.

Execution Sample

AWS

1. Client sends HTTP request to API Gateway
2. API Gateway triggers Lambda function
3. Lambda runs code and returns result
4. API Gateway sends response to client

This sequence shows the steps of Lambda integration with API Gateway handling a client request.

Process Table

Step	Component	Action	Input	Output
1	Client	Sends HTTP request	GET /hello	Request sent to API Gateway
2	API Gateway	Receives request	GET /hello	Triggers Lambda function
3	Lambda	Executes function code	Event with request data	Returns response payload
4	API Gateway	Receives Lambda response	Response payload	Sends HTTP response to client
5	Client	Receives response	HTTP 200 OK with message	Displays response

💡 Execution stops after client receives the response from API Gateway.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Request	None	HTTP GET /hello	Event passed to Lambda	Response payload from Lambda	HTTP 200 OK response
Response	None	None	Lambda response generated	API Gateway response ready	Displayed to client

Key Moments - 3 Insights

Why does API Gateway trigger the Lambda function instead of the client calling Lambda directly?

API Gateway acts as a secure front door that accepts client requests and triggers Lambda. This allows managing access, routing, and protocols. See execution_table step 2.

What does the Lambda function receive as input?

Lambda receives an event object containing request details from API Gateway, not the raw HTTP request. See execution_table step 3.

How does the client get the Lambda function's response?

Lambda returns a response to API Gateway, which formats and sends it back to the client as an HTTP response. See execution_table steps 4 and 5.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the Lambda function execute its code?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Check the 'Component' and 'Action' columns in execution_table row for step 3.

At which step does the client receive the HTTP response?

A Step 5

B Step 3

C Step 1

D Step 4

💡 Hint

Look for when the client 'Receives response' in the execution_table.

If the Lambda function fails to return a response, which step would be affected?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Consider when API Gateway 'Receives Lambda response' in execution_table step 4.

Concept Snapshot

Lambda integration with API Gateway:
- Client sends HTTP request to API Gateway
- API Gateway triggers Lambda function with event data
- Lambda executes code and returns response
- API Gateway sends HTTP response back to client
- Enables serverless backend for web APIs

Full Transcript

This visual execution shows how a client request flows through API Gateway to trigger a Lambda function. The client sends an HTTP request which API Gateway receives. API Gateway then triggers the Lambda function by passing an event object. The Lambda function runs its code and returns a response payload. API Gateway receives this response and sends it back to the client as an HTTP response. The client finally receives and displays the response. Key points include API Gateway acting as a secure front door, Lambda receiving event data, and the response flow back to the client.

Practice

1. What is the main purpose of Lambda integration in AWS API Gateway?

easy

A. To create virtual machines in the cloud

B. To store data permanently in AWS

C. To connect API Gateway to Lambda functions so they run code on requests

D. To monitor network traffic between AWS services

Solution

1. Step 1: Understand Lambda integration role

   Lambda integration allows API Gateway to trigger Lambda functions when an API request is made.

2. Step 2: Compare options with this role

   Only To connect API Gateway to Lambda functions so they run code on requests describes connecting API Gateway to Lambda to run code on requests.

3. Final Answer:

   To connect API Gateway to Lambda functions so they run code on requests -> Option C

4. Quick Check:

   Lambda integration = connect API Gateway to Lambda [OK]

Hint: Lambda integration runs code on API requests [OK]

Common Mistakes:

• Confusing Lambda integration with data storage
• Thinking it creates virtual machines
• Mixing it with monitoring services

2. Which of the following is the correct way to specify Lambda integration in an API Gateway method using AWS CLI?

easy

A. --integration-type HTTP --integration-uri http://example.com

B. --integration-type AWS_PROXY --integration-uri arn:aws:lambda:region:account-id:function:function-name

C. --integration-type MOCK --integration-uri arn:aws:s3:::bucket/key

D. --integration-type AWS --integration-uri arn:aws:ec2:region:account-id:instance/instance-id

Solution

1. Step 1: Identify Lambda integration syntax

   Lambda integration uses type AWS_PROXY and the Lambda function ARN as URI.

2. Step 2: Match options with correct syntax

   --integration-type AWS_PROXY --integration-uri arn:aws:lambda:region:account-id:function:function-name correctly uses AWS_PROXY and Lambda ARN format.

3. Final Answer:

   --integration-type AWS_PROXY --integration-uri arn:aws:lambda:region:account-id:function:function-name -> Option B

4. Quick Check:

   AWS_PROXY + Lambda ARN = correct syntax [OK]

Hint: AWS_PROXY + Lambda ARN = Lambda integration syntax [OK]

Common Mistakes:

• Using HTTP or MOCK integration types for Lambda
• Incorrect ARN format for Lambda function
• Confusing AWS integration with EC2 ARN

3. Given this AWS SAM template snippet, what will happen when the API Gateway endpoint is called?

{
  "Resources": {
    "MyFunction": {
      "Type": "AWS::Serverless::Function",
      "Properties": {
        "Handler": "index.handler",
        "Runtime": "nodejs18.x",
        "Events": {
          "ApiEvent": {
            "Type": "Api",
            "Properties": {
              "Path": "/hello",
              "Method": "get"
            }
          }
        }
      }
    }
  }
}

medium

A. API Gateway triggers the Lambda function on GET /hello requests

B. API Gateway returns a static response without calling Lambda

C. Lambda function is triggered only on POST requests

D. The template will fail to deploy due to missing integration

Solution

1. Step 1: Analyze the SAM template event

   The event defines an API trigger on path /hello with GET method linked to the Lambda function.

2. Step 2: Understand behavior on API call

   Calling GET /hello triggers the Lambda function as defined.

3. Final Answer:

   API Gateway triggers the Lambda function on GET /hello requests -> Option A

4. Quick Check:

   API event with GET triggers Lambda [OK]

Hint: API event with GET method triggers Lambda [OK]

Common Mistakes:

• Assuming static response without Lambda call
• Confusing HTTP methods for trigger
• Thinking deployment fails without explicit integration

4. You set up API Gateway with Lambda integration but get a 403 Forbidden error when calling the API. What is the most likely cause?

medium

A. API Gateway lacks permission to invoke the Lambda function

B. The API Gateway endpoint URL is incorrect

C. The Lambda function code has syntax errors

D. The Lambda function timeout is too short

Solution

1. Step 1: Understand 403 Forbidden meaning

   403 means access denied, often due to missing permissions.

2. Step 2: Check Lambda invoke permissions

   API Gateway must have permission to invoke Lambda; missing this causes 403 errors.

3. Final Answer:

   API Gateway lacks permission to invoke the Lambda function -> Option A

4. Quick Check:

   403 Forbidden = missing invoke permission [OK]

Hint: 403 Forbidden means missing invoke permission [OK]

Common Mistakes:

• Blaming Lambda code errors for 403
• Assuming wrong URL causes 403
• Thinking timeout causes 403 instead of 504

5. You want API Gateway to forward the full client request (headers, query, body) to your Lambda function and receive the Lambda response directly. Which integration type and setup should you use?

hard

A. Use AWS integration type with manual mapping templates

B. Use MOCK integration type and configure static responses

C. Use HTTP integration type pointing to Lambda's REST endpoint

D. Use AWS_PROXY integration type with Lambda and grant invoke permissions

Solution

1. Step 1: Identify integration type for full request forwarding

   AWS_PROXY integration forwards the entire client request to Lambda and returns Lambda's response directly.

2. Step 2: Confirm permissions and setup

   Granting API Gateway permission to invoke Lambda is required for this integration to work.

3. Final Answer:

   Use AWS_PROXY integration type with Lambda and grant invoke permissions -> Option D

4. Quick Check:

   AWS_PROXY + invoke permission = full request forwarding [OK]

Hint: AWS_PROXY forwards full request and response [OK]

Common Mistakes:

• Using MOCK for dynamic Lambda calls
• Using HTTP integration which doesn't call Lambda directly
• Using AWS integration without proxy causing manual mapping