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Application Load Balancer

Elastic Load Balancing invokes your Lambda function synchronously with an event that contains the request body and metadata.


  • The Lambda function and target group must be in the same account and in the same Region.
  • The maximum size of the request body that you can send to a Lambda function is 1 MB. For related size limits, see HTTP header limits.
  • The maximum size of the response JSON that the Lambda function can send is 1 MB.
  • WebSockets are not supported. Upgrade requests are rejected with an HTTP 400 code.
  • Local Zones are not supported.
Tip: Reasons to use Application Load Balancers over API Gateway
  • Nearly unlimited tps vs 10,000 requests per second (RPS) (Can be increased)
  • No specific function timeout vs 30 seconds for API Gateway
  • DDOS protection via AWS Shield support vs no support for API Gateway
  • Application Load Balancer pricing favors high transactions per second
Tip: Reasons to use API Gateway over Application Load Balancer
  • API Gateway has a 10 MB request body limit
  • API Gateway has support for WebSockets
  • API Gateway has built in support for rate limiting support
  • API Gateway has built in support authentication support
  • API Gateway pricing allow for scale to zero
  • Application Load Balancer would require a VPC


Request Fields

targetGroupArn (String)
Target group arn for your Lambda function
httpMethod (String)
The HTTP method used. Valid values include: DELETE, GET, HEAD, OPTIONS, PATCH, POST, and PUT.
path (String)
Http request path
multiValueQueryStringParameters (Optional, map of string to list of strings)
If you enable multi-value headers, the load balancer uses both key values sent by the client and sends you an event that includes query string parameters using multiValueQueryStringParameters
queryStringParameters (Optional, map of string to string)
Query string parameters sent by the client.
headers (Optional, map of string to string)
Http header sent by the client.
isBase64Encoded (Boolean)
A Boolean flag to indicate if the request body is Base64-encoded.
body (Optional, string)
The request body sent by the client.

Getting the correlation id

JSON path to correlation id: headers."x-amzn-trace-id"

Generating sample events

sam local generate-event alb request --body {"test":"body"} --path foo --method POST

Request Example

Application Load Balance GET request
  "requestContext": {
    "elb": {
      "targetGroupArn": "arn:aws:elasticloadbalancing:us-east-2:123456789012:targetgroup/lambda-279XGJDqGZ5rsrHC2Fjr/49e9d65c45c6791a"
  "httpMethod": "GET",
  "path": "/lambda",
  "queryStringParameters": {
    "query": "1234ABCD"
  "multiValueQueryStringParameters": {
    "myKey": [
  "headers": {
    "accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8",
    "accept-encoding": "gzip",
    "accept-language": "en-US,en;q=0.9",
    "connection": "keep-alive",
    "host": "",
    "upgrade-insecure-requests": "1",
    "user-agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36",
    "x-amzn-trace-id": "Root=1-5c536348-3d683b8b04734faae651f476",
    "x-forwarded-for": "",
    "x-forwarded-port": "80",
    "x-forwarded-proto": "http",
    "x-imforwards": "20"
  "body": "Test",
  "isBase64Encoded": false


Response Fields

isBase64Encoded (Boolean)
A Boolean flag to indicate if the response body is Base64-encoded.
statusCode (Integer)
The HTTP status code.
statusDescription (String)
The HTTP status description.
headers (Optional, map of string to string)
Http header to be sent in the response.
body (Optional, string)
The response body sent by the server.

Response Examples

Example 200 html response
    "statusCode": 200,
    "statusDescription": "200 OK",
    "isBase64Encoded": False,
    "headers": {
        "Content-Type": "text/html"
    "body": "<h1>Hello from Lambda!</h1>"


If you enable multi-value headers, you must specify multiple cookies as follows

        "cookie-name=cookie-value;Expires=May 8, 2019"


Typed Lambda handlers by Language

Convenient Lambda Handlers

Code Examples


Last update: 2022-12-27