When we deploy a web API on AWS Lambda leveraging the benefit of server less architecture, we have two options for the front-end service for Lambda: API Gateway (API GW) or Application Load Balancer (ALB). This articles studies the difference between them to help you to decide which to choose.

Features unique to API Gateway

• It is obvious, but API Gateway specific features such as Request Validation, Data Mapping, Access Control and SDK Generation cannot be used with ALB.

Protocols and Port Numbers

• API GW supports only HTTPS (TLS 1.2) on 443 port. Note that though API GW supports VPC Endpoint, you still need to use HTTPS in this case too.
• ALB supports both HTTP and HTTPS on arbitrary port numbers and TLS version also can be chosen from various options.

Request Timeout

• API GW has a severe limitation of 29 seconds.
• ALB itself doesn't have a limitation, thus the timeout of Lambda (up to 15 minutes) is applied.

Request and Response Sizes

• API GW can handle up to 6MB payload, which is the ceiling of Lambda's payload size.

  • Note that 6MB includes headers and Base64-encoded body as payloads are requests and responses of Lambda.
  • The limitation of API GW's payload size is 10MB.

• With ALB, the size of request and response bodies cannot be larger than 1MB. 413 error will be returned when a request body exceeds the limit, and 502 error for response body.

Request Event（Reference）

• evt.headers (Request)
  • API GW sets header names and values with keeping the original upper and lower cases, whereas ALB decapitalize header names.
• evt.queryStringParameters (Request)
  • When there is no query string parameter, API GW sets null, whereas ALB sets {}.
  • API Gateway sets the URL decoded value for query string parameter key and value, whereas ALB sets them without URL decoding.
• evt.multiValueHeaders / evt.multiValueQueryStringParameters (Request)
  • API Gateway sets these parameters by default, whereas ALB requires that they are enabled in its configuration.

Response Event（Reference）

• ALB response event must include statusDescription, isBaseEncoded and headers (when "Multi Value Header" is enabled in ALB configuration).

Cost

• Up to a certain request frequency, API GW is cheaper than ALB, that has a time-based charge. However for the services that receive a large amount of requests, ALB is more reasonable. (Reference)

Other References

• Amazon Web Services Blog
  • It only mentions as follows:there’s also a lot of overlap and commonality between the parameters used by Amazon API Gateway for triggering Lambda functions and ALB’s parameters.
• API Gateway vs Application Load Balancer—Technical Details
• API Gateway vs. ALB - 持田システム
  • It covers the difference in request timeout and limitation on response size.