Lambda+ALB vs Lambda+API GW

Question

Lambda can be invoked by http endpoint via ALB as per the links pasted below. On the other hand, API GW can also be used to invoke the lambda (this approach is recommended widely over online). When to use ALB and when to use API GW ? Are there any recommendation on this ? Are there any key differentiating factors to pick one over the other ?

Please find the few below differences that could figure out. (Pls correct me if i am wrong)
1. API GW aggregates multiple service end points or multiple lambdas whereas one ALB can point to only one lambda.
2. All requests through that ALB will go the same lambda and the code inside the lambda need to identify which functionality to execute.

ALB Links
1. https://docs.aws.amazon.com/lambda/latest/dg/services-alb.html
2. https://aws.amazon.com/blogs/networking-and-content-delivery/lambda-functions-as-targets-for-application-load-balancers/

Accepted Answer

In case of relative low traffic (around 5 million requests per month), I would discourage the use of ALB + Lambda for services that are more normal REST API and when you don't need any of functionalities offered by ALB (like the openID authentication, etc), as the cost of having ALB+Lambda is much higher for .

Anyway, you could use ALB + Lambda, when you are already using ALB for your EC2 instances.
For example you have a domain for your application "www.myapp.com" pointing to your ALB, and you want to extend some of the functionalities of your domain using lambda.
You can create new ALB targets pointing to lambda.
For example you can create the target "/billing" to point to lambda, and the rest of the request to go to EC2, doing this you spread the traffic of your domain between different backends.

**Major differences between API GW and ALB**

|  | API GW | ALB |
| --- | --- | --- |
| Request/seconds | Limited to 10000 | virtually unlimited |
| Integration | Lambda and many others AWS services | EC2, ECS, private IP addresses, Lambda |
| Routing | Only on path based | Path base and others like Requesters IP, Http Headers, Http method, etc |
| Cost | Pay per request | Pay by time + Request (LCU) |
| Access Logging | Cloudwatch | S3 bucket |

	API GW	ALB
Request/seconds	Limited to 10000	virtually unlimited
Integration	Lambda and many others AWS services	EC2, ECS, private IP addresses, Lambda
Routing	Only on path based	Path base and others like Requesters IP, Http Headers, Http method, etc
Cost	Pay per request	Pay by time + Request (LCU)
Access Logging	Cloudwatch	S3 bucket

Lambda+ALB vs Lambda+API GW

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