Is AWS Lambda good for real-time API Rest?

Question

I'm learning about AWS Lambda and I'm worried about synchronized real-time requests. The fact the lambda has a "cold start" it doesn't sounds good for handling GET petitions.

Imagine a user is using the application and do a GET HTTP Request to get a Product or a list of Products, if the lambda is sleeping, then it will take 10 seconds to respond, I don't see this as an acceptable response time. Is it good or bad practice to use AWS Lambda for classic (sync responses) API Rest?

Answer 1

As an AWS Lambda + API Gateway user (with Serverless Framework) I had to deal with this too.

The problem I faced:

• Few requests per day per lambda (not enough to keep lambdas warm)
• Time critical application (the user is on the phone, waiting for text-to-speech to answer)

How I worked around that:

The idea was to find a way to call the critical lambdas often enough that they don't get cold.
If you use the Serverless Framework, you can use the serverless-plugin-warmup plugin that does exactly that.
If not, you can copy it's behavior by creating a worker that will invoke the lambdas every few minutes to keep them warm. To do this, create a lambda that will invoke your other lambdas and schedule CloudWatch to trigger it every 5 minutes or so. Make sure to call your to-keep-warm lambdas with a custom event.source so you can exit them early without running any actual business code by putting the following code at the very beginning of the function:

if (event.source === 'just-keeping-warm) {
  console.log('WarmUP - Lambda is warm!');
  return callback(null, 'Lambda is warm!');
}

Depending on the number of lamdas you have to keep warm, this can be a lot of "warming" calls. AWS offers 1.000.000 free lambda calls every month though.

Answer 2

Like most things, I think you should measure before deciding. A lot of AWS customers use Lambda as the back-end for their webapps quite successfully.

There's a lot of discussion out there on Lambda latency, for example:

• 2017-04 comparing Lambda performance using Node.js, Java, C# or Python
• 2018-03 Lambda call latency
• 2019-09 improved VPC networking for AWS Lambda
• 2019-10 you're thinking about cold starts all wrong

In December 2019, AWS Lambda introduced Provisioned Concurrency, which improves things. See:

• 2019-12 AWS Lambda announces Provisioned Concurrency
• 2020-09 AWS Lambda Cold Starts: Solving the Problem

You should measure latency for an environment that's representative of your app and its use.

A few things that are important factors related to request latency:

• cold starts => higher latency
• request patterns are important factors in cold starts
• if you need to deploy in VPC (attachment of ENI => higher cold start latency)
• using CloudFront --> API Gateway --> Lambda (more layers => higher latency)
• choice of programming language (Java likely highest cold-start latency, Go lowest)
• size of Lambda environment (more RAM => more CPU => faster)
• Lambda account and concurrency limits
• pre-warming strategy

Update 2019-12: see Predictable start-up times with Provisioned Concurrency.

Update 2021-08: see Increasing performance of Java AWS Lambda functions using tiered compilation.