What is serverless

How it works

You write a handler: one function that takes an event and returns an answer. The event can be a web request, a file landing in storage, a message in a queue or a timer. The provider runs your handler when the event arrives, then freezes or removes the environment.

AWS Lambda is a serverless compute service: you run code without provisioning or managing servers. The provider handles server maintenance, capacity provisioning, scaling and patching. There is no upfront commitment, and you pay only for the resources used. On AWS Lambda, 1 million requests and 400,000 GB-seconds a month are free; after that each million requests costs $0.20.

# the whole "server" is this function
def lambda_handler(event, context):
    name = event.get("name", "world")
    return {"statusCode": 200, "body": "Hello, " + name}

Most of the time nobody calls, so most of the time nothing runs and nothing is billed. That is the appeal. A weekend project or a rarely used webhook can cost close to zero.

What you give up

A cold start is the delay on a call that finds no ready environment: the service downloads your code, starts the environment and runs your initialization code before the handler. AWS says it lasts from under 100 ms to over 1 second, and that cold starts occur in under 1% of invocations. A function that runs rarely meets it more often.

The limits are by design. A function that lives for seconds and forgets everything is easy to copy a thousand times. Serverless is moving: AWS now also sells Lambda MicroVMs, isolated environments for untrusted code that run up to 8 hours per session and keep memory and disk state on suspend. Those are a different product from a plain function.

Do you need it?

The two meet in the middle: a VPS is the server you keep, a function is the server you rent by the second, and this comparison sets them next to a platform service with prices.

Questions