To use a custom runtime, you have to specify that you're providing one when deploying your function. When the function is invoked, AWS Lambda will bootstrap your runtime code and communicate with it over Runtime API to execute the function code.That's enough about custom runtimes. What are AWS Lambda Layers? They're a new type of artifact that can contain arbitrary code and data. It can be referenced by multiple functions at the same time. That's so awesome! Your functions usually share common dependencies like SDKs, prebuilt modules, libraries and frameworks. Here's the kicker, now you can share runtimes as well!.