AWS Lambda
Developer Guide

Custom AWS Lambda Runtimes

You can implement an AWS Lambda runtime in any programming language. A runtime is a program that runs a Lambda function's handler method when the function is invoked. You can include a runtime in your function's deployment package in the form of an executable file named bootstrap.

A runtime is responsible for running the function's setup code, reading the handler name from an environment variable, and reading invocation events from the Lambda runtime API. The runtime passes the event data to the function handler, and posts the response from the handler back to Lambda.

Your custom runtime runs in the standard Lambda execution environment. It can be a shell script, a script in a language that's included in Amazon Linux, or a binary executable file that's compiled in Amazon Linux.

To get started with custom runtimes, see Tutorial – Publishing a Custom Runtime. You can also explore a custom runtime implemented in C++ at awslabs/aws-lambda-cpp on GitHub.

Using a Custom Runtime

To use a custom runtime, set your function's runtime to provided. The runtime can be included in your function's deployment package, or in a layer.


. ├── bootstrap ├──

If there's a file named bootstrap in your deployment package, Lambda executes that file. If not, Lambda looks for a runtime in the function's layers. If the bootstrap file isn't found or isn't executable, your function returns an error upon invocation.

Building a Custom Runtime

A custom runtime's entry point is an executable file named bootstrap. The bootstrap file can be the runtime, or it can invoke another file that creates the runtime. The following example uses a bundled version of Node.js to execute a JavaScript runtime in a separate file named runtime.js.

Example bootstrap

#!/bin/sh cd $LAMBDA_TASK_ROOT ./node-v11.1.0-linux-x64/bin/node runtime.js

Your runtime code is responsible for completing some initialization tasks. Then it processes invocation events in a loop until it's terminated. The initialization tasks run once per instance of the function to prepare the environment to handle invocations.

Initialization Tasks

  • Retrieve settings – Read environment variables to get details about the function and environment.

    • _HANDLER – The location to the handler, from the function's configuration. The standard format is file.method, where file is the name of the file without an extension, and method is the name of a method or function that's defined in the file.

    • LAMBDA_TASK_ROOT – The directory that contains the function code.

    • AWS_LAMBDA_RUNTIME_API – The host and port of the runtime API.

    See Environment Variables Available to Lambda Functions for a full list of available variables.

  • Initialize the function – Load the handler file and run any global or static code that it contains. Functions should create static resources like SDK clients and database connections once, and reuse them for multiple invocations.

  • Handle errors – If an error occurs, call the initialization error API and exit immediately.

Initialization counts towards billed execution time and timeout. When an execution triggers the initialization of a new instance of your function, you can see the initialization time in the logs and AWS X-Ray trace.

Example Log

REPORT RequestId: f8ac1208... Init Duration: 48.26 ms Duration: 237.17 ms Billed Duration: 300 ms Memory Size: 128 MB Max Memory Used: 26 MB

        Initialization time in an X-Ray trace.

While it runs, a runtime uses the Lambda runtime interface to manage incoming events and report errors. After completing initialization tasks, the runtime processes incoming events in a loop.

Processing Tasks

  • Get an event – Call the next invocation API to get the next event. The response body contains the event data. Response headers contain the request ID and other information.

  • Propagate the tracing header – Get the X-Ray tracing header from the Lambda-Runtime-Trace-Id header in the API response. Set the _X_AMZN_TRACE_ID environment variable with the same value for the X-Ray SDK to use.

  • Create a context object – Create an object with context information from environment variables and headers in the API response.

  • Invoke the function handler – Pass the event and context object to the handler.

  • Handle the response – Call the invocation response API to post the response from the handler.

  • Handle errors – If an error occurs, call the invocation error API.

  • Cleanup – Release unused resources, send data to other services, or perform additional tasks before getting the next event.

You can include the runtime in your function's deployment package, or distribute the runtime separately in a function layer. For an example walkthrough, see Tutorial – Publishing a Custom Runtime.