Building a custom runtime for AWS Lambda - AWS Lambda

Building a custom runtime for AWS Lambda

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 the runtime in your function's deployment package or distribute it in a layer. When you create the Lambda function, choose an OS-only runtime (the provided runtime family).


Creating a custom runtime is an advanced use case. If you're looking for information about compiling to a native binary or using a third-party off-the-shelf runtime, see When to use Lambda's OS-only runtimes.

For a walkthrough of the custom runtime deployment process, see Tutorial: Building a custom runtime. You can also explore a custom runtime implemented in C++ at awslabs/aws-lambda-cpp on GitHub.


Custom runtimes must complete certain initialization and processing tasks. A runtime runs the function's setup code, reads the handler name from an environment variable, and reads 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.

Intitialization tasks

The initialization tasks run once per instance of the function to prepare the environment to handle invocations.

  • 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.

    For a full list of available variables, see Defined runtime environment 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

Processing tasks

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. In your runtime code, perform the following steps in order.

  • 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 locally with the same value. The X-Ray SDK uses this value to connect trace data between services.

  • 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.


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. If the root of your deployment package doesn't contain a file named bootstrap, Lambda looks for the file in the function's layers. If the bootstrap file doesn't exist or isn't executable, your function returns a Runtime.InvalidEntrypoint error upon invocation.

Here's an example bootstrap file that uses a bundled version of Node.js to run 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

Implementing response streaming in a custom runtime

For response streaming functions, the response and error endpoints have slightly modified behavior that lets the runtime stream partial responses to the client and return payloads in chunks. For more information about the specific behavior, see the following:

  • /runtime/invocation/AwsRequestId/response – Propagates the Content-Type header from the runtime to send to the client. Lambda returns the response payload in chunks via HTTP/1.1 chunked transfer encoding. The response stream can be a maximum size of 20 MiB. To stream the response to Lambda, the runtime must:

    • Set the Lambda-Runtime-Function-Response-Mode HTTP header to streaming.

    • Set the Transfer-Encoding header to chunked.

    • Write the response conforming to the HTTP/1.1 chunked transfer encoding specification.

    • Close the underlying connection after it has successfully written the response.

  • /runtime/invocation/AwsRequestId/error – The runtime can use this endpoint to report function or runtime errors to Lambda, which also accepts the Transfer-Encoding header. This endpoint can only be called before the runtime begins sending an invocation response.

  • Report midstream errors using error trailers in /runtime/invocation/AwsRequestId/response – To report errors that occur after the runtime starts writing the invocation response, the runtime can optionally attach HTTP trailing headers named Lambda-Runtime-Function-Error-Type and Lambda-Runtime-Function-Error-Body. Lambda treats this as a successful response and forwards the error metadata that the runtime provides to the client.


    To attach trailing headers, the runtime must set the Trailer header value at the beginning of the HTTP request. This is a requirement of the HTTP/1.1 chunked transfer encoding specification.

    • Lambda-Runtime-Function-Error-Type – The error type that the runtime encountered. This header consists of a string value. Lambda accepts any string, but we recommend a format of <category.reason>. For example, Runtime.APIKeyNotFound.

    • Lambda-Runtime-Function-Error-Body – Base64-encoded information about the error.