Building custom runtimes

You can use the sam build command to build custom runtimes required for your Lambda function. You declare your Lambda function to use a custom runtime by specifying Runtime: provided for the function.

To build a custom runtime, declare the Metadata resource attribute with a BuildMethod: makefile entry. You provide a custom makefile, where you declare a build target of the form build-function-logical-id that contains the build commands for your runtime. Your makefile is responsible for compiling the custom runtime if necessary, and copying the build artifacts into the proper location required for subsequent steps in your workflow. The location of the makefile is specified by the CodeUri property of the function resource, and must be named Makefile.

Examples

Example 1: Custom runtime for a function written in Rust

Note

We recommend building Lambda functions with Cargo Lambda. To learn more, see Building Rust Lambda functions with Cargo Lambda.

The following AWS SAM template declares a function that uses a custom runtime for a Lambda function written in Rust, and instructs sam build to execute the commands for the build-HelloRustFunction build target.

Resources:
  HelloRustFunction:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: HelloRust
      Handler: bootstrap.is.real.handler
      Runtime: provided
      MemorySize: 512
      CodeUri: .
    Metadata:
      BuildMethod: makefile

The following makefile contains the build target and commands that will be executed. Note that the CodeUri property is set to ., so the makefile must be located in the project root directory (that is, the same directory as the application's AWS SAM template file). The filename must be Makefile.

build-HelloRustFunction:
  cargo build --release --target x86_64-unknown-linux-musl
  cp ./target/x86_64-unknown-linux-musl/release/bootstrap $(ARTIFACTS_DIR)

For more information about setting up your development environment in order to execute the cargo build command in the previous makefile, see the Rust Runtime for AWS Lambda blog post.

Example 2: Makefile builder for Python3.12 (alternative to using the bundled builder)

You might want to use a library or module that is not included in a bundled builder. This example shows a AWS SAM template for a Python3.12 runtime with a makefile builder.

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: hello_world/
      Handler: app.lambda_handler
      Runtime: python3.12
    Metadata:
      BuildMethod: makefile

The following makefile contains the build target and commands that will be executed. Note that the CodeUri property is set to hello_world, so the makefile must be located in the root of the hello_world subdirectory, and the filename must be Makefile.

build-HelloWorldFunction:
    cp *.py $(ARTIFACTS_DIR)
    cp requirements.txt $(ARTIFACTS_DIR)
    python -m pip install -r requirements.txt -t $(ARTIFACTS_DIR)
    rm -rf $(ARTIFACTS_DIR)/bin