Building applications - AWS Serverless Application Model

Building applications

To build your serverless application, use the sam build command. This command also gathers the build artifacts of your application's dependencies and places them in the proper format and location for next steps, such as locally testing, packaging, and deploying.

You specify your application's dependencies in a manifest file, such as requirements.txt (Python) or package.json (Node.js), or by using the Layers property of a function resource. The Layers property contains a list of AWS Lambda layer resources that the Lambda function depends on.

The format of your application's build artifacts depends on each function's PackageType property. The options for this property are:

  • Zip – A .zip file archive, which contains your application code and its dependencies. If you package your code as a .zip file archive, you must specify a Lambda runtime for your function.

  • Image – A container image, which includes the base operating system, runtime, and extensions, in addition to your application code and its dependencies.

For more information about Lambda package types, see Lambda deployment packages in the AWS Lambda Developer Guide.

Building a .zip file archive

To build your serverless application as a .zip file archive, declare PackageType: Zip for your serverless function.

AWS SAM builds your application for the architecture that you specify. If you don't specify an architecture, AWS SAM uses x86_64 by default.

If your Lambda function depends on packages that have natively compiled programs, use the --use-container flag. This flag locally compiles your functions in a Docker container that behaves like a Lambda environment, so they're in the right format when you deploy them to the AWS Cloud.

When you use the --use-container option, by default AWS SAM pulls the container image from Amazon ECR Public. If you would like to pull a container image from another repository, for example DockerHub, you can use the --build-image option and provide the URI of an alternate container image. Following are two example commands for building applications using container images from the DockerHub repository:

# Build a Node.js 20 application using a container image pulled from DockerHub sam build --use-container --build-image amazon/aws-sam-cli-build-image-nodejs20.x # Build a function resource using the Python 3.12 container image pulled from DockerHub sam build --use-container --build-image Function1=amazon/aws-sam-cli-build-image-python3.12

For a list of URIs you can use with --build-image, see Image repositories which contains DockerHub URIs for a number of supported runtimes.

For additional examples of building a .zip file archive application, see the Examples section later in this topic.

Building a container image

To build your serverless application as a container image, declare PackageType: Image for your serverless function. You must also declare the Metadata resource attribute with the following entries:

Dockerfile

The name of the Dockerfile associated with the Lambda function.

DockerContext

The location of the Dockerfile.

DockerTag

(Optional) A tag to apply to the built image.

DockerBuildArgs

Build arguments for the build.

The following is an example Metadata resource attribute section:

Metadata: Dockerfile: Dockerfile DockerContext: ./hello_world DockerTag: v1

To download a sample application that's configured with the Image package type, see Tutorial: Deploying a Hello World application in Tutorial: Deploying a Hello World application. At the prompt asking which package type you want to install, choose Image.

Note

If you specify a multi-architecture base image in your Dockerfile, AWS SAM builds your container image for your host machine's architecture. To build for a different architecture, specify a base image that uses the specific target architecture.

Container environment variable file

To provide a JSON file that contains environment variables for the build container, use the --container-env-var-file argument with the sam build command. You can provide a single environment variable that applies to all serverless resources, or different environment variables for each resource.

Format

The format for passing environment variables to a build container depends on how many environment variables you provide for your resources.

To provide a single environment variable for all resources, specify a Parameters object like the following:

{ "Parameters": { "GITHUB_TOKEN": "TOKEN_GLOBAL" } }

To provide different environment variables for each resource, specify objects for each resource like the following:

{ "MyFunction1": { "GITHUB_TOKEN": "TOKEN1" }, "MyFunction2": { "GITHUB_TOKEN": "TOKEN2" } }

Save your environment variables as a file, for example, named env.json. The following command uses this file to pass your environment variables to the build container:

sam build --use-container --container-env-var-file env.json

Precedence

  • The environment variables that you provide for specific resources take precedence over the single environment variable for all resources.

  • Environment variables that you provide on the command line take precedence over environment variables in a file.

Speed up build times by building your project in the source folder

For supported runtimes and build methods, you can use the --build-in-source option to build your project directly in the source folder. By default, the AWS SAM CLI builds in a temporary directory, which involves copying over source code and project files. With --build-in-source, the AWS SAM CLI builds directly in your source folder, which speeds up the build process by removing the need to copy files to a temporary directory.

For a list of supported runtimes and build methods, see --build-in-source.

Examples

Example 1: .zip file archive

The following sam build commands build a .zip file archive:

# Build all functions and layers, and their dependencies sam build # Run the build process inside a Docker container that functions like a Lambda environment sam build --use-container # Build a Node.js 20 application using a container image pulled from DockerHub sam build --use-container --build-image amazon/aws-sam-cli-build-image-nodejs20.x # Build a function resource using the Python 3.12 container image pulled from DockerHub sam build --use-container --build-image Function1=amazon/aws-sam-cli-build-image-python3.12 # Build and run your functions locally sam build && sam local invoke # For more options sam build --help

Example 2: Container image

The following AWS SAM template builds as a container image:

Resources: HelloWorldFunction: Type: AWS::Serverless::Function Properties: PackageType: Image ImageConfig: Command: ["app.lambda_handler"] Metadata: Dockerfile: Dockerfile DockerContext: ./hello_world DockerTag: v1

The following is an example Dockerfile:

FROM public.ecr.aws/lambda/python:3.12 COPY app.py requirements.txt ./ RUN python3.12 -m pip install -r requirements.txt # Overwrite the command by providing a different command directly in the template. CMD ["app.lambda_handler"]

Example 3: npm ci

For Node.js applications, you can use npm ci instead of npm install to install dependencies. To use npm ci, specify UseNpmCi: True under BuildProperties in your Lambda function's Metadata resource attribute. To use npm ci, your application must have a package-lock.json or npm-shrinkwrap.json file present in the CodeUri for your Lambda function.

The following example uses npm ci to install dependencies when you run sam build:

Resources: HelloWorldFunction: Type: AWS::Serverless::Function Properties: CodeUri: hello-world/ Handler: app.handler Runtime: nodejs20.x Architectures: - x86_64 Events: HelloWorld: Type: Api Properties: Path: /hello Method: get Metadata: BuildProperties: UseNpmCi: True

Building functions outside of AWS SAM

By default, when you run sam build, AWS SAM builds all of your function resources. Other options include:

  • Build all function resources outside of AWS SAM – If you build all of your function resources manually or through another tool, sam build is not required. You can skip sam build and move on to the next step in your process, such as performing local testing or deploying your application.

  • Build some function resources outside of AWS SAM – If you want AWS SAM to build some of your function resources while having other function resources built outside of AWS SAM, you can specify this in your AWS SAM template.

Build some function resources outside of AWS SAM

To have AWS SAM skip a function when using sam build, configure the following in your AWS SAM template:

  1. Add the SkipBuild: True metadata property to your function.

  2. Specify the path to your built function resources.

Here is an example, with TestFunction configured to be skipped. Its built resources are located at built-resources/TestFunction.zip.

TestFunction: Type: AWS::Serverless::Function Properties: CodeUri: built-resources/TestFunction.zip Handler: TimeHandler::handleRequest Runtime: java11 Metadata: SkipBuild: True

Now, when you run sam build, AWS SAM will do the following:

  1. AWS SAM will skip functions configured with SkipBuild: True.

  2. AWS SAM will build all other function resources and cache them in the .aws-sam build directory.

  3. For skipped functions, their template in the .aws-sam build directory will automatically be updated to reference the specified path to your built function resources.

    Here is an example of the cached template for TestFunction in the .aws-sam build directory:

    TestFunction: Type: AWS::Serverless::Function Properties: CodeUri: ../../built-resources/TestFunction.zip Handler: TimeHandler::handleRequest Runtime: java11 Metadata: SkipBuild: True