The AWS Serverless Application Model (AWS SAM) is an open-source framework for building serverless applications. Built on AWS CloudFormation, AWS SAM provides shorthand syntax to declare serverless resources. During deployment, AWS SAM transforms the serverless resources into CloudFormation syntax, enabling you to build serverless applications faster.

As a companion to AWS SAM, the AWS SAM CLI is a command line tool that operates on AWS SAM templates. It provides developers local tooling to create, develop, debug, and deploy serverless applications. AWS SAM has been open-source and generally available since April 2018. Today, the AWS SAM CLI is now also generally available (GA).

The AWS SAM CLI offers a rich set of tools that enable developers to build serverless applications quickly. This blog post summarizes the different tools available.

Init

The sam init command creates the folder structure and basic resources for a new serverless application. Additionally, you can choose a starter template for the serverless applications from one of the AWS managed templates, or create your own. Some runtimes also offer multiple dependency manager options to choose from. In this case, you choose Maven for Java11.

Demonstration of sam init

Local

The sam local command provides tooling for locally testing serverless applications. It has four commands available.

  1. sam local invoke: Invokes an AWS Lambda function locally once and quits after the invocation is complete. This is great for testing asynchronous invocations from services like Amazon S3 or Amazon EventBridge. Invoke allows you to pass parameters for environment settings, event sources, debugging, Docker network settings, and more.
  2. sam local start-api: Invokes a Lambda function using a local emulation of an Amazon API Gateway REST API. Using a Docker container, the service starts and listens on a specific port allowing you to repeatedly invoke the Lambda function via an HTTP request. Like invoke, start-api allows you to set flags for request and configuration data.
  3. sam local start-lambda: Starts a service that emulates Lambda, allowing you to test Lambda function invocations from the AWS CLI or an SDK. Using a Docker container, the service starts and listens on a specific port. It is available for repeated testing invocations.
  4. sam local generate-event: Generates a mock event to use with the local invocation commands. This is useful if you are working with an asynchronous service call and need to understand what the event looks like. Here is an example of a generated Amazon S3 put event:

Demonstration of sam local generate

Build

The sam build command provides contextual build capabilities based on the selected runtime. The build process prepares the code and dependencies into a deployable artifact and updates the AWS SAM template accordingly. While the build process has been around since the AWS SAM CLI was created, new features have been added for the general release. These features are covered in depth later in this post.

Deploy

The sam deploy command packages and deploys a serverless application with the following steps:

  1. Compresses the application resources into a zip file and uploads to a AWS SAM-managed S3 bucket. AWS SAM creates the S3 bucket if it does not exist and uses it for any other applications in the same Region.
  2. AWS SAM calls AWS CloudFormation with an updated AWS SAM template and requests a new CloudFormation change set be created.
  3. If the change set is valid, CloudFormation then creates or updates the application resources as needed.

The first time you run sam deploy, it is recommended you use the -g or –guided flag. This indicates to the AWS SAM CLI that the deployment configurations must be set or updated.

Demonstration of sam deploy with guided option on

By using the guided deploy, you are prompted for the required application and configuration information. You also have the option to save the information to a configuration file for subsequent deployments.

Package

The sam package command is generally only required in a continuous integration and continuous delivery (CI/CD) scenario, where the deployment is not handled by sam deploy.

This command expedites the process of packaging the deployment artifacts and uploading to an Amazon S3 bucket for deployment by CloudFormation. This command compresses the application resources referenced in the AWS SAM template and uploads to the specified S3 bucket. It then outputs a new AWS SAM template with the updated resource location for deployment.

Publish

The sam publish command enables you to publish applications to the AWS Serverless Application Repository. This command expects the AWS SAM template to include application metadata required for publishing. You can use the same build and package tools to prepare the artifacts.

Logs

The sam logs command enables you to explore Amazon CloudWatch Logs for a deployed Lambda function. You can identify the function by the stack name and logical identifier of the function, or directly by the physical name of the Lambda function. You can filter the results by start time, end time, and keywords. You can also add the -t or –tail flag to have AWS SAM fetch new logs as they become available.

Demonstration of sam logs

Validate

The sam validate command validates a AWS SAM template file. This allows you to quickly identify template errors before sending to CloudFormation.

New features with GA

In preparation for the this release, the tooling team is making improvements to the build process for Lambda functions and Lambda Layers.

Building Lambda functions

When creating Lambda functions in AWS SAM templates, you now have an optional MetaData parameter with a nested parameter called BuildMethod. The build method can be set to a supported runtime (node12.x, java11, etc.) and AWS SAM uses the default build process for the selected runtime. The BuildMethod can also be set to makefile, which allows a MakeFile to customize the build for supported runtimes or automate the build for custom runtimes.

Demonstration of a Lambda function with the makefile build option

Building Lambda Layers

Lambda layers also benefit from the new build process as well. Previously, a Lambda Layer had to be fully built and packaged to deploy, and the files had to be structured according to runtimes.

Demonstration of the old layer structure

With the new AWS SAM CLI build, a layer only needs to contain the manifest file. AWS SAM build fetches the dependencies and builds the layer for deployment. AWS SAM build for layers works with all runtimes supported by AWS SAM build.

Demonstration of the updated layer structure

A new Docker container image

With the general availability of the AWS SAM CLI, sam local commands now use an AWS managed Docker image. Previously, AWS SAM used the docker-lambda image created and maintained by AWS Serverless Hero, Michael Hart (@hichaelmart). The AWS Serverless team thanks Michael for his tireless work on this project and for his unfailing leadership and support in the serverless space. We also appreciate how Michael selflessly worked with us to make our new container images a reality.

What does GA mean?

The term generally available means that the software is no longer considered in beta and is released as stable, from version 1.0.0. It also means that the team is actively building new features and security updates on a regular cadence.

Chart explaining three code versions

After this launch, for any major version updates, AWS will first release a beta and then promote to the stable version. The stable version is used for non-breaking feature updates and the beta version is used for any breaking changes. The old major version will be supported in maintenance mode for a period of time. Today’s v1.0.0 release is the first stable version. The following semantic version patterns are used:

  1. MAJOR version for incompatible changes (1.0.0)
  2. MINOR version for functionality in a backward compatible manner (1.1.0)
  3. PATCH version for backward compatible bug fixes and security updates (1.1.1)

Today’s v1.0.0 release marks the first stable version.

Conclusion

AWS SAM CLI is a powerful tool for accelerating serverless development and helping developers improve their time to market. Now that AWS SAM CLI is generally available, we continue to add features and make service improvements that are generally available as well. As always, your feedback is important to us. If you have ideas or comments, submit an issue at https://github.com/awslabs/aws-sam-cli.

Now, build something serverless!