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Enhanced Serverless Development with Terraform and AWS SAM

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AWS announced support for local development and testing of AWS Serverless Application Model (AWS SAM) projects defined in HashiCorp Terraform configuration files. The functionality is provided via the AWS SAM Command Line Interface (AWS SAM CLI) as of version 1.63.0 and is currently released as a public-preview feature.

AWS SAM is an open-source framework for defining serverless application infrastructure as code. It operates as an extension to AWS’ Infrastructure-as-code service, Cloudformation, by enabling AWS Lambda functions, AWS DynamoDB tables and AWS API Gateways to be described and connected together succinctly. The AWS SAM CLI is a tool that facilitates the easy creation, build, and deployment of AWS SAM projects and their associated infrastructure. To build an application, the CLI applies transformations to the resources specific to SAM, e.g. AWS::Serverless::Function, and converts them into their CloudFormation equivalents according to AWS’ recommended practices.

Terraform is an open-source framework for provisioning, changing, and versioning infrastructure resources in cloud environments. Although the Terraform HCL syntax is cloud-vendor neutral, the AWS resource provider allows for a declarative specification of AWS infrastructure in a similar fashion to Cloudformation.

Previously, SAM applications could only be built and run locally by the CLI if they had been defined with AWS SAM syntax templates. Executing the commands sam build && sam local invoke in a compatible project would instantiate the SAM application’s Lambda function on the user’s computer and make it available for testing and debugging. With the newly released feature, serverless applications defined using Terraform can also be executed locally using the CLI by including the flags: --hook-name terraform --beta-features.

A common pattern for serverless application development has historically been to provide Lambda function application code alongside infrastructure definitions, with the AWS SAM CLI being a simplified mechanism for packaging and releasing both types of code changes together. As Terraform became more widely used for managing infrastructure, community discussion arose, for example, on r/aws or r/serverless, on the best way of managing serverless applications. The open-source and community-developed framework Serverless.tf was eventually created as,

an organic response to the accidental complexity of many existing tools used by serverless developers

with the goal of enabling infrastructure and application development using only Terraform modules.

By adding support for a SAM metadata resource within Terraform, the CLI can now locate and package Lambda function source code, easing some of the pain-points Serverless.tf was created to fix.

Terraform support in the AWS SAM CLI is available as of v1.63.0, and its source code can be found on GitHub.

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