Serverless Application Model

Describing a serverless app in raw CloudFormation is verbose — a single Lambda-behind-an-API needs a function, a role, permissions, an API, stages, and integrations, dozens of lines each. The AWS Serverless Application Model (SAM) is a CloudFormation transform: you write compact resource types like AWS::Serverless::Function, AWS::Serverless::Api, AWS::Serverless::HttpApi, AWS::Serverless::SimpleTable, and AWS::Serverless::StateMachine, and at deploy time SAM expands them into full CloudFormation. The SAM CLI adds sam build, local testing (sam local invoke / start-api in Docker), guided deploys, and log tailing.

• A SAM deploy is a CloudFormation stack — Regional, with the same change-set, rollback, and drift behaviour. Multi-Region means deploying the stack in each Region (or via StackSets).
• The resources it creates (Lambda, API Gateway, DynamoDB, Step Functions, EventBridge rules) carry their own availability characteristics — SAM is packaging, not runtime.
• Built artifacts are uploaded to an S3 bucket (or ECR for container-image functions) in the target Region before the stack deploys.

graph LR
TMPL["template.yaml (SAM: Function, Api, Table)"] --> BUILD["sam build"]
BUILD --> PKG["sam deploy (package -> S3 / ECR)"]
PKG --> XFORM["CloudFormation transform (expand to full CFN)"]
XFORM --> STACK
subgraph STACK["CloudFormation stack"]
  FN["Lambda functions + roles"]
  API["API Gateway"]
  DB["DynamoDB table"]
end
classDef sam fill:#f3ecfb,stroke:#8b3de0,stroke-width:2px,color:#3a1a5c
classDef res fill:#e8f0fe,stroke:#3b82f6,stroke-width:2px,color:#173a70
class TMPL,XFORM sam
class FN,API,DB res

SAM and CloudFormation are free. You pay only for the resources the stack provisions (Lambda GB-seconds, API Gateway requests, DynamoDB, etc.) and the small S3 cost for build artifacts. Because everything is defined as code, you can tear the whole app down (sam delete) between experiments to stop all charges.

• SAM templates start with Transform: AWS::Serverless-2016-10-31 — that line is what makes it a SAM template rather than plain CloudFormation. You can mix normal CloudFormation resources in the same file.
• sam local invoke / sam local start-api run functions in a Docker container that mimics the Lambda runtime — the standard “test serverless locally before deploying” answer.
• SAM has built-in support for safe deployments: AutoPublishAlias + DeploymentPreference (Canary / Linear) wires up CodeDeploy traffic shifting and automatic rollback on CloudWatch alarms.
• SAM vs CDK vs Serverless Framework: SAM is AWS-native, YAML, CloudFormation under the hood, serverless-focused; CDK is general-purpose infrastructure in a real programming language; both synthesize to CloudFormation.
• The Serverless Application Repository (SAR) publishes and shares packaged SAM apps across accounts.