The AWS Serverless Application Model (AWS SAM) allows you to easily create and manage resources used in your serverless application using AWS CloudFormation. You can define your serverless application as a SAM template - a JSON or YAML configuration file that describes Lambda function, API endpoints and other resources in your application. Using a variety of nifty commands, you upload this template to CloudFormation, which in turn creates all the individual resources and groups them into a CloudFormation Stack for ease of management. When you update your SAM template, you will re-deploy the changes to this stack. AWS CloudFormation will take care of updating the individual resources for you.
The remainder of this document explains how to write SAM templates and deploy them via AWS CloudFormation.
Check out the latest specification for details on how to write a SAM template
You could also use the aws-sam-cli to get started
$ sam init --runtime python3.7
Before you can deploy a SAM template, you should first upload your Lambda
function code zip and API's OpenAPI File to S3. Set the CodeUri
and
DefinitionUri
properties to the S3 URI of uploaded files. You
can choose to do this manually or use aws cloudformation package
CLI command to automate the task of uploading local artifacts to S3 bucket. The command returns a copy of your template, replacing references to local artifacts with S3 location where the command uploaded your artifacts.
To use this command, set CodeUri
property to be the path to your
source code folder/zip/jar and DefinitionUri
property to be a path to
your OpenAPI file, as shown in the example below
MyLambdaFunction:
Type: AWS::Serverless::Function
Properties:
CodeUri: ./code
...
MyApi:
Type: AWS::Serverless::Api
Properties:
DefinitionUri: ./specs/swagger.yaml
...
Run the following command to upload your artifacts to S3 and output a packaged template that can be readily deployed to CloudFormation.
$ aws cloudformation package \
--template-file /path_to_template/template.yaml \
--s3-bucket bucket-name \
--s3-prefix appname/branchname/version \
--output-template-file packaged-template.yaml
Or using the aws-sam-cli
$ sam package \
--template-file /path_to_template/template.yaml \
--s3-bucket bucket-name \
--s3-prefix appname/branchname/version \
--output-template-file packaged-template.yaml
The packaged template will look something like this:
MyLambdaFunction:
Type: AWS::Serverless::Function
Properties:
CodeUri: s3://<mybucket>/<my-zipfile-path>
...
MyApi:
Type: AWS::Serverless::Api
Properties:
DefinitionUri: s3://<mybucket>/<my-openapi-file-path>
...
SAM template is deployed to AWS CloudFormation by creating a changeset using the SAM template followed by executing the changeset. Think of a ChangeSet as a diff between your current stack template and the new template that you are deploying. After you create a ChangeSet, you have the opportunity to examine the diff before executing it. Both the AWS Console and AWS CLI provide commands to create and execute a changeset.
Alternatively, you can use aws cloudformation deploy
CLI command to deploy the SAM template. Under the hood it creates and executes a changeset and waits until the deployment completes. It also prints debugging hints when the deployment fails. Run the following command to deploy the packaged template to a stack called my-new-stack
:
$ aws cloudformation deploy \
--template-file /path_to_template/packaged-template.yaml \
--stack-name my-new-stack \
--capabilities CAPABILITY_IAM
Or using aws-sam-cli
$ sam deploy \
--template-file /path_to_template/packaged-template.yaml \
--stack-name my-new-stack \
--capabilities CAPABILITY_IAM
Refer to the cloudformation documentation and samcli for more details.
CloudFormation provides handy functions that you can use to generate values at runtime. These are called Intrinsic Functions. Since SAM is deployed using CloudFormation, you can use these intrinsic functions within SAM as well. Here are some examples:
Transform: 'AWS::Serverless-2016-10-31'
# Parameters are CloudFormation features to pass input
# to your template when you create a stack
Parameters:
BucketName:
Type: String
CodeKey:
Type: String
Resources:
MyFunction:
Type: AWS::Serverless::Function
Properties:
Handler: index.handler
Runtime: nodejs4.3
CodeUri:
# !Ref function allows you to fetch value
# of parameters and other resources at runtime
Bucket: !Ref BucketName
Key: !Ref CodeKey
Transform: 'AWS::Serverless-2016-10-31'
# Parameters are CloudFormation features to pass input
# to your template when you create a stack
Parameters:
FunctionNameSuffix:
Type: String
Resources:
MyFunction:
Type: AWS::Serverless::Function
Properties:
# !Sub performs string substitution
FunctionName: !Sub "mylambda-${FunctionNameSuffix}"
Handler: index.handler
Runtime: nodejs4.3
CodeUri: s3://bucket/key
ImportValue
allows one stack to refer to the value of properties from another stack. ImportValue is supported on most properties, except the very few that SAM needs to parse. The following properties are not supported:
RestApiId
ofAWS::Serverless::Function
Policies
ofAWS::Serverless::Function
StageName
ofAWS::Serverless::Api