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Read this on the main serverless docs site

Variables

Variables allow users to dynamically replace config values in serverless.yml config.

They are especially useful when providing secrets for your service to use and when you are working with multiple stages.

Syntax

To use variables, you will need to reference values enclosed in ${} brackets.

# serverless.yml file
yamlKeyXYZ: ${variableSource} # see list of current variable sources below
# this is an example of providing a default value as the second parameter
otherYamlKey: ${variableSource, defaultValue}

You can define your own variable syntax (regex) if it conflicts with CloudFormation's syntax.

Note: You can only use variables in serverless.yml property values, not property keys. So you can't use variables to generate dynamic logical IDs in the custom resources section for example.

Current variable sources:

Recursively reference properties

You can also Recursively reference properties with the variable system. This means you can combine multiple values and variable sources for a lot of flexibility.

For example:

provider:
  name: aws
  stage: ${opt:stage, 'dev'}
  environment:
    MY_SECRET: ${file(./config.${self:provider.stage}.json):CREDS}

If sls deploy --stage qa is ran, the option stage=qa is used inside the ${file(./config.${self:provider.stage}.json):CREDS} variable and it will resolve the config.qa.json file and use the CREDS key defined.

How that works:

  1. stage is set to qa from the option supplied to the sls deploy --stage qa command
  2. ${self:provider.stage} resolves to qa and is used in ${file(./config.${self:provider.stage}.json):CREDS}
  3. ${file(./config.qa.json):CREDS} is found & the CREDS value is read
  4. MY_SECRET value is set

Likewise, if sls deploy --stage prod is ran the config.prod.json file would be found and used.

If no --stage flag is provided, the second parameter defined in ${opt:stage, 'dev'} a.k.a dev will be used and result in ${file(./config.dev.json):CREDS}.

Reference Properties In serverless.yml

To self-reference properties in serverless.yml, use the ${self:someProperty} syntax in your serverless.yml. someProperty can contain the empty string for a top-level self-reference or a dotted attribute reference to any depth of attribute, so you can go as shallow or deep in the object tree as you want.

service: new-service
provider: aws
custom:
  globalSchedule: rate(10 minutes)
  newService: ${self:}
  # the following will resolve identically in other serverless.yml files so long as they define
  # `custom.newService: ${file(<relative-path-to-this-file>/serverless.yml)}`
  exportName: ${self:custom.newService.service}-export

functions:
  hello:
      handler: handler.hello
      events:
        - schedule: ${self:custom.globalSchedule}
  world:
      handler: handler.world
      events:
        - schedule: ${self:custom.globalSchedule}
resources:
  Outputs:
    NewServiceExport:
      Value: 'A Value To Export'
      Export:
        Name: ${self:custom.exportName}

In the above example you're setting a global schedule for all functions by referencing the globalSchedule property in the same serverless.yml file. This way, you can easily change the schedule for all functions whenever you like.

Referencing Environment Variables

To reference environment variables, use the ${env:SOME_VAR} syntax in your serverless.yml configuration file. It is valid to use the empty string in place of SOME_VAR. This looks like "${env:}" and the result of declaring this in your serverless.yml is to embed the complete process.env object (i.e. all the variables defined in your environment).

Note:

Keep in mind that sensitive information which is provided through environment variables can be written into less protected or publicly accessible build logs, CloudFormation templates, et cetera.

service: new-service
provider: aws
functions:
  hello:
    name: ${env:FUNC_PREFIX}-hello
    handler: handler.hello
  world:
    name: ${env:FUNC_PREFIX}-world
    handler: handler.world

In the above example you're dynamically adding a prefix to the function names by referencing the FUNC_PREFIX env var. So you can easily change that prefix for all functions by changing the FUNC_PREFIX env var.

Referencing CLI Options

To reference CLI options that you passed, use the ${opt:some_option} syntax in your serverless.yml configuration file. It is valid to use the empty string in place of some_option. This looks like "${opt:}" and the result of declaring this in your serverless.yml is to embed the complete options object (i.e. all the command line options from your serverless command).

service: new-service
provider: aws
functions:
  hello:
      name: ${opt:stage}-hello
      handler: handler.hello
  world:
      name: ${opt:stage}-world
      handler: handler.world

In the above example, you're dynamically adding a prefix to the function names by referencing the stage option that you pass in the CLI when you run serverless deploy --stage dev. So when you deploy, the function name will always include the stage you're deploying to.

Reference CloudFormation Outputs

You can reference CloudFormation stack output values as the source of your variables to use in your service with the cf:stackName.outputKey syntax. For example:

service: new-service
provider: aws
functions:
  hello:
      name: ${cf:another-service-dev.functionPrefix}-hello
      handler: handler.hello
  world:
      name: ${cf:another-stack.functionPrefix}-world
      handler: handler.world

In that case, the framework will fetch the values of those functionPrefix outputs from the provided stack names and populate your variables. There are many use cases for this functionality and it allows your service to communicate with other services/stacks.

You can reference CloudFormation stack outputs export values as well. For example:

# Make sure you set export value in StackA.

  Outputs:
    DynamoDbTable:
      Value:
        "Ref": DynamoDbTable
      Export:
        Name: DynamoDbTable-${self:custom.stage}

# Then you can reference the export name in StackB

provider:
  environment:
    Table:
        'Fn::ImportValue': 'DynamoDbTable-${self:custom.stage}'

Referencing S3 Objects

You can reference S3 values as the source of your variables to use in your service with the s3:bucketName/key syntax. For example:

service: new-service
provider: aws
functions:
  hello:
      name: ${s3:myBucket/myKey}-hello
      handler: handler.hello

In the above example, the value for myKey in the myBucket S3 bucket will be looked up and used to populate the variable.

Reference Variables using the SSM Parameter Store

You can reference SSM Parameters as the source of your variables with the ssm:/path/to/param syntax. For example:

service: ${ssm:/path/to/service/id}-service
provider:
  name: aws
functions:
  hello:
    name: ${ssm:/path/to/service/myParam}-hello
    handler: handler.hello

In the above example, the value for the SSM Parameters will be looked up and used to populate the variables.

You can also reference encrypted SSM Parameters, of type SecureString, using the extended syntax, ssm:/path/to/secureparam~true.

service: new-service
provider: aws
functions:
  hello:
    name: hello
    handler: handler.hello
custom:
  supersecret: ${ssm:/path/to/secureparam~true}

In this example, the serverless variable will contain the decrypted value of the SecureString.

Reference Variables in Other Files

You can reference variables in other YAML or JSON files. To reference variables in other YAML files use the ${file(./myFile.yml):someProperty} syntax in your serverless.yml configuration file. To reference variables in other JSON files use the ${file(./myFile.json):someProperty} syntax. It is important that the file you are referencing has the correct suffix, or file extension, for its file type (.yml for YAML or .json for JSON) in order for it to be interpreted correctly. Here's an example:

# myCustomFile.yml
globalSchedule: rate(10 minutes)
# serverless.yml
service: new-service
provider: aws
custom: ${file(./myCustomFile.yml)} # You can reference the entire file
functions:
  hello:
      handler: handler.hello
      events:
        - schedule: ${file(./myCustomFile.yml):globalSchedule} # Or you can reference a specific property
  world:
      handler: handler.world
      events:
        - schedule: ${self:custom.globalSchedule} # This would also work in this case

In the above example, you're referencing the entire myCustomFile.yml file in the custom property. You need to pass the path relative to your service directory. You can also request specific properties in that file as shown in the schedule property. It's completely recursive and you can go as deep as you want. Additionally you can request properties that contain arrays from either YAML or JSON reference files. Here's a YAML example for an events array:

myevents:
  - schedule:
     rate: rate(1 minute)

and for JSON:

{
  "myevents": [{
    "schedule" : {
      "rate" : "rate(1 minute)"
    }
  }]
}

In your serverless.yml, depending on the type of your source file, either have the following syntax for YAML:

functions:
  hello:
    handler: handler.hello
    events: ${file(./myCustomFile.yml):myevents}

or for a JSON reference file use this sytax:

functions:
  hello:
    handler: handler.hello
    events: ${file(./myCustomFile.json):myevents}

Note: If the referenced file is a symlink, the targeted file will be read.

Reference Variables in Javascript Files

You can reference JavaScript files to add dynamic data into your variables.

References can be either named or unnamed exports. To use the exported someModule in myFile.js you'd use the following code ${file(./myFile.js):someModule}. For an unnamed export you'd write ${file(./myFile.js)}. The first argument to your export will be a reference to the Serverless object, containing your configuration.

Here are other examples:

// scheduleConfig.js
module.exports.rate = () => {
   // Code that generates dynamic data
   return 'rate (10 minutes)';
}
// config.js
module.exports = (serverless) => {
  serverless.cli.consoleLog('You can access Serverless config and methods as well!');

  return {
    property1: 'some value',
    property2: 'some other value'
  }
}
# serverless.yml
service: new-service
provider: aws

custom: ${file(./config.js)}

functions:
  hello:
      handler: handler.hello
      events:
        - schedule: ${file(./scheduleConfig.js):rate} # Reference a specific module

You can also return an object and reference a specific property. Just make sure you are returning a valid object and referencing a valid property:

# serverless.yml
service: new-service
provider: aws
functions:
  scheduledFunction:
      handler: handler.scheduledFunction
      events:
        - schedule: ${file(./myCustomFile.js):schedule.ten}
// myCustomFile.js
module.exports.schedule = () => {
   // Code that generates dynamic data
   return {
     ten: 'rate(10 minutes)',
     twenty: 'rate(20 minutes)',
     thirty: 'rate(30 minutes)'
   };
}

If your use case requires handling dynamic/async data sources (ie. DynamoDB, API calls...etc), you can also return a Promise that would be resolved as the value of the variable:

# serverless.yml
service: new-service
provider: aws
functions:
  scheduledFunction:
      handler: handler.scheduledFunction
      events:
        - schedule: ${file(./myCustomFile.js):promised}
// myCustomFile.js
module.exports.promised = () => {
   // Async code that fetches the rate config...
   return Promise.resolve('rate(10 minutes)');
}

Multiple Configuration Files

Adding many custom resources to your serverless.yml file could bloat the whole file, so you can use the Serverless Variable syntax to split this up.

resources:
  Resources: ${file(cloudformation-resources.json)}

The corresponding resources which are defined inside the cloudformation-resources.json file will be resolved and loaded into the Resources section.

Nesting Variable References

The Serverless variable system allows you to nest variable references within each other for ultimate flexibility. So you can reference certain variables based on other variables. Here's an example:

service: new-service
provider: aws
custom:
  myFlexibleArn: ${env:${opt:stage}_arn}

functions:
  hello:
      handler: handler.hello

In the above example, if you pass dev as a stage option, the framework will look for the dev_arn environment variable. If you pass production, the framework will look for production_arn, and so on. This allows you to creatively use multiple variables by using a certain naming pattern without having to update the values of these variables constantly. You can go as deep as you want in your nesting, and can reference variables at any level of nesting from any source (env, opt, self or file).

Overwriting Variables

The Serverless framework gives you an intuitive way to reference multiple variables as a fallback strategy in case one of the variables is missing. This way you'll be able to use a default value from a certain source, if the variable from another source is missing.

For example, if you want to reference the stage you're deploying to, but you don't want to keep on providing the stage option in the CLI. What you can do in serverless.yml is:

service: new-service
provider:
  name: aws
  stage: dev
custom:
  myStage: ${opt:stage, self:provider.stage}
  myRegion: ${opt:region, 'us-west-1'}

functions:
  hello:
      handler: handler.hello

What this says is to use the stage CLI option if it exists, if not, use the default stage (which lives in provider.stage). So during development you can safely deploy with serverless deploy, but during production you can do serverless deploy --stage production and the stage will be picked up for you without having to make any changes to serverless.yml.

You can have as many variable references as you want, from any source you want, and each of them can be of different type and different name.

Using Custom Variable Syntax

In some cases, the ${xxx} variable syntax conflicts with some CloudFormation functionality. In that case you can provide a custom syntax to overwrite our default ${xxx} syntax by setting the provider.variableSyntax property to the desired regex:

service: new-service

provider:
  name: aws
  runtime: nodejs6.10
  variableSyntax: "\\${{([ ~:a-zA-Z0-9._\\'\",\\-\\/\\(\\)]+?)}}" # notice the double quotes for yaml to ignore the escape characters!
#  variableSyntax: "\\${((?!AWS)[ ~:a-zA-Z0-9._'\",\\-\\/\\(\\)]+?)}" # Use this for allowing CloudFormation Pseudo-Parameters in your serverless.yml -- e.g. ${AWS::Region}. All other Serverless variables work as usual.

custom:
  myStage: ${{opt:stage}}

In this example, we're overwriting the default regex for our variable syntax. So whenever you define variables, you now need to use ${{}} instead of ${} (double curly brackets).

Migrating serverless.env.yml

Previously we used the serverless.env.yml file to track Serverless Variables. It was a completely different system with different concepts. To migrate your variables from serverless.env.yml, you'll need to decide where you want to store your variables.

Using a config file: You can still use serverless.env.yml, but the difference now is that you can structure the file however you want, and you'll need to reference each variable/property correctly in serverless.yml. For more info, you can check the file reference section above.

Using the same serverless.yml file: You can store your variables in serverless.yml if they don't contain sensitive data, and then reference them elsewhere in the file using self:someProperty. For more info, you can check the self reference section above.

Using environment variables: You can instead store your variables in environment variables and reference them with env.someEnvVar. For more info, you can check the environment variable reference section above.

Making your variables stage/region specific: serverless.env.yml allowed you to have different values for the same variable based on the stage/region you're deploying to. You can achieve the same result by using the nesting functionality of the new variable system. For example, if you have two different ARNs, one for dev stage and the other for prod stage, you can do the following: ${env:${opt:stage}_arn}. This will make sure the correct env var is referenced based on the stage provided as an option. Of course you'll need to export both dev_arn and prod_arn env vars on your local system.

Now you don't need serverless.env.yml at all, but you can still use it if you want. It's just not required anymore. Migrating to the new variable system is easy and you just need to know how the new system works and make small adjustments to how you store & reference your variables.

Pseudo Parameters Reference

You can reference AWS Pseudo Parameters

Here's an example:

     Resources:
        - 'Fn::Join':
          - ':'
          -
            - 'arn:aws:logs'
            - Ref: 'AWS::Region'
            - Ref: 'AWS::AccountId'
            - 'log-group:/aws/lambda/*:*:*'