variables.md

Read this on the main serverless docs site

Google - Variables

The Serverless framework provides a powerful variable system which allows you to add dynamic data into your serverless.yml. With Serverless Variables, you'll be able to do the following:

• Reference & load variables from environment variables
• Reference & load variables from CLI options
• Recursively reference properties of any type from the same serverless.yml file
• Recursively reference properties of any type from other YAML / JSON files
• Recursively nest variable references within each other for ultimate flexibility
• Combine multiple variable references to overwrite each other

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.

Reference Properties In serverless.yml

To self-reference properties in serverless.yml, use the ${self:someProperty} syntax in your serverless.yml. This functionality is recursive, so you can go as deep in the object tree as you want.

service: new-service
provider: google

custom:
  resource: projects/*/topics/my-topic

functions:
  first:
    handler: firstPubSub
    events:
      - event:
          eventType: providers/cloud.pubsub/eventTypes/topics.publish
          resource: ${self:custom.resource}
  second:
    handler: secondPubSub
    events:
      - event:
          eventType: providers/cloud.pubsub/eventTypes/topics.publish
          resource: ${self:custom.resource}

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

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
topic: projects/*/topics/my-topic

# serverless.yml
service: new-service
provider: google

custom: ${file(./myCustomFile.yml)} # You can reference the entire file

functions:
  hello:
    handler: pubSub.hello
    events:
      - event:
          eventType: providers/cloud.pubsub/eventTypes/topics.publish
          resource: ${file(./myCustomFile.yml):topic} # Or you can reference a specific property
  world:
      handler: pubSub.hello
      events:
          eventType: providers/cloud.pubsub/eventTypes/topics.publish
          resource: ${self:custom.topic} # 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 topic 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:
  - event:
      eventType: providers/cloud.pubsub/eventTypes/topic.publish
      resource: projects/*/topics/my-topic

and for JSON:

{
  "myevents": [{
    "event" : {
      "eventType": "providers/cloud.pubsub/eventTypes/topic.publish",
      "resource" : "projects/*/topics/my-topic"
    }
  }]
}

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

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

or for a JSON reference file use this sytax:

functions:
  hello:
    handler: pubSub.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)}.

// resources.js
module.exports.topic = () => {
   // Code that generates dynamic data
   return 'projects/*/topics/my-topic';
}

// config.js
module.exports = () => {
  return {
    property1: 'some value',
    property2: 'some other value'
  }
}

# serverless.yml
service: new-service

provider: google

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

functions:
  first:
    handler: pubSub
    events:
      - event:
          eventType: providers/cloud.pubsub/eventTypes/topics.publish
          resource: ${file(./resources.js):topic} # 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:

// myCustomFile.js
module.exports.pubSub = () => {
   // Code that generates dynamic data
   return {
     resource: 'projects/*/topics/my-topic'
   };
}

# serverless.yml
service: new-service

provider: google

functions:
  first:
    handler: pubSub
    events:
      - event:
          eventType: providers/cloud.pubsub/eventTypes/topics.publish
          resource: ${file(./myCustomFile.js):pubSub.resource} # Reference a specific module

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(google-cloud-resources.json)}

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

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.

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.