Using functions to get an IAM token for a service account

If the function version was created with a service account, you can get an IAM token for it from:

• The handler context. The IAM token is in the access_token field of the context parameter.
• The metadata service in Google Compute Engine via the API.

To get an IAM token:

1. Create a function.

2. Select the programming language and create a version of the function:

   Node.js

   Python

   1. Prepare a ZIP archive with the function code:
      1. Save the following code to a file named index.js to get the IAM token:
         • From the handler context.

           exports.main = async function (event, context) {
               return {
                   'statusCode': 200,
                   'headers': {
                       'Content-Type': 'text/plain'
                   },
                   'isBase64Encoded': false,
                   'body': context.token
               }
           };
           

         • Using the API.

           const fetch = require("node-fetch");
           let url = 'http://169.254.169.254/computeMetadata/v1/instance/service-accounts/default/token';
           let headers = {'Metadata-Flavor': 'Google'};
           
           exports.main = async function (event) {
               const resp = await fetch(url, {
                   headers: headers,
               });
               return {
                   code: resp.status,
                   body: await resp.text()
               };
           };
           

      2. If you get your IAM token using the API, save the following code to a file named package.json:

         {
             "name": "my-app",
             "dependencies": {
                 "node-fetch": "2.x"
             }
         }
         

      3. Add index.js and package.json (if you get your IAM token using the API) to a ZIP file called index-js.zip.
   2. Create a function version. Indicate the following:
      • Runtime environment: nodejs16.
      • Code upload method: ZIP archive.
      • File: index-js.zip.
      • Entry point: index.main.
      • The service account to get the IAM token for.

   1. Prepare a ZIP archive with the function code:
      1. Save the following code to a file named index.py to get the IAM token:
         • From the handler context.

           def main(event, context):
           
               return {
                   'statusCode': 200,
                   'headers': {
                       'Content-Type': 'text/plain'
                   },
                   'isBase64Encoded': False,
                   'body': context.token
               }
           

         • Using the API.

           import requests
           
           url = 'http://169.254.169.254/computeMetadata/v1/instance/service-accounts/default/token'
           headers = {'Metadata-Flavor': 'Google'}
           
           
           def main(event, context):
           
               resp = requests.get(url, headers=headers)
           
               return {
                   'statusCode': 200,
                   'headers': {
                       'Content-Type': 'text/plain'
                   },
                   'isBase64Encoded': False,
                   'body': resp.content.decode('UTF-8')
               }
           

      2. Add index.py to the index-py.zip archive.
   2. Create a function version. Indicate the following:
      • Runtime environment: python37.
      • Code upload method: ZIP archive.
      • File: index-py.zip.
      • Entry point: index.main.
      • The service account to get the IAM token for.

3. Run the function.

   The function response looks like this:

   {
       "access_token": "CggVAgAAABoBMRKABHGgpZ......",
       "expires_in": 42299,
       "token_type": "Bearer"
   }