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Form rendering callbacks

During rendering of a Task in the browser, we may send calls to the server-side for additional data. In Mephisto, API views servicing such requests are called "remote procedures".

Presigned S3 URLs​

An example of a remote procedure that gets called during the initial form loading, is getPresignedUrl and getMultiplePresignedUrls functions. These functions allow to generate short-lived S3 URLs, in order to limit exposure of sensitive resources.

The below command auto-generates config token values that presign themselves during rendering of the Task page:

mephisto form_composer config --update-file-location-values "https://s3.amazonaws.com/..." --use_presigned_urls

This is how URL pre-signing works:

  • When a worker opens the Task page and the form HTML is generated, it will contain so-called "procedure tokens", i.e. token values that look like this: {{getMultiplePresignedUrls(<S3_FILE_URL>)}}
    • the "wrapper" part of a procedure token is the name of a Javascript function that will render itself dynamically (e.g. by calling some remote API to receive additional data)
    • the argument part is the argument value provided suring the function call
  • As soon as the form HTML is in place, the remote procedure gets called
  • Mephisto's predefined remote procedure generates presigned URL, and its expiration starts ticking

Presigned S3 URLs use the following environment variables:

  • Required: valid AWS credentials: AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and AWS_DEFAULT_REGION (form_composer config command)
  • Optional: URL expiration time S3_URL_EXPIRATION_MINUTES (if missing the default value is 60 minutes)

Custom callbacks​

You can write your own remote procedures. A good place to start is looking at how S3 URL presigning is implemented in the examples/form_composer_demo example project.