I have a web app written by golang. There is also an independent data fetcher to be run separately as a standalone app to collect data and store in the database/datastore. How to run this data fetcher as a worker in the hosting environment e.g. App Engine/Heroku
In appengine you would refactor your loader as one or more tasks. For example https://github.com/mjibson/goread/blob/master/tasks.go and https://github.com/mjibson/goread/blob/master/app/queue.yaml use tasks to update a datastore from RSS feeds.
The docs at https://cloud.google.com/appengine/docs/go/taskqueue/ explain the API.