How the callback is registered

watch.py

    logging.info(f'Watching Firestore collection {firestore_prefix}users/<user_id>/jobs '
                 f'for documents with status=={statuses_of_interest}')
    sync_client = firestore.Client(credentials=creds, project=project_id)
    # a Firebase index on collection_id and field status MUST exist for this query to work
    watched_jobs = sync_client.collection_group(job_collection_id).where('status', 'in', statuses_of_interest)

    ...
    
    watched_jobs.on_snapshot(callback)