def __init__(self, client, collection, doc_id, field_name='<STR_LIT>'):
super().__init__()<EOL>self.client = client<EOL>self.collection = collection<EOL>self.doc_id = doc_id<EOL>self.doc_ref = client.collections(collection).document(doc_id)<EOL>self.field_name = field_name<EOL>
Init Backend
:param client firestore.Client: the firestore Client instance
:param collection str: the firestore collection where to store tokens (can be a field_path)
:param doc_id str: # the key of the token document. Must be unique per-case.
:param field_name: the name of the field that stores the token in the document