Our GAE app makes a local copy of another website's relational database in the NDB. There are 4 entity types - User, Table, Row, Field. Each user has a bunch of tables, each table has a bunch of rows, each row has a bunch of fields.
SomeUser > SomeTable > ARow > AField
Thus, each User becomes one entity group. I need a feature where I can clear out all the tables (and their rows) for a certain user. What is the right way to delete all the tables and all the rows, while avoiding the contention limit of ~5 operations/second.
The current code is getting TransactionFailedErrors because of contention on the Entity Group.
(detail that I overlooked is that we only want to delete tables with the attribute 'service' set to a certain value)
def delete_tables_for_service(user, service):
tables = Tables.query(Tables.service == service, ancestor=user.key).fetch(keys_only=True)
for table in tables:
keys = []
keys += Fields.query(ancestor=table).fetch(keys_only=True)
keys += TableRows.query(ancestor=table).fetch(keys_only=True)
keys.append(table)
ndb.delete_multi(keys)
If all of the entities you're deleting are in one entity group, try deleting them all in one transaction. Without an explicit transaction, each delete is occurring in its own transaction, and all of the transactions have to line up (via contention and retries) to change the entity group.