Flask-SQLAlchemy 不会始终如一地增加列

Question

I've build small web app and noticed when there are multiple actions at the same time where it requires database to get last record it doesn't get newest results

For example:

add_amount = 1

record = Table.query.filter_by(id=1).first()

record.amount += add_amount
db.session.commit()

if multiple people add record in same time it won't get newest data

• at 12:00:01 record.amount == 3 someone adds 1
• at 12:00:02 record.amount == 3 someone adds 1
• at 12:00:03 record.amount == 4 instead of 5

What would be best solution to take care of it? create new db session?

Answer 1

What you're running into is called a race condition . It's a heavily studied subject within concurrency in general. It happens, for example, when the following order of operations occurs:

• Process A fetches the row with ID 1 (amount = 10)
• Process A increments the amount (amount = 11)
• Process B fetches the row with ID 1 (amount = 10)
• Process A saves the updated amount (amount = 11)
• Process B increments the amount (amount = 11)
• Process B saves the updated amount (amount = 11)

Now you can see that the change made by A has been overwritten completely. There is no record that it ever happened and it did not influence the current value of amount .

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As per this answer , you can apply a lock that will allow only one process to have the row fetched at a time, and the lock is released on db.session.commit() . The lock locks only that one row, so any other calls to any other records should be fine.

record = Table.query.filter_by(id=1).with_for_update().first()

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With the locking, the process would become:

• Process A tries to fetch the row with ID 1 and acquires the lock (amount = 10)
• Process A increments the amount (amount = 11)
• Process B tries to fetch the row with ID 1, but has to wait for the lock
• Process A saves the updated amount and releases the lock (amount = 11)
• Process B acquires the lock (amount = 11)
• Process B increments the amount (amount = 12)
• Process B saves the updated amount and releases the lock (amount = 12)