Should I use select_for_update in my django application with postgresql database?

Question

I am currently building a booking tool (django and postgresql) where people book spaces in a location. The location has limited capacity and therefore might run out of space while a user is trying to book a space. I imagine that this could happen when another user books a place slightly before the current user is booking, e.g. if only 1 space is left, the other user might book too and the database cannot handle it.

So my question is would you strongly advise using select_for_update or is there something else that might help me overcome this issue?

Snake9920 · Accepted Answer · 2021-04-06 20:18:40Z

1

Yes, that's a correct use of select_for_update. You would be blocking a specific location row (apply a filter before calling select_for_update). That means that 2 different locations can be booked concurrently, but if there are 2 bookings for the same location happening at exactly the same second they would be called.

This creates a critical section and you can sure that it won't overlap with a critical section of another request. In within the critical section, you will have to validate that the selected time slot is free - without that validation select_for_update would have no effect.

I could imagine another approach based on unique constraints, it's not universal but might be easier to implement. Let's imagine that you are booking a resource for a specific day. You could have a "unique together" combination for the resource_id and date. A subsequent save would raise an IntegrityError and you could catch it and inform the user that the resource was just booked for the selected date.

answered Apr 6, 2021 at 20:18

Snake9920

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2 Comments

manfredobelugo Over a year ago

Thank you so much @Snake9920 when I use select_for_update on one table is it only locking this one, or also all other tables in the database?

Snake9920 Over a year ago

It is locking specific rows (the ones you selected) rather than entire tables. I'd recommend to read more about it in the Postgres docs as Django is simply adding the "FOR UPDATE" clause to the SQL query and the database engine is doing the actual locking.

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