If you make 2 updates, check the balance to ensure > 0, and rollback if < 0, and... | Hacker News

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mangeletti on May 30, 2015 | parent | context | favorite | on: Hacking Starbucks for unlimited coffee

If you make 2 updates, check the balance to ensure > 0, and rollback if < 0, and you do all of this in a transaction, doesn't concurrency no longer matter? If another transaction beats you to the punch, won't the balance check query reflect that?

TheLoneWolfling on May 30, 2015 [–]

Nope - this can happen with cards with a balance above zero too.

Look at what happens if you start two transfers of 1/2 the money from A to B.

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