Concurrency is hard
We ran into a problem last month around concurrent mutations of our credit ledger which lead us to adopt a really cool feature of Postgres.
At Peachjar, we track the purchase, usage, and refund of credits as a log of transactions (e.g. a ledger). The ledger system is really just a table in Postgres and a microservice (with many deployments) that contain the rules for interacting with that table.
Image the table looking something like this:
- Press the purchase button twice (and the button didn't disable after the first push).
- The user refreshes the page after a form submittal (classic replay of a POST).
- The user is malicious and purposely executes two concurrent purchase requests.
- A timeout is reached and a retry is executed (this was our actual problem).
[2]. Spring AOP transaction management in Hibernate – Mkyong.com
[3]. Quick Guide to Spring Bean Scopes | Baeldung
