ACID Transaction Ledger
How can a fund-transfer system protect balances when a transaction fails or multiple transfers happen at once?
01 / THE QUESTION
How can a fund-transfer system protect balances when a transaction fails or multiple transfers happen at once?
02 / THE APPROACH
Build a PostgreSQL and Python ledger with debit, credit, and transaction logging inside one database transaction. Add a non-negative balance constraint and a concurrency test comparing isolation levels.
03 / THE RESULT
Implemented schema and atomic transfer logic, with concurrency and durability tests still being run and documented in the repository.
[ Project insights ]
EVIDENCE / FINDINGSA transfer is one unit of work
The transfer function wraps debit, credit, and logging in conn.begin(). If a step fails, the transaction rolls back instead of leaving a partially completed transfer.
Guardrails belong in the database
CHECK (balance >= 0) protects the balance invariant at the database level, regardless of which application writes to the table.
Concurrency is a separate question
The repository includes a multithreaded scenario comparing READ COMMITTED and SERIALIZABLE. Published test outcomes are not complete, so neither isolation nor crash durability is presented as fully verified.
CONTEXT / LIMITATIONS
The repository explicitly marks this project In Progress. Concurrency and durability evidence is incomplete; the illustration is concept artwork, not proof of a successful test run.