Topic
Unique constraints, key locking and 409 responses used to stop near-simultaneous duplicate requests from both being processed.
Effect 4 moves STM into Effect as TxRef and Effect.tx, and a transfer ported without the Effect.tx wrapper runs as four separate transactions. Effect 3 teams porting TRef code must place each boundary by hand, since the types barely track it.
Reality
Developer sunny56's ledger-core harness runs 8 threads of 250 transfers each and asserts that its naive read-modify-write strategy loses money. The same balance-total check, run under concurrency, shows whether the locking strategies conserve it.
Reality
Every booking in a flash-sale seat design on dev.to runs through one atomic Redis Lua script, yet the counter still needed four separate fixes. The last one holds each booking until a Redis replica confirms the write.
Reality
Writing on dev.to, Ben McCarthy describes rebuilding the guest-messaging agents across his 23 holiday lets around a typed per-request view of state, with world facts held apart from the policy about what the agent may promise.
Reality
A small FastAPI repro turns 20 simultaneous requests for one tenant into 20 database queries while the dashboard still reads healthy. The metric that catches it is in-flight loads per key.
Reality