Concurrency
Concurrency means operations overlap and can act on stale state.
What it is
Concurrency means operations overlap and can act on stale state.
When to use it
Protect shared invariants using constraints, locks, or conditional updates.
Syntax
UPDATE accounts SET balance_cents = balance_cents - ?
WHERE id = ? AND balance_cents >= ?;Example
// Laravel conditional update:
$changed = DB::table("accounts")->where("id", $id)
->where("balance_cents", ">=", $amountCents)
->decrement("balance_cents", $amountCents);
if ($changed !== 1) throw new InsufficientFunds();Common mistakes
Checking a balance then updating without protection races; group related writes in a transaction.