Technical Glossary

• Atomic Conditional Update: A single SQL statement (UPDATE ... WHERE condition) that evaluates availability and applies decrements atomically inside the database engine without holding open application-level locks.
• Conservation Invariant: The mathematical law $Available + Reserved + Sold == Total Allocated$ that must evaluate to true across all points in time.
• Fencing Token: A monotonically increasing integer issued by a distributed lock manager to detect and reject stale writes from paused or partitioned clients.
• Idempotency Key: A unique client-supplied token (UUID) ensuring an operation (e.g. payment or order creation) is executed exactly once even when retried multiple times.
• Optimistic Concurrency Control (OCC): A concurrency management method that assumes conflicts are rare, checking a record's version column at commit time and retrying if conflicts occur.
• Pessimistic Locking: Locking a database row (SELECT FOR UPDATE) to prevent any other transaction from reading or writing until the lock is released.
• Token Bucket: A rate-limiting algorithm where tokens refill at a steady rate and burst requests consume tokens up to bucket capacity.
• Virtual Waiting Room: A FIFO queue admission gatekeeper that throttles traffic surges during sale openings, admitting users in metered batches.