Selling seats is harder than it looks. At 10 AM when Taylor Swift tickets go on sale, you have 50,000 seats and 500,000 concurrent buyers. You need to sell each seat exactly once. Overselling is a business disaster. Underselling (seats going unsold because they were locked but not purchased) is revenue loss. The core problem: how do you keep seat inventory consistent under massive concurrent load? Why This Is Hard A seat has three states: available, held, sold.

Two users edit the same document simultaneously. User A inserts “X” at position 5. User B deletes the character at position 3. Apply both naively and the result is corrupted. The positions shifted when B’s deletion ran first, and A’s insertion lands in the wrong place. The Position Problem Operations encode positions at generation time, not application time. When document state changes between generation and application, positions are stale. Operational Transformation (OT) transforms an incoming op relative to already-applied ops before executing it.

Redis locks expire after a TTL. If your process crashes, you wait up to 30 seconds for the lock to become available. ZooKeeper takes a different approach: lock it to the session, not a timer. Ephemeral Nodes ZooKeeper has two kinds of nodes: persistent (survive until explicitly deleted) and ephemeral (automatically deleted when the client session expires). A session is kept alive by a heartbeat. If the client crashes, heartbeats stop, the session expires after a configurable timeout, and the ephemeral node vanishes.

A single Redis instance holds your lock. Redis crashes. The lock entry is gone. But your client already received “acquired” before the crash and is happily running. Another client acquires the same lock on the recovered instance. Two lock holders. The single-instance Redis lock has a fundamental flaw. Quorum Locking Redlock is Redis creator Antirez’s answer. Instead of one Redis, use N independent instances (typically 5). To acquire the lock:

Two services start the same batch job at the same time. Both read the same data, both process it, both write conflicting results. Your database row lock didn’t help because the services are on different JVMs. This is the distributed lock problem. Why Database Locks Don’t Work Here A SELECT FOR UPDATE on a MySQL row holds a lock only for the lifetime of that connection. Cross-service, that’s useless. You’d need a shared coordination point, something every instance can talk to.