Which concurrency control protocol prevents deadlock and ensures…

Concept

A concurrency control protocol is judged on two independent properties: whether it guarantees serializability (the concurrent schedule is equivalent to some serial order) and whether it can deadlock (transactions blocked in a cyclic wait). A protocol that NEVER makes a transaction wait for a lock cannot form a wait-cycle, so it is deadlock-free by construction.

Application

Under timestamp-ordering, every transaction is assigned a unique timestamp on arrival, and each data item carries read- and write-timestamps. Conflicting operations are forced into timestamp order:

• A transaction whose operation would violate the timestamp order is aborted and restarted with a new timestamp — it is never blocked waiting.

• Because no transaction ever waits on another, no wait-for cycle can form, so deadlock is impossible.

• Serializing every conflicting pair by timestamp yields a schedule equivalent to the serial order of the timestamps, so the schedule is conflict-serializable.

Both required properties — serializability and freedom from deadlock — hold together only for timestamp-ordering among the choices.

Contrast

2PL guarantees conflict-serializability but uses blocking locks, so two transactions can wait on each other and deadlock. Strict 2PL adds cascadeless recovery on top of 2PL but is still a locking protocol and is equally deadlock-prone. “Optimistic” validation defers conflict checks to a validation phase and resolves conflicts by aborting, but it is not the classical waiting-free protocol named for this guarantee, and shared-locking alone is only one lock mode, not a complete protocol that ensures serializability.

Result

Timestamp ordering protocol is the protocol that both prevents deadlock and ensures serializability.