sharedlock

Shared lock, or S-lock, is a locking mechanism used in concurrent systems, particularly database engines, to coordinate access to a data resource such as a row, page, or table. An S-lock allows multiple transactions to read the resource simultaneously, but it prevents write operations. A resource that is held by one or more S-locks cannot be written to until all S-locks are released and an exclusive lock can be granted.

In practice, S-locks are compatible with other S-locks but incompatible with exclusive locks (X-locks). Some systems also use update locks (U-locks), which are compatible with S-locks and can be promoted to an X-lock when a write occurs. Many implementations follow a two-phase locking protocol, acquiring locks in a growing phase and releasing them in a shrinking phase to ensure transactional serializability.

Lifecycle and usage: a read operation typically acquires an S-lock on the data it reads. If a subsequent write is needed, the transaction may upgrade to an X-lock or perform an update using a U-lock, depending on the database’s locking rules. Locks are held for the duration of the transaction or a defined scope and are released at commit or rollback.

Considerations: shared locking increases read concurrency but can block writers, potentially leading to longer wait times or deadlocks in high-contention scenarios. Mitigations include deadlock detection, lock timeouts, lock escalation strategies, or using alternative concurrency control models such as multiversion concurrency control (MVCC) or optimistic locking.