Compare optimistic and pessimistic locking strategies.

Pessimistic locking locks resources when accessed, preventing conflicts but reducing concurrency - SELECT FOR UPDATE. Good when conflicts are frequent. Optimistic locking assumes conflicts are rare, checks at commit time (using version/timestamp) - if data changed, reject and retry. Better for high-read systems with infrequent writes. ORM frameworks often support optimistic locking. Choice depends on conflict frequency and acceptable retry rate.