bcquality/microsoft/knowledge/performance/prefer-readisolation-over-locktable-for-reads.md

bc-version	domain	keywords	technologies	countries	application-area
all	performance	readisolation locktable updlock read-only transaction-scope isolation-level	al	w1	all

Prefer ReadIsolation over LockTable for read-only scenarios

Description

Without read scale-out, LockTable causes subsequent reads of that table in the transaction to use UPDLOCK. With read scale-out, those reads use REPEATABLEREAD on the replica instead. ReadIsolation selects an isolation level for one record instance. A helper that only reads should not broaden locking for the table merely to request committed data.

Best Practice

For a read-only operation that specifically requires committed data, set Rec.ReadIsolation := IsolationLevel::ReadCommitted immediately before the read. If the default isolation is sufficient, set neither property. ReadCommitted can still block behind writers and does not guarantee that repeated reads stay unchanged; use the isolation level required by the operation. Reserve update locks for read-before-write logic, not read-only helpers.

See sample: prefer-readisolation-over-locktable-for-reads.good.al.

Anti Pattern

Rec.LockTable(); at the top of a helper that only reads, perhaps to "make sure the read is consistent". It takes stronger isolation than the helper needs and changes later reads of that table in the surrounding transaction or read-scale-out session.

See sample: prefer-readisolation-over-locktable-for-reads.bad.al.