bcquality/microsoft/knowledge/performance/findset-true-applies-updlock-on-read.md

bc-version	domain	keywords	technologies	countries	application-area
all	performance	findset updlock readisolation locking modify obsolete-syntax	al	w1	all

FindSet(true) applies UpdLock on the read; the two-parameter form is obsolete

Description

FindSet() and FindSet(false) are read-only — no locking. Per the upstream guidance, FindSet(true) "signifies the intent is to modify records" and "sets ReadIsolation::UpdLock on the record before finding rows." That is exactly the right shape when the loop body modifies each row: the read takes the same lock the modification will need, avoiding the deadlock window between an unlocked read and a later upgrade. The older two-parameter form FindSet(ForUpdate, UpdateKey) is obsolete — only the single-parameter signature should appear in new code.

Best Practice

Use FindSet(true) only when the loop body genuinely modifies the iterated rows; use FindSet() (or FindSet(false)) when the loop only reads. Do not write FindSet(true, true) or FindSet(true, false) — the two-parameter form is the obsolete signature.

See sample: findset-true-applies-updlock-on-read.good.al.

Anti Pattern

FindSet(true) on a loop that does not modify the iterated rows takes an UpdLock the work does not need; competing readers and writers stall against a lock the loop never uses. The mirror anti-pattern is FindSet() (no parameter) on a loop that does modify each row — the read takes a shared lock, the Modify then needs to upgrade, and the gap between them is a deadlock candidate.

See sample: findset-true-applies-updlock-on-read.bad.al.