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a9f3c50863
The previous LLM-generated knowledge files contained factual
hallucinations. The most visible was the claim that `FindFirst` /
`FindLast` "forces a full-table scan" on an unfiltered record - it does
not; those APIs return a single row via the current key.
Other inaccuracies the audit found and fixed:
* `FindSet(true)` was described as "taking a LockTable". The correct
upstream phrasing is that `FindSet(true)` sets
`ReadIsolation::UpdLock` on the read. UpdLock and LockTable are
related but distinct mechanisms.
* The list of production-scale tables had been invented beyond the
upstream source (e.g. "Detailed Cust. Ledg. Entry") without a
citation. The regenerated list matches the ten tables upstream lists
with their P95 row counts.
* `SetLoadFields` guidance had been augmented with an extra mechanism
claim ("the database resolves the filter using the index without
hydrating the value") not present in upstream.
Approach: full regeneration of `microsoft/knowledge/` from the six
upstream BCApps Code Review instruction files, with Microsoft Learn /
the AL language reference as a secondary source. Every claim in every
regenerated file is anchored to a verbatim upstream quote (or a Learn
URL); the audit trail lives in artifacts/trace-<domain>.json on the
session workspace.
The PR #11 transaction/error-handling cluster is preserved verbatim:
* performance/understand-implicit-transaction-boundary.md
* performance/codeunit-run-as-atomic-sub-operation.{md,good.al,bad.al}
* performance/codeunit-run-requires-prior-commit-inside-transaction.{md,good.al,bad.al}
* performance/use-tryfunction-for-error-catching-not-rollback.{md,good.al,bad.al}
* performance/avoid-commit-inside-loops.{md,good.al,bad.al}
* security/commitbehavior-attribute-scopes-explicit-commits.{md,good.al,bad.al}
* testing/transactionmodel-attribute-governs-test-transactions.{md,good.al,bad.al}
These articles already cite Microsoft Learn and were carefully
cross-referenced; the regeneration skips their topics rather than
duplicating them.
File counts after regeneration:
performance 35 .md (5 preserved + 30 new)
privacy 17 .md
security 18 .md (1 preserved + 17 new)
style 33 .md
testing 1 .md (preserved)
ui 19 .md
upgrade 18 .md
Total 141 atomic knowledge files, each strictly one rule. All pass
.github/scripts/validate_frontmatter.py with 0 errors and 0 warnings.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
26 lines
1.9 KiB
Markdown
26 lines
1.9 KiB
Markdown
---
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bc-version: [all]
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domain: performance
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keywords: [readisolation, locktable, updlock, read-only, transaction-scope, isolation-level]
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technologies: [al]
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countries: [w1]
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application-area: [all]
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---
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# Prefer ReadIsolation over LockTable for read-only scenarios
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## Description
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`LockTable` and `ReadIsolation` solve different problems with different blast radii. Per the upstream guidance, "`LockTable` ensures that all READS against that table will happen with UPDLOCK for the remainder of the transaction." `ReadIsolation` "only pertains to the current record instance, while `LockTable` affects the lockstate of the entire transaction." `ReadIsolation` is also more expressive: it can heighten or lower the isolation level inside an already-established transaction. Reaching for `LockTable` when only a single read needs guarding therefore poisons every later read on that table — including reads in other code paths that share the transaction.
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## Best Practice
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For a read-only operation, or a single read that needs a higher isolation level than the surrounding transaction, set `Rec.ReadIsolation := IsolationLevel::ReadCommitted;` (or the level the call requires) immediately before the read. The hint applies only to that record instance. Save `LockTable` for code that genuinely needs every subsequent read on the table to acquire an update lock (see `findset-true-applies-updlock-on-read.md` for the alternative narrower mechanism on iterated reads).
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See sample: `prefer-readisolation-over-locktable-for-reads.good.al`.
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## Anti Pattern
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`Rec.LockTable();` at the top of a helper that only reads, perhaps to "make sure the read is consistent". Every subsequent read on that table for the rest of the transaction acquires `UPDLOCK`, including reads from unrelated code paths fused into the same transaction. The contention surfaces in unrelated user sessions, not in the helper that introduced it.
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See sample: `prefer-readisolation-over-locktable-for-reads.bad.al`.
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