satellite-provider/_docs/LESSONS.md

Engineering Lessons

Recurring bugs, surprising library behaviors, and process insights extracted from completed cycles. Newest at the top. Keep entries short — this is for fast scanning at the start of new cycles, not exhaustive history.

This file has two layers:

• Deep engineering lessons (L-NNN): library bugs, architectural insights, multi-paragraph context. Persist forever.
• Ring buffer at the bottom: most recent 15 single-sentence lessons emitted by the retrospective skill, consumed by new-task / plan / decompose / autodev Step 0. Oldest entries drop off the top.

Categories: estimation · architecture · testing · dependencies · tooling · process

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L-001 — Dapper TypeHandler<T> is bypassed for enum types during read deserialization

Cycle: 1 (AZ-484) Discovered by: integration test failure (Error parsing column 12 (source=google_maps - String)); root-caused via web search to long-standing Dapper issue #259. Affects: Dapper 2.1.35 (and most other versions until the proposed Settings.PreferTypeHandlersForEnums opt-in in PR #2200, not yet merged).

What happens Registering SqlMapper.AddTypeHandler(new MyEnumHandler()) for an enum type — even via SqlMapper.TypeHandler<TEnum> — works for writes (the handler's SetValue is invoked for parameter binding) but is silently bypassed for reads. Dapper's IL-emitted deserializer checks IsEnum first and falls back to Enum.TryParse(string, ignoreCase: true).

Why this is dangerous If the enum's wire string happens to match a member name case-insensitively (e.g., RegionStatus.Failed ↔ "failed"), the bypass goes unnoticed and round-trip works accidentally. The bug only surfaces when the wire format diverges from the C# member name (e.g., TileSource.GoogleMaps ↔ "google_maps" — Enum.TryParse("google_maps") does not match GoogleMaps because of the underscore).

Recommended approach

• Do not rely on SqlMapper.TypeHandler<TEnum> for read-side enum mapping unless the wire values match the enum member names case-insensitively.
• For enums whose wire format diverges (snake_case, kebab-case, custom IDs), store the entity field as string and provide an explicit converter (*Converter.ToWireValue / FromWireValue) for use at the service-layer boundary. This is what AZ-484 does for TileEntity.Source ↔ TileSourceConverter.
• Unit-test the converter directly. Do not assume that round-tripping through Dapper proves anything for enums.

Detection

• Unit tests of the type handler in isolation will pass even when the handler is bypassed at runtime.
• Failure surfaces only at integration-test time when the actual SELECT runs.
• If you must keep an enum-typed field, write at minimum one integration test that reads the enum back through Dapper from a real database row.

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Ring buffer (last 15 entries — newest at top)

• [2026-05-11] [testing] Persisted enums need a Dapper read-roundtrip integration test — unit-testing the type handler in isolation does not prove read-side behavior (see L-001). Source: _docs/06_metrics/retro_2026-05-11.md
• [2026-05-11] [process] NFR test-spec additions must include the runner-script implementation in the same step, or be tagged "Deferred — harness work tracked in "; otherwise scenarios accumulate as Unverified across cycles. Source: _docs/06_metrics/retro_2026-05-11.md
• [2026-05-11] [estimation] Task-spec test-site-count estimates must be backed by an explicit grep evidence block, not pattern-matched against neighboring code (AZ-484 spec said ~3 sites in RegionServiceTests; actual = 0). Source: _docs/06_metrics/retro_2026-05-11.md