# WP-60 — Write-divergence resilience (local + ZGW writes)

Status: done
Phase: 10 — OpenZaak production hardening

## Why

A citizen action today does a local `Aanvraag`/`Document` write and a paired ZGW write
(create zaak/status/document); these aren't transactional. If the ZGW call fails after the
local write succeeds (or vice versa), the two diverge silently —
`openzaak-integration.md` flags this explicitly as "acceptable for a demo backend; a
production arc needs retry/reconciliation or an outbox." This is the one genuine
correctness gap standing between the current integration and something safe to call
production.

## Read first

- [openzaak-integration.md](../reference/openzaak-integration.md) (the section discussing
  this gap)
- `backend/src/BigRegister.Api/Data/ApplicationStore.cs`,
  `Zgw/OpenZaakZaakSource.cs` (the two write sides)
- [ADR-0005 — OpenZaak behind the BFF](../reference/architecture/0005-openzaak-behind-bff.md)

## Decisions

Picked **(b), narrowed further: bounded synchronous retry + flag, no reconcile job.** The
`planner` agent's kickoff review found the write side smaller than either option assumed:

- The only ZGW writes are `OpenZaakZaakSource.CreateZaak` (zaak/status/rol, one POST sequence
  per submit) and `OpenZaakDocumentSource.Upload`/`LinkToZaak` (DRC + zaakinformatieobject).
  There is no standalone status-transition write path yet (that's WP-66) — Step 2 below is
  corrected accordingly.
- Every path already does the local write first and never rolls it back on a ZGW failure — "the
  ZGW half fails, local succeeded" is the only real scenario; the reverse can't happen.
- An outbox was rejected: three request-triggered write paths don't justify a persisted queue,
  and a ZGW call's `CallerIdentity` (needed for the JWT's audit claims, WP-53) would mean PII
  sitting in a new table — the "generic outbox framework" this WP's own Risks section warns
  against.
- A reconcile job was judged unnecessary for the acceptance criteria: flagging (not silent
  divergence) is sufficient, and repair is always possible on demand because a zaak's
  `identificatie` equals the aanvraag's `Referentie` — no reconcile job ships in this WP.

Shipped: bounded retry (3 attempts, doubling backoff from 200ms) in `ZgwHttpClient` for
transport-shaped failures only (429/502/503/504/408 + connection errors/timeouts — deliberately
**not** 500, which can follow a partial commit on the non-idempotent `/statussen`/`/rollen`
POSTs); `Aanvraag.ZgwError` + a `zgw:divergence` audit row when a ZGW write still fails after
retry (`Program.cs`'s submit endpoint, two separate try/catches so a create-zaak failure doesn't
also skip the still-local document link); `OpenZaakDocumentSource.Upload` catches and logs
without a separate flag column (`DrcUrl == null` already means "not registered in ZGW yet").
Full reasoning + rejected sub-options: [openzaak-integration.md](../reference/openzaak-integration.md)'s
"Write resilience" section.

## Files

`Zgw/ZgwHttpClient.cs` (retry), `Data/ApplicationStore.cs` (`ZgwError` column + migration),
`Program.cs` (submit endpoint rewire + `RecordZgwDivergence` + HttpClient timeouts),
`Zgw/OpenZaakDocumentSource.cs` (non-throwing upload). No new file for a mechanism — no
outbox/background worker shipped (see Decisions).

## Steps

1. Design review with the `planner` agent — pick outbox vs. retry+reconcile. Done: retry+flag
   (see Decisions).
2. Implement the chosen mechanism for the create-zaak and document (upload + link) write
   paths — not "status-transition" as originally scoped here; that path doesn't exist yet
   (arrives with WP-66).
3. Add a test that simulates a ZGW failure mid-write and asserts the system recovers
   (retries successfully, or is left in a detectably-inconsistent-but-flagged state)
   rather than silently diverging.

## Acceptance criteria

- [x] A simulated ZGW failure after a successful local write no longer leaves permanent
      silent divergence — either it retries to consistency or the divergence is
      detectable/flagged.
- [x] No new synchronous latency added to the happy path beyond what the chosen mechanism
      requires.

## Verification

A new integration test that fails a stubbed ZGW call mid-write and asserts
recovery/flagging behavior; `cd backend && dotnet test`.

## Out of scope

A general-purpose outbox framework reusable beyond this one write pair (YAGNI unless a
second write pair appears — note WP-66 is exactly that second pair, so revisit scope if
WP-66 lands first); UI surfacing of reconciliation state (backend-only fix for now).

## Risks

Over-building this (a generic outbox/saga framework) for a POC's actual write volume —
ladder check at kickoff: does a bounded retry + periodic reconcile job cover it before
reaching for an outbox table?