ng-eventually/docs/incidents/2026-07-14-write-loss-on-disconnect.md

# Write loss on socket death (`SerializationError`)

**Post-mortem — 2026-07-14 · Status: OPEN (not addressed).**

An entity written just before a period of inactivity can be **silently lost**: it is absent on reconnection. *(Whether the write never durably reached the broker, or reached it and is not read back on a cold reconnection, is **not settled** — see Epistemic caveat below. The wording here deliberately states only the observed symptom.)* The **account / identity survives** (no fork). Observed in real conditions (Festipod, Firefox) during a pause after login/creation.

## Symptom

1. The user logs in, the app creates an entity (a Festipod event).
2. A period of inactivity follows (idle, tab in the background…).
3. The broker socket dies spontaneously with `SOCKET IS CLOSED Some(Left(SerializationError))`.
4. On reconnection, the created entity has disappeared; the app reads back its own scope **empty**.

## Evidence (VERIFIED — live Firefox logs, verbatim)

```
… REPLAY TOPIC NOT FOUND <topic> IN OVERLAY <overlay>
… NEED REPLAY true
… SENDING EVENTS FROM OUTBOX RETURNED: Err(TopicNotFound)
[user1][polyfill] resolveAccount(user1) → 1 record          ← the account SURVIVES (no fork)
[user1][polyfill] readScopeIndex(…) → 0 entities            ← but the scope is EMPTY
… set reçu: 0 objets Event (public)
… SOCKET IS CLOSED Some(Left(SerializationError)) [51, 3, 223, …]
```

Interpretation (**plausible mechanism, not settled**): the write was pushed into the local **outbox**, but the socket died before it was **durably flushed** into the broker topic; on reconnection, the outbox replay fails (`Err(TopicNotFound)`) because the topic was **never created on the broker side** → the event is abandoned. The account, for its part, had already been durably resolved (`resolveAccount → 1 record`): it is neither lost nor forked.

> **Epistemic caveat.** The evidence establishes the *symptom* (loss + `Err(TopicNotFound)` + the scope read returning 0 — logged above as `readScopeIndex`, since renamed `readUserStore`). The exact *mechanism* is not settled between **(i) loss at write time** (the write never durably reaches the broker) and **(ii) cold-rehydration failure** (the write *is* on the broker but a fresh session does not reopen its own scope). The `Err(TopicNotFound)` on the outbox replay leans toward **(i) in this Firefox case**. See the @data repro below, which exhibits a neighboring symptom but **does not settle** (i) vs (ii).

## Causal chain (TRACED — reading of the NextGraph core, to be re-verified)

- The `SerializationError` closes the socket. The core emits the disconnection in **two** places, and they are not the same file: `engine/net/src/broker.rs:1074` sends `LocalBrokerMessage::Disconnected`, which the SDK turns into `disconnections_sender.send(...)` at `sdk/rust/src/local_broker.rs:648`. Navigate by symbol — the line numbers are volatile, and the earlier note in this file put the `send` in `broker.rs`, which it never was.
- This disconnection is **pushed** to subscribers via `disconnections_subscribe(cb)` (PUSH stream).
- **NextGraph reconnection is an unimplemented `// TODO`** (≈ `broker.rs:1051-1076`): nothing re-establishes the socket nor re-flushes the outbox.
- `user_connect` returns a **snapshot** `{ server_id, server_ip, error, since }` at call time — not a stream, unusable for detecting a later drop.
- **No write-durability confirmation API**: a caller cannot `await` the guarantee that a write has reached the broker.

## What the SDK exposes but does not consume

`disconnections_subscribe` **does fire** on this failure — but neither the polyfill (`@ng-eventually/polyfill`) nor the consumer app subscribes to it. The signal exists, nobody listens to it; on the app side, no mechanism retries or warns the user.

## Scope & not reproduced

- **Observed on Firefox only** to date. A manual test on another browser did not trigger the `SerializationError` nor its consequences.
- **@data reproduction (Chromium, real broker) — 2026-07-14, decisive.** The existing @data reconnection test (`reconnexion-meme-identite`) was a **false green**: it read A's repos back from the persistent profile's **local IndexedDB**, never from the broker. A **genuinely cold** reader (non-persistent `freshBrowser` context, the **same** wallet/account A, no local state — seeded from the wallet captured before the event) reads **0** events from A (`BARRIER timed-out (8000ms)`, `CONNECTION ESTABLISHED`). A **different** signature from the Firefox case (no socket death; the `OUTBOX empty` is the reader's, trivially empty) and it **does not settle** (i) vs (ii) — an empty barrier is compatible with both. Established on the other hand: **@data has never verified the broker durability of A's own reads**, and cold rehydration from the broker fails. Repro: `src/modules/event/features/reconnexion-froide-sans-local.feature` (Festipod).
- **To settle (i) vs (ii)**: independently verify that A's write reaches the broker — e.g. a *warm* reader / a second identity reads the event's public doc (the two-identity isolation scenario). If it sees it → the write is durable → the cold reader's 0 is a **(ii)** (rehydration). Otherwise → **(i)**.

## Fix leads (not arbitrated)

1. **Core** — fix the `SerializationError` **and** implement the reconnection TODO (re-establish the socket + re-flush the outbox).
2. **SDK / polyfill** — consume `disconnections_subscribe` → reconnection + outbox re-flush as a mitigation, independently of the core.
3. **Durability API** — expose a confirmation that a write has reached the broker, so that the caller can `await` it.

## Links

- `docs/nextgraph-current-state.md` — current state of the core (disconnection / reconnection to be cross-referenced here).
- Product impact + consumer-side caveat: Festipod concept `data-layer` → `caveat_write-durability-across-disconnect`.