# t3 drop attribution — "is it infra or my network?"

When a t3 user reports "disconnects, then self-recovers after a few seconds",
that is the t3 **client watchdog**: the browser heartbeats every 10s and force-
reconnects after >20s without a response. Any stall or break anywhere on
browser → Cloudflare → tunnel → Traefik → t3-dispatch → `t3 serve` produces
the identical symptom. This runbook attributes a drop to a segment in minutes.

## 1. Check the probe (first stop)

The in-cluster `t3-probe` (stacks/t3code, scrape job `t3-probe`) holds three
permanent legs that differ only in path segment:

| leg | path under test | drop means |
|---|---|---|
| `cloudflare` | WAN → CF edge → tunnel → cloudflared → Traefik → dispatch | Cloudflare/WAN segment |
| `internal` | Traefik LB (10.0.20.203) → dispatch (no Cloudflare) | Traefik / dispatch / devvm network |
| `t3serve` | HTTP straight to devvm:3773 (`t3 serve` process) | the serve process itself (event-loop stall) |

Prometheus queries:

```promql
increase(t3probe_disconnects_total[1h])          # drops per leg+reason
t3probe_connected                                # current state per leg
histogram_quantile(0.99, rate(t3probe_rtt_seconds_bucket{leg="t3serve"}[15m]))
```

Attribution table:

- `cloudflare` drops, `internal` clean → Cloudflare edge / QUIC tunnel / WAN.
- both WS legs drop together → Traefik, dispatch, or devvm reachability.
- `t3serve` RTT spikes / timeouts → the user's `t3 serve` stalled (see §3).
- **all legs clean while the user dropped → their last mile / device. Infra
  is exonerated, with data.**

Alerts `T3ProbeLegDown` / `T3ProbeDropBurst` fire on sustained breakage.

## 2. Server-side log recipe (per-event forensics)

On devvm (timestamps in UTC):

```bash
# dispatch view — error class identifies which side died:
#   "context canceled"                      = front/client side tore down
#   "connection reset by peer 127.0.0.1:PORT" = that user's serve closed
#   "connection refused"                    = that user's serve was down
journalctl -u t3-dispatch --since "1 hour ago" | grep "proxy error"

# mass-cancel bursts (many same-second cancels = shared-segment break):
journalctl -u t3-dispatch --since "6 hours ago" \
  | grep -oE '^.* [0-9:]+ http: proxy error: context canceled' \
  | awk '{print $6}' | sort | uniq -c | awk '$1>=5'

# serve-side starvation markers (git taking >5s = devvm frozen):
journalctl -u t3-serve@<user> --since "6 hours ago" | grep "timed out"

# tunnel-side: cloudflared pod restarts + per-connection events
kubectl -n cloudflared get pods
kubectl -n cloudflared logs <pod> --since=6h | grep -E "ERR|reconnect"
```

## 3. devvm pressure correlation

devvm node_exporter is scraped as job `devvm` (since 2026-06-10). The known
high-frequency drop mechanism is **memory+IO pressure on devvm**: agent
processes live inside `t3-serve@<user>`'s cgroup; a runaway agent swap-thrashes
the spinning root disk and freezes the box in multi-10s windows — every
connected client's watchdog fires at once (2026-06-10: a 10.8G agent → global
OOM → 8.5min hard outage).

```promql
rate(node_pressure_io_stalled_seconds_total{instance="devvm"}[5m])
rate(node_pressure_memory_stalled_seconds_total{instance="devvm"}[5m])
node_memory_SwapFree_bytes{instance="devvm"}
```

Guardrails in place (2026-06-10, `scripts/t3-serve@.service`): per-unit
`MemoryHigh=12G`, `MemoryMax=16G`, `MemorySwapMax=0`, `OOMPolicy=continue` —
a runaway agent now OOMs alone inside the cgroup instead of taking the box
(and the WS server) with it.

## 4. Known root causes (2026-06-10 investigation)

1. **devvm memory/IO storms** (high-frequency mechanism) — §3.
2. **cloudflared in-place autoupdate** — fixed: `--no-autoupdate`
   (stacks/cloudflared). Before the fix every CF release exited all 3 pods
   (code 11), severing all tunnel WebSockets.
3. **QUIC tunnel churn** (~1–2/day, "no recent network activity") — inherent;
   visible as `cloudflare`-leg-only blips.
4. **t3 nightly autoupdate** — pinned after the 2026-06-09 outage, see
   `docs/post-mortems/2026-06-09-t3-nightly-autoupdate-auth-outage.md`.