terdut-server/README.md

# Terminal Duty (terdut-server)

Incident management server for teams using Prometheus Alertmanager.

- Receives Alertmanager webhooks directly — no adapter needed
- Turns alerts into **incidents**, correlated by Alertmanager's own `groupKey`
- Incident workflow: acknowledge, assign, snooze, note, resolve, with a full timeline
- On-call schedule management, with new incidents auto-assigned to whoever is on call
- Alert and incident statistics, including MTTA and MTTR
- REST API with per-user API key authentication
- Single binary, SQLite storage — trivial to self-host

---

## Quick start

**Prerequisites:** Go 1.21+

```bash
git clone https://git.ryuvia.com/niklas/terdut-server
cd terdut-server
go run ./cmd/terdut
```

The server starts on `:8080` with a `terdut.db` file in the working directory.

### Create the first user

```bash
curl -X POST http://localhost:8080/api/bootstrap \
  -H "Content-Type: application/json" \
  -d '{"username": "admin", "email": "admin@example.com"}'
```

Save the `api_key.key` value from the response — it is shown **once only**.

Use it as a bearer token for all subsequent requests:

```bash
export KEY=<your-key>
curl -H "Authorization: Bearer $KEY" http://localhost:8080/api/users
```

### Docker

```bash
docker build -t terdut-server .
docker run -p 8080:8080 -v $(pwd)/data:/data \
  -e TERDUT_DB_PATH=/data/terdut.db \
  terdut-server
```

### Kubernetes

A Helm chart is published from this repository as an OCI artifact, versioned in lockstep
with the app — chart `x.y.z` is always app `vx.y.z`:

```bash
helm upgrade --install terdut-server oci://git.ryuvia.com/niklas/terdut-server \
  --version 0.9.0 \
  --namespace terdut-server --create-namespace \
  --set networking.hostname=terdut.example.com
```

The chart expects a [Gateway API](https://gateway-api.sigs.k8s.io/) Gateway named `envoy-main` in
the `envoy-gateway-system` namespace to already exist — it renders an `HTTPRoute` against it rather
than an `Ingress`. TLS is terminated at the gateway, so the server itself never sees a certificate.

| Value | Default | Description |
|---|---|---|
| `networking.hostname` | `terdut.example.com` | Hostname the `HTTPRoute` serves |
| `networking.listener` | `""` | Gateway listener (`sectionName`) to bind to. Empty attaches to every matching listener, **including plaintext HTTP** — set it to the HTTPS listener's name to serve TLS only |
| `networking.servicePort` | `8080` | Port the route forwards to; keep in sync with `service.port` |
| `bootstrap.enabled` | `true` | Runs a post-install hook that creates the first user and stores its API key in the `<release>-admin-key` Secret. Already-bootstrapped servers are left alone |
| `backupSidecar.enabled` | `true` | Adds an idle `python` sidecar and the [k8up](https://k8up.io/) annotations that dump the database through it |

The API key travels in an `Authorization: Bearer` header, so set `networking.listener` whenever the
hostname is reachable outside a trusted network.

#### Backups

The server image is `FROM scratch` — the binary and nothing else — so there is no interpreter to
run a database dump in, and the database runs in WAL mode, where a file-level copy of the volume is
not crash-consistent. The chart therefore ships an idle `python:*-alpine` sidecar that shares the
data volume, and points k8up's `backupcommand` at it with `k8up.io/backupcommand-container`. Without
that annotation k8up execs into `.spec.containers[0]` and the dump fails.

The dump is buffered and sanity-checked before its first byte reaches stdout, because k8up streams
stdout straight into Restic: a dump that dies partway is otherwise stored as a silently truncated
snapshot that k8up still reports as successful.

Set `backupSidecar.enabled=false` if you back the volume up some other way.

---

## Configuration

| Variable | Default | Description |
|---|---|---|
| `TERDUT_ADDR` | `:8080` | TCP address to listen on |
| `TERDUT_DB_PATH` | `terdut.db` | Path to the SQLite database file |
| `TERDUT_ARCHIVE_AFTER` | `168h` (7d) | How long a resolved alert or incident stays in the default list before being auto-archived |
| `TERDUT_STALE_AFTER` | `6h` | How long a firing alert may go without a refreshing webhook before it is treated as resolved — **must exceed your Alertmanager `repeat_interval`** |
| `TERDUT_DEADMAN_MATCHERS` | `alertname=Watchdog` | Which alerts are [dead man's switches](#dead-mans-switch). `;` separates matchers, `,` the label conditions within one, `=` is exact equality. Every matcher must name an `alertname` |
| `TERDUT_DEADMAN_TIMEOUT` | `15m` | How long a heartbeat may go unheard before its switch is declared dead — **must be shorter than the `repeat_interval` of the route carrying it**. `0` disables dead man's switch handling |
| `TERDUT_DEADMAN_SEVERITY` | `critical` | Severity a dead man's switch incident opens at |
| `TERDUT_NTFY_URL` | — | ntfy server to publish push notifications to. Empty disables notifications entirely |
| `TERDUT_NTFY_TOKEN` | — | Bearer token for an access-controlled ntfy |
| `TERDUT_NTFY_FALLBACK_TOPIC` | — | Topic used when nobody is on call |
| `TERDUT_PUBLIC_URL` | — | Base URL a phone uses to reach this server, for the link and Acknowledge button inside a notification |
| `TERDUT_NOTIFY_REPEAT` | `15m` | How long an incident may sit unacknowledged before it is paged again. `0` notifies once and never repeats |

Durations use Go syntax (`30m`, `12h`, `168h`). An unparseable value falls back to the default.

Note that `TERDUT_STALE_AFTER` and `TERDUT_DEADMAN_TIMEOUT` point in opposite directions. Staleness
is a generous grace period around a `repeat_interval` you do not control; a dead man's switch is a
deadline you set deliberately, and the heartbeat's route is configured to beat faster than it.

In the Helm chart the two sweeper durations are set via `sweeper.staleAfter` and `sweeper.archiveAfter`, dead man's switches via the `deadman.*` values, and notifications via the `notify.*` values.

---

## Alertmanager configuration

Add terdut-server as a webhook receiver in your `alertmanager.yml`:

```yaml
receivers:
  - name: terdut
    webhook_configs:
      - url: http://terdut-server:8080/api/alertmanager/webhook
        send_resolved: true

route:
  receiver: terdut
```

The webhook endpoint requires no authentication.

If you use the [dead man's switch](#dead-mans-switch) — and the default configuration does — give
the heartbeat a route of its own, because the deadline is only as tight as the interval feeding it:

```yaml
route:
  receiver: terdut
  repeat_interval: 4h
  routes:
    - matchers: [ 'alertname = "Watchdog"' ]
      receiver: terdut
      group_wait: 0s
      group_interval: 1m
      repeat_interval: 1m
```

That delivers a heartbeat every **2 minutes**, not every minute. Alertmanager only reconsiders a
group every `group_interval`, and at exactly one elapsed interval `repeat_interval` has not *quite*
passed, so the send slips to the next tick — equal values give 2×. Two minutes against the 15 minute
default is seven heartbeats per window, which is the point; use `group_interval: 30s` if you want
the numbers to mean what they say.

kube-prometheus-stack users get the `Watchdog` alert (`expr: vector(1)`) for free; it just needs
routing to terdut rather than to `null`.

---

## Alerts and incidents

There are two objects, and the difference between them is the whole design.

**An alert is Alertmanager's record.** It has two states, `firing` and
`resolved`, one row per fingerprint, and no human ever writes to it. The API
exposes alerts read-only.

**An incident is the work item.** It goes `triggered → acknowledged → resolved`,
carries an assignee, a snooze, notes and a timeline, and is the only thing people
act on. Many alerts belong to one incident.

### Correlation uses Alertmanager's `groupKey`

Alertmanager has already grouped alerts according to the `group_by` routing tree
you configured, and it sends the resulting `groupKey` and `groupLabels` on every
webhook. Incidents adopt that answer rather than re-grouping alerts a second
time — if you want different correlation, change `group_by` in
`alertmanager.yml` and terdut follows.

At most one incident is open per `groupKey` at a time. Alerts firing in a group
that already has an open incident join it. The incident's `severity` is a
high-water mark — the highest `severity` label any of its alerts has carried — so
an incident that hit `critical` still reads as critical after the critical alert
clears.

### An incident opens only on a new occurrence

An incident opens when an alert **transitions into firing**: a fingerprint that
was never seen, an alert with a newer `startsAt`, or a resolved alert that
started again. The unchanged firing notifications Alertmanager re-sends every
`repeat_interval` are none of those, and open nothing.

This is what makes closing an incident by hand mean something. Without the rule,
`POST /api/incidents/{id}/resolve` would be undone by the next re-send of an
alert that never stopped firing.

### Leaving the open state

- **Automatically**, once every alert under the incident has stopped firing —
  whether by a resolved webhook or by the sweeper's
  [stale-alert expiry](#stale-alert-expiry). The incident gets
  `"resolution_source": "alerts"`.
- **By hand**, via `POST /api/incidents/{id}/resolve`
  (`"resolution_source": "manual"`). This is **terminal**: a later occurrence in
  that group opens a *new* incident rather than reopening this one. If the alert
  underneath never stops firing, the incident stays closed — that is what
  resolving by hand asserts.
- **On recovery**, for a [dead man's switch](#dead-mans-switch) incident whose
  heartbeat started arriving again (`"resolution_source": "recovered"`). These
  incidents have no member alerts, so the automatic cascade above cannot reach
  them.

To quieten an incident you expect to come back, snooze it instead
(`POST /api/incidents/{id}/snooze`). A snooze hides the incident from the default
list without closing it, and expires by simply falling into the past.

### On-call assignment

A new incident is assigned to whoever holds today's schedule entry at the moment
it opens (`GET /api/schedule/current`). If nobody is scheduled it opens
unassigned. Reassign with `POST /api/incidents/{id}/assign`.

One person holds a given day, so `POST /api/schedule` refuses a date somebody
already has: taking a shift off the person expecting to be paged for it should
not be something a plain call does by accident. Pass `"replace": true` to take
them anyway. Either way the whole request is one transaction — a week where some
days are free and some are taken moves as a unit, and a failure leaves the rota
exactly as it was rather than with a hole in it.

### Push notifications

With `TERDUT_NTFY_URL` set, an incident that opens is pushed to the on-call
person's phone through [ntfy](https://ntfy.sh). Set each user's topic with
`PUT /api/users/{id}/notify`; a user with no topic falls back to
`TERDUT_NTFY_FALLBACK_TOPIC`, as does an incident that opens with nobody on call.
If neither yields a topic, nothing is queued.

Three things get pushed:

- **triggered** — an incident opened. Priority follows severity (`critical` maps
  to ntfy's max priority, the one that overrides the phone's quiet settings).
- **reminder** — the incident is still `triggered` after `TERDUT_NOTIFY_REPEAT`.
  Repeats until somebody acts. Acknowledging, snoozing, resolving or archiving
  all stop it — snooze is the mute button.
- **resolved** — every alert under the incident stopped firing. Only sent to
  whoever was paged in the first place, and only for the automatic cascade:
  resolving by hand pushes nothing, since the person who did it already knows.

Notifications carry an **Acknowledge** button that acknowledges the incident
without opening anything. It POSTs to `/api/notify/ack/{token}`, an
unauthenticated route authorised by the 256-bit token in its path — minted fresh
per notification, scoped to one incident and one action, and valid for 24 hours.
A real API key is never put in a notification, because the message is stored on
the ntfy server and cached on the device.

The token is **not** consumed by use. Acknowledging is idempotent, so a token
stays valid for its full 24 hours and a second tap is a no-op that reports the
incident's current state rather than an error — which is what you want when a
tap is retried on a flaky mobile connection. What bounds it is scope, not a use
count: one incident, one action, one day. Expired tokens are purged by the
sweeper.

Two consequences worth planning for:

- `/api/notify/ack/{token}` **must stay publicly reachable**, or the button will
  not work when the responder is off your network.
- Notifications sent to the fallback topic carry **no** Acknowledge button. The
  topic is shared, and a button on it would let any subscriber acknowledge as
  somebody else.

Delivery is a queue, not an inline call: the webhook writes a row and a
background notifier sends it within 30 seconds, retrying with exponential
backoff up to 8 attempts. Nothing about ingestion blocks on ntfy being reachable.

Every delivery is recorded on the incident's timeline: a `notified` event once
ntfy accepts the publish, and a `notify_failed` event when a notification
exhausts its retries. Written from the result rather than at enqueue, so the
timeline says what actually happened — and a page that never landed is visible
instead of looking the same as one that did.

### Stale alert expiry

A resolved webhook is the only signal that an alert has stopped firing, so a
notification that is dropped, silenced, or lost to a restart would otherwise pin
that alert as firing forever. A background sweeper resolves firing alerts that
Alertmanager has stopped refreshing, using either signal:

- the `endsAt` watermark on the last notification has passed, or
- no webhook has refreshed the alert within `TERDUT_STALE_AFTER`.

Alertmanager re-sends firing notifications every `repeat_interval`, which is what
keeps a live alert fresh — so `TERDUT_STALE_AFTER` must be comfortably larger
than your `repeat_interval` (default 4h), or live alerts will be resolved
prematurely. Alerts resolved this way are marked `"resolution_source": "expiry"`
to distinguish them from a real Alertmanager resolve (`"alertmanager"`).

An expiry cascades: once it leaves an incident with nothing firing under it, the
incident resolves too, in the same sweep.

### Dead man's switch

Everything above assumes alerts arrive. If Prometheus stops evaluating, or
Alertmanager cannot reach this server, nothing arrives — and silence looks
exactly like everything being fine. A dead man's switch inverts the handling for
one designated alert so that silence is the signal:

- **receiving** it opens no incident, and
- the **absence** of it does.

kube-prometheus-stack already ships the alert for this. `Watchdog` is
`expr: vector(1)`, so it fires permanently and is re-sent forever; it is worth
nothing unless something downstream notices it stop. That is what
`TERDUT_DEADMAN_MATCHERS` defaults to.

A matcher is a set of exact label conditions, one of which must be the
`alertname`:

```
TERDUT_DEADMAN_MATCHERS="alertname=Watchdog,cluster=prod; alertname=EdgeHeartbeat"
```

**The unit of monitoring is the fingerprint, not the alert name.** Two clusters
sending the same `Watchdog` are two independent switches, so a healthy one can
never mask a dead one.

#### The lifecycle

A switch is **dormant** until its first heartbeat arrives. A configured matcher
that has never been heard from opens nothing, so a fresh deploy or a restored
database does not page. It also means a matcher that never matches anything is
silently inert — check the startup log line, which lists the matchers that
survived parsing.

Once armed, the sweeper declares it **dead** when either the heartbeat has not
been refreshed within `TERDUT_DEADMAN_TIMEOUT`, or Alertmanager explicitly
resolved it — the sender saying the heartbeat stopped needs no further waiting.
That opens an incident at `TERDUT_DEADMAN_SEVERITY`, assigned and paged like any
other, and marks the heartbeat alert `"resolution_source": "deadman"` so the
alert list stops claiming a dead switch is firing.

It **recovers** when the heartbeat starts arriving again: the incident resolves
with `"resolution_source": "recovered"` and the all-clear goes to whoever was
paged.

Resolving the incident by hand sticks, the same way it does for an alert-backed
one. While the switch stays silent nothing new opens — so a decommissioned
source is a one-time page rather than a nag. The switch **re-arms** on the next
heartbeat: come back and die again, and that is a new incident.

#### Two things to know

`TERDUT_DEADMAN_TIMEOUT` must be **shorter** than the `repeat_interval` of the
route carrying the heartbeat, which is the exact opposite of
`TERDUT_STALE_AFTER`. Inheriting a default `repeat_interval` of 4h gives you a
switch that takes four hours to notice anything, so give the heartbeat
[its own route](#alertmanager-configuration). Matched alerts are exempt from
stale-alert expiry — a heartbeat answers to its own timeout and nothing else.

A dead man's switch incident has **no member alerts**:
`GET /api/incidents/{id}/alerts` returns an empty list. There is no alert
describing the problem, because the problem is that no alert arrived. What
happened is on the timeline instead, as a `deadman_silent` event carrying the age
of the last heartbeat, and the heartbeat's labels are on the incident's
`group_labels`.

---

## API reference

### Authentication

All endpoints except `/api/bootstrap` and `/api/alertmanager/webhook` require:

```
Authorization: Bearer <api-key>
```

### Users

| Method | Path | Description |
|---|---|---|
| `POST` | `/api/bootstrap` | Create first user + API key (only works on empty DB) |
| `GET` | `/api/users` | List users |
| `POST` | `/api/users` | Create user `{"username","email"}` |
| `DELETE` | `/api/users/{id}` | Delete user (cascades to keys) |
| `PUT` | `/api/users/{id}/notify` | Set push notification target `{"ntfy_topic"}` — empty string clears it |
| `POST` | `/api/users/{id}/api-keys` | Issue API key `{"name"}` — key shown once |
| `DELETE` | `/api/users/{id}/api-keys/{keyID}` | Revoke API key |

### Alert ingestion

| Method | Path | Description |
|---|---|---|
| `POST` | `/api/alertmanager/webhook` | Alertmanager v4 webhook receiver (no auth) |

### Notifications

| Method | Path | Description |
|---|---|---|
| `POST` | `/api/notify/ack/{token}` | Acknowledge an incident from a push notification's Acknowledge button. No auth: the token in the path is the credential — one incident, one action, 24 hours, idempotent. Must stay publicly reachable |

### Incidents

| Method | Path | Description |
|---|---|---|
| `GET` | `/api/incidents` | List incidents. Filters: `?status=triggered\|acknowledged\|resolved`, `?severity=`, `?assigned_to=<user id>`, `?archived=true`, `?snoozed=true`, `?from=YYYY-MM-DD`, `?to=YYYY-MM-DD`, `?sort=severity`, `?limit=` (default 50, max 500) |
| `GET` | `/api/incidents/{id}` | Get single incident, with its alerts inline |
| `GET` | `/api/incidents/{id}/alerts` | Alerts under this incident |
| `GET` | `/api/incidents/{id}/timeline` | Full event history, chronological |
| `POST` | `/api/incidents/{id}/acknowledge` | Acknowledge (stamps authed user + time) |
| `DELETE` | `/api/incidents/{id}/acknowledge` | Clear acknowledgement, back to `triggered` |
| `POST` | `/api/incidents/{id}/resolve` | Close by hand — **terminal**, see above |
| `POST` | `/api/incidents/{id}/assign` | Reassign `{"user_id"}` |
| `POST` | `/api/incidents/{id}/snooze` | Hide until `{"until": RFC3339}` or `{"duration": "2h"}` |
| `DELETE` | `/api/incidents/{id}/snooze` | Un-snooze |
| `POST` | `/api/incidents/{id}/archive` | Archive (hides from the default list) |
| `DELETE` | `/api/incidents/{id}/archive` | Un-archive |
| `POST` | `/api/incidents/{id}/notes` | Add a note `{"content"}` |
| `DELETE` | `/api/incidents/{id}/notes/{eventID}` | Delete own note |

With no `?status=` filter, `GET /api/incidents` returns **open** incidents only —
the queue an on-call person wants. Currently snoozed and archived incidents are
excluded unless asked for. Actions that only make sense on an open incident
return `409` once it is resolved.

Notes are ordinary timeline events of type `note`; only they are deletable, and
only by their author. The rest of the timeline is a record of what happened.

#### The incident object

| Field | Type | Notes |
|---|---|---|
| `id` | integer | Server-assigned |
| `group_key` | string | Alertmanager's `groupKey` — opaque, treat as an identifier |
| `title` | string | Rendered from `groupLabels` |
| `group_labels` | object | String→string, as sent by Alertmanager |
| `status` | string | `"triggered"`, `"acknowledged"` or `"resolved"` |
| `severity` | string | *optional* — high-water mark across the incident's alerts; never lowered |
| `triggered_at` | timestamp | When the incident opened |
| `acknowledged_by_id` / `acknowledged_by` / `acknowledged_at` | | *optional* — user id, username, time |
| `assigned_to_id` / `assigned_to` | | *optional* — user id, username |
| `snoozed_until` | timestamp | *optional* — a value in the past reads as not snoozed |
| `resolved_at` | timestamp | *optional* |
| `resolution_source` | string | *optional* — `"alerts"`, `"manual"` or `"recovered"` |
| `archived_at` | timestamp | *optional* |
| `alerts` | array | Only on `GET /api/incidents/{id}` |

Treat `resolution_source` as an open set, as with the alert field of the same
name: degrade unknown values to "resolved, reason unknown".

#### The timeline event object

| Field | Type | Notes |
|---|---|---|
| `id` | integer | |
| `incident_id` | integer | |
| `type` | string | See below — treat as an open set |
| `user_id` / `username` | | *optional* — absent when the server acted rather than a person |
| `alert_id` | integer | *optional* — the alert an `alert_added` / `alert_resolved` event refers to |
| `detail` | string | *optional* — the note body, the snooze deadline, etc. |
| `created_at` | timestamp | |

Types written today: `triggered`, `alert_added`, `alert_resolved`,
`acknowledged`, `unacknowledged`, `assigned`, `snoozed`, `unsnoozed`, `resolved`,
`note`, `notified`, `notify_failed`, `deadman_silent`. On an `assigned` event
`user_id` is the **assignee**, not the actor. New types may be added; render
unknown ones generically rather than dropping them.

On `notified` and `notify_failed`, `detail` carries the notification kind
(`triggered` | `reminder` | `resolved`), and on a failure the reason after it.
`user_id` is who was paged — absent means the page went to the shared fallback
topic and so belongs to nobody. The topic itself is never written to the
timeline: it is a shared secret with the ntfy server, and every API key can read
this.

### Alerts

Alerts are read-only. Everything a person does happens on the incident.

| Method | Path | Description |
|---|---|---|
| `GET` | `/api/alerts` | List alerts. Filters: `?status=firing\|resolved`, `?name=`, `?incident_id=`, `?archived=true`, `?from=YYYY-MM-DD`, `?to=YYYY-MM-DD`, `?limit=` (default 50, max 500) |
| `GET` | `/api/alerts/{id}` | Get single alert |

Archived alerts are hidden from `GET /api/alerts` unless `?archived=true` is
passed; alert archiving is automatic housekeeping by the sweeper, not a user
action. Resolved alerts carry `resolution_source`: `"alertmanager"` for a real
resolved webhook, `"expiry"` when the sweeper inferred it (see
[Stale alert expiry](#stale-alert-expiry)), `"deadman"` for a heartbeat declared
dead (see [Dead man's switch](#dead-mans-switch)).

#### The alert object

Returned by `GET /api/alerts` (as an array) and `GET /api/alerts/{id}`.
Timestamps are RFC 3339 in UTC. Fields marked *optional* are omitted entirely
when unset, so clients must treat them as nullable.

| Field | Type | Notes |
|---|---|---|
| `id` | integer | Server-assigned; stable for the life of the row |
| `fingerprint` | string | Alertmanager's fingerprint — the upsert key |
| `name` | string | From the `alertname` label |
| `status` | string | `"firing"` or `"resolved"` |
| `labels` | object | String→string, as sent by Alertmanager |
| `annotations` | object | String→string, as sent by Alertmanager |
| `starts_at` | timestamp | When the alert instance began, **per Prometheus** |
| `ends_at` | timestamp | *optional* — absent while no end is known |
| `generator_url` | string | Link back to the originating Prometheus |
| `received_at` | timestamp | When the server last accepted a webhook for this alert — see below |
| `incident_id` | integer | *optional* — the most recent incident this alert belongs to |
| `resolution_source` | string | *optional* — `"alertmanager"`, `"expiry"` or `"deadman"` |
| `archived_at` | timestamp | *optional* — set while archived |

##### `received_at` is a liveness heartbeat

`starts_at` comes from Prometheus and **never changes** for the lifetime of an
alert instance. It says when the problem began, not whether it is still
happening — an alert that started twelve days ago looks identical whether
Alertmanager refreshed it a minute ago or went silent a week ago.

`received_at` is the field that answers "is this still live". It is set to the
server's clock on **every accepted webhook** for that fingerprint, including the
unchanged firing notifications Alertmanager re-sends every `repeat_interval`.
Clients may rely on this:

- **A firing alert whose `received_at` is advancing is still being refreshed.**
  Stale-dating it against `repeat_interval` is a valid liveness check, and it is
  what the built-in sweeper does (see
  [Stale alert expiry](#stale-alert-expiry)).
- **`received_at` tracks accepted payloads, not delivery attempts.** A retry
  that describes an older instance than the stored one is discarded, and a
  discarded payload does not move `received_at`.
- **It stops advancing once the alert resolves,** because Alertmanager stops
  re-sending. On an alert resolved by the sweeper
  (`"resolution_source": "expiry"`) it therefore marks the last time
  Alertmanager was actually heard from, which is earlier than `ends_at`.

`GET /api/alerts` is ordered by `received_at` descending — most recently
refreshed first — and the `?from=` / `?to=` filters on both the alert and stats
endpoints select on `received_at`, not `starts_at`.

##### `resolution_source` says how much to trust `ends_at`

An alert can leave the firing state two ways, and `resolution_source` records
which happened. Clients may rely on this:

- **Absent while firing.** It is set only on resolve, and a re-fire under the
  same fingerprint clears it again, so its presence always agrees with
  `"status": "resolved"`.
- **`"alertmanager"` — a real resolved webhook arrived.** `ends_at` is the end
  time Alertmanager reported. It is an observed value and can be displayed as
  fact.
- **`"expiry"` — the sweeper inferred the resolve** because Alertmanager stopped
  refreshing the alert (see [Stale alert expiry](#stale-alert-expiry)). Nothing
  ever reported an end, so **`ends_at` is approximate**: it is either the stale
  `endsAt` watermark from the last notification, or — when that notification
  carried none — the time the sweep ran, which lags the last real contact by up
  to `TERDUT_STALE_AFTER` plus a sweep interval. Treat it as "no later than",
  not as when the problem stopped.

  On these alerts `received_at` is the more truthful signal: it marks the last
  time Alertmanager was actually heard from. Surfacing the distinction is
  worthwhile, since `"expiry"` can also mean the alert is still firing and the
  notification path broke.

- **`"deadman"` — a heartbeat was declared dead** (see
  [Dead man's switch](#dead-mans-switch)). Like `"expiry"`, an inference from
  silence rather than an observed end, so `ends_at` is approximate — but a much
  tighter one, bounded by `TERDUT_DEADMAN_TIMEOUT`. It is also the one resolution
  a re-fire under the same `starts_at` can undo, since the switch coming back is
  exactly the evidence that the inference was wrong.

Treat the value as an open set and tolerate ones you do not recognise — new
sources may be added, and unknown values should degrade to "resolved, reason
unknown" rather than being rejected.

### On-call schedule

| Method | Path | Description |
|---|---|---|
| `POST` | `/api/schedule` | Assign user to dates `{"user_id", "dates":["YYYY-MM-DD",...], "replace"}` — all-or-nothing |
| `GET` | `/api/schedule` | List entries. Filters: `?from=YYYY-MM-DD`, `?to=YYYY-MM-DD` |
| `GET` | `/api/schedule/current` | Today's on-call user (UTC), 404 if none |
| `DELETE` | `/api/schedule/{id}` | Remove schedule entry |

### Statistics

All stat endpoints accept optional `?from=YYYY-MM-DD` and `?to=YYYY-MM-DD`, and exclude archived rows to match the default list views. Alert stats filter on `received_at`; incident stats filter on `triggered_at`.

| Method | Path | Description |
|---|---|---|
| `GET` | `/api/stats/incidents` | `{total, triggered, acknowledged, resolved, mtta_seconds, mttr_seconds}` |
| `GET` | `/api/stats/alerts` | `{total, firing, resolved}` counts |
| `GET` | `/api/stats/alerts/top` | Most frequent alert names. `?limit=` (default 10, max 100) |
| `GET` | `/api/stats/alerts/by-hour` | Count per hour-of-day (UTC), all 24 slots returned |
| `GET` | `/api/stats/alerts/by-day` | Count per day-of-week, all 7 slots with names returned |

`mtta_seconds` (time to acknowledge) and `mttr_seconds` (time to resolve) are
averages over incidents that have actually been acknowledged or resolved, and are
**null** until there are any — null means "no data", not zero.

---

## Upgrading to incidents

The incidents release moves the workflow off alerts, which is a **breaking API
change**. These endpoints are gone:

| Removed | Replacement |
|---|---|
| `POST`/`DELETE` `/api/alerts/{id}/acknowledge` | `POST`/`DELETE` `/api/incidents/{id}/acknowledge` |
| `POST`/`DELETE` `/api/alerts/{id}/archive` | `POST`/`DELETE` `/api/incidents/{id}/archive` (alert archiving is now sweeper-only) |
| `GET`/`POST` `/api/alerts/{id}/comments` | `GET /api/incidents/{id}/timeline`, `POST /api/incidents/{id}/notes` |
| `DELETE /api/alerts/{id}/comments/{commentID}` | `DELETE /api/incidents/{id}/notes/{eventID}` |

The alert object also drops `acknowledged_by_id`, `acknowledged_by` and
`acknowledged_at`, and gains `incident_id`.

Migration `008_incidents.sql` runs automatically on start and preserves existing
data: every alert gets a backfilled incident carrying its acknowledgement, and
comments become timeline notes. Backfilled incidents have a `group_key` of
`backfill:<fingerprint>` — there is no historical `groupKey` to correlate on, so
they are one-per-alert rather than grouped.

Nothing about the two documented alert contracts changes: `received_at` is still
advanced on every accepted webhook, and `resolution_source` still means what it
did.

## Upgrading to dead man's switches

Dead man's switch handling is **on by default**, watching `alertname=Watchdog`
with a 15 minute timeout. If you already route `Watchdog` to this server, the
behaviour of that alert changes on upgrade, in both directions:

- it stops opening incidents when it arrives, and
- it starts opening one when it stops arriving.

**Check your `repeat_interval` before upgrading.** The switch pages whenever a
heartbeat has not been refreshed within `TERDUT_DEADMAN_TIMEOUT`, so a `Watchdog`
route inheriting a 4h or 12h `repeat_interval` will page constantly against the
15 minute default. Either give the heartbeat
[its own fast route](#alertmanager-configuration) — the point of the feature — or
set `TERDUT_DEADMAN_TIMEOUT` above your current `repeat_interval` until you have.
`TERDUT_DEADMAN_TIMEOUT=0` turns the whole thing off.

There is no migration and no schema change. An existing open incident from a
`Watchdog` that arrived under the old behaviour is unaffected; resolve it by hand.

---

## Development

```bash
go test ./...        # run all tests
go build ./...       # compile all packages
go run ./cmd/terdut  # run locally
```