# Alert Rules

This page explains when go-web-monitor sends an offline alert, how the failure count accumulates and resets, and what the alert email contains.

## Failure Count

Each site has a `count` field in `.webMonitor.json` that records consecutive offline checks. After each round it updates as follows (`internal/service/monitor.go:159`):

| Result this round | Previous count | Action | New count |
|-------------------|----------------|--------|-----------|
| Online | Any | None | 0 |
| Offline | 0–3 | None | Previous + 1 |
| Offline | 4 | Send alert | 0 |

The condition is written as `(count+1)%5 == 0`; because the count resets on the 5th failure, the effective rule is "one email per 5 consecutive offline checks".

```mermaid
stateDiagram-v2
    [*] --> Healthy
    Healthy: Online (count = 0)
    Failing: Offline (count 1–4)
    Healthy --> Failing: Offline
    Failing --> Failing: Offline and count < 4
    Failing --> Healthy: Online
    Failing --> Alerted: 5th offline
    Alerted: Alert sent, count reset
    Alerted --> Failing: Still offline
    Alerted --> Healthy: Online
```

## Alert Frequency

| Situation | Result |
|-----------|--------|
| Site goes offline for 1–4 minutes, then recovers | No email |
| Site stays offline | One email at roughly minute 5, 10, 15, and so on |
| Manual `refresh` | Counts as a round and speeds up accumulation |

No "recovered" email is sent when a site comes back online.

## Alert Email

| Field | Value |
|-------|-------|
| Subject | `Website Offline Alert: <url>` |
| Format | HTML (`Content-Type: text/html; charset=UTF-8`) |
| Body | URL, `Status: Offline`, `Reason: HTTP <code>`, check time |

On a connection failure the status code in `Reason` is `HTTP 0`.

## Delivery Conditions

Alerts call `SMTP.SendEmail()` in a background goroutine. When any of the following is false, no email goes out and the error is written only to stderr (`slog`):

- `enabled` is `true`
- `host` is set
- `to` has at least one recipient

See [Email Alert Setup](/smtp-setup) for configuration and [SMTP Connection](/smtp-connection) for connection details.