receivers can't receive the alerts from alert manager if the firing time is short

[Maple Wang]

Hi,

I met a problem about alert sending. I found that if firing time of alert is short(<30s), the alertmanager won't send it to receivers. Here is debug log from alertmanager:

```

level=debug ts=2020-10-30T09:19:23.450Z caller=dispatch.go:135 component=dispatcher msg="Received alert" alert=CPUloadIsExtremelyHigh[1826b54][active]
level=debug ts=2020-10-30T09:19:38.452Z caller=dispatch.go:135 component=dispatcher msg="Received alert" alert=CPUloadIsExtremelyHigh[1826b54][resolved]

```

the firing time of this alert is 15s, which match the prometheus statistics, but receivers can't receive either firing alert or resolved alert. I tried webhook/mail/wechat, all of them can't get this alert, so I think this is because alertmanager didn't send this short alert.

Anyone know why this happened? alertmanager version is 0.20.0.

best regards. 

[Matthias Rieber]

On Fri, 30 Oct 2020, Maple Wang wrote:

[..]

> the firing time of this alert is 15s, which match the prometheus
> statistics, but receivers can't receive either firing alert or resolved
> alert. I tried webhook/mail/wechat, all of them can't get this alert, so I
> think this is because alertmanager didn't send this short alert.
>
> Anyone know why this happened? alertmanager version is 0.20.0.

Check the group_wait[1] setting in your alertmanager configuration:

> # How long to initially wait to send a notification for a group
> # of alerts. Allows to wait for an inhibiting alert to arrive or collect
> # more initial alerts for the same group. (Usually ~0s to few minutes.)
> [ group_wait: <duration> | default = 30s ]

best regards,
Matthias

[1] https://prometheus.io/docs/alerting/latest/configuration/#route

[Maple Wang]

Hi Matthias,

Thanks, it works now,  group_wait and group_interval should be less than smallest duration of alert fired by prometheus, otherwise, it can't be routed to receivers.