LastSeenDocs

What LastSeen watches

LastSeen tells you when something goes quiet. Here's what it can watch, and which kind of monitor to use.

Everything LastSeen watches is a monitor. There are two ways a monitor can work: something tells LastSeen it's alive, or LastSeen goes and looks.

Heartbeat monitors: it tells us #

A device, cron job, backup script or server pings a URL of its own, over and over. While the pings keep coming, it's up. When they stop, LastSeen tells you.

Use a heartbeat monitor for anything that runs on its own and can reach the internet:

  • an ESP32 or Raspberry Pi in the garage
  • a nightly backup, a cron job, a scheduled task
  • a server, a queue worker, a Home Assistant box

Pings can carry a reading too (a temperature, a queue length, "OK"), and LastSeen can alert on the value as well as on silence.

Start with Sending heartbeats, then Late and down for how LastSeen decides something has gone quiet. Jobs that run at set times, like "02:00 on weekdays", can be watched on their cron schedule.

Checks: we go and look #

LastSeen checks something from the outside, on a timer:

  • Website checks fetch a URL and look at the answer: the status code, a word on the page, the certificate. See Website checks.
  • TCP checks see whether a port answers, like a database or a mail server.
  • DNS checks ask a name server for a name, and can expect a particular address.
  • Ping checks send ICMP pings to a host on your own network.

Checks can run from several places in the world, so one bad network path doesn't wake you up.

Agents: inside your network #

An agent is a small LastSeen program you run on your own network, in Docker or as a Home Assistant add-on. It can check things the internet can't reach (a NAS, a printer, a router), and it relays heartbeats from devices that have no internet of their own. If the agent itself goes quiet, you get one alert, and what's behind it waits rather than alerting all at once.

Where alerts go #

When a monitor goes down, and when it comes back, LastSeen tells you on the channels you add: email, ntfy, Slack, Discord, Telegram or a webhook of your own. See Alert channels.

Next #

Quick start: your first monitor and your first alert, in about five minutes.