---
title: "The first monitor"
description: "A monitor runs a definition on every beat. Each beat reads what entered the record since the last beat, records each match with its evidence, and adds to the signals by subject and strength."
section: start
address: /start/the-first-monitor
contract: 0476e35e6e275db5
---

# The first monitor

A monitor runs a definition on every beat. Each beat reads what entered the record since the last beat, records each match with its evidence, and adds to the signals by subject and strength.

[figure: On every beat the monitor reads the new data, records each match with its evidence, and adds the matches to the signals.]

Arm a definition once its preview returns a useful count. A monitor is the definition with a window and a target. The monitor runs on every beat, the fixed interval at which the platform checks new data.

## Monitor beat

1. The beat reads what entered the record since the last beat. New facts, observations and signals arrive from every source that the tenant reads.
2. The beat runs the monitor's predicate over the new data. The preview ran the same predicate.
3. Each hit is a match. A match records which condition matched, on which fact, from which source, and when the platform saw the fact.
4. The matches add to the signals. The platform groups signals by subject, kind and strength over the window.

## Monitor routes

| route | returns |
|---|---|
| [`/v1/act/arm_monitor`](/reference/act/arm_monitor) | the armed monitor, with its version |
| [`/v1/read/signals`](/reference/read/signals) | the signals in a scope and a window, by subject |
| [`/v1/read/explain_catch`](/reference/read/explain_catch) | the reasons for one match, back to the fact and its source |
| [`/v1/read/monitor_strength`](/reference/read/monitor_strength) | the strength of the monitor's evidence, and its calibration |

## Monitor page in the app

The monitor's page in the app shows each match as it arrives. The page also shows the monitor's strength and the matches that the plan withheld. You can correct a match on the page. The platform records the correction as an event, and the monitor's rule learns from it.