> For the complete documentation index, see [llms.txt](https://docs.mediarithmics.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.mediarithmics.io/audience-and-activation/automate/automations-reference/triggers.md).

# Triggers

The three ways a user enters an automation, and the re-entry rule that governs all of them.

## What a trigger is

The **trigger** is what makes a user enter an automation. Each automation has **one and only one** trigger, and it is always the first node - which is why entering the automation builder starts by asking which type of trigger you want.

## The three types

<table data-search="false"><thead><tr><th width="220">Trigger</th><th>The user enters when</th><th width="230">Configured with</th></tr></thead><tbody><tr><td><strong>On segment entry</strong></td><td>They enter a specific segment. The automation is linked to segment computation.</td><td>A <a href="/audience-and-activation/build-your-audiences/segment-reference/segment-types/query-segment.md">Query</a> or <a href="/audience-and-activation/build-your-audiences/segment-reference/segment-types/list-import-segment.md">List</a> segment</td></tr><tr><td><strong>On segment exit</strong></td><td>They leave a specific segment. Also linked to segment computation.</td><td>A <a href="/audience-and-activation/build-your-audiences/segment-reference/segment-types/query-segment.md">Query</a> segment</td></tr><tr><td><strong>React to an event</strong></td><td>They cause a specified event.</td><td>A query on incoming activities</td></tr></tbody></table>

{% hint style="info" %}
The first two run at the pace of segment computation - by default daily. **React to an event** runs at the pace of ingestion. If the scenario has to act within minutes of a behaviour, it needs an event trigger, not a segment trigger.
{% endhint %}

## React to an event, in detail

The configuration specifies which events trigger the automation. In the **Advanced** view you can add field conditions - for example to trigger only on events taking place after a certain date.

Two constraints on the query:

* **Only fields carrying the `@TreeIndex` directive** in the runtime schema can be used.
* The query is evaluated **in the ingestion pipeline**, on incoming activities - after session aggregation and processing by activity analysers, and before storage in the datamart.

{% hint style="info" %}
That evaluation point is why an event trigger is fast, and also why it sees the activity as the pipeline produced it rather than as the datamart later stores it.
{% endhint %}

## Re-entry

{% hint style="danger" %}
**If a user enters a scenario, any attempt to make that user enter the same scenario again fails until the user exits it.**
{% endhint %}

This single rule explains most surprising automation behaviour, and it is the reason [exit conditions](/audience-and-activation/automate/automations-reference/flow-control.md#exit-conditions) matter as much as triggers:

* A user sitting in a 2-hour wait node cannot be re-triggered by a second qualifying event during those 2 hours.
* A scenario with a long wait and no exit condition can hold a user for a long time, silently excluding them from re-entry.
* A user who leaves through an exit condition becomes eligible again immediately.

Design the exit before you design the wait.

## Related pages

<table data-view="cards" data-search="false"><thead><tr><th></th><th></th><th data-hidden data-card-target data-type="content-ref"></th></tr></thead><tbody><tr><td><strong>Flow control</strong></td><td>Wait, If, Split, and the exit conditions that free a user.</td><td><a href="/audience-and-activation/automate/automations-reference/flow-control.md">Flow control</a></td></tr><tr><td><strong>Build your audience from live user activity</strong></td><td>Choosing an automation over Edge or a campaign segment.</td><td><a href="/audience-and-activation/build-your-audiences/target-your-audience-based-on-criteria/build-your-audience-from-live-user-activity.md">Build your audience from live user activity</a></td></tr></tbody></table>


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