# MongoDB Atlas Triggers

A MongoDB Atlas Trigger is a feature in MongoDB Atlas that lets you run serverless functions automatically when specific events occur in your database (such as insert, update, delete, or schedule-based time events).

It listens for events and then runs code automatically.

Every Database Trigger is internally implemented using a MongoDB Change Streams connection.

## **How do Atlas Triggers use Change Streams?**

![](https://cdn.hashnode.com/uploads/covers/63577a91dcf7c3f71621c3aa/9255b1cd-63f1-4941-ae38-72205b9d8513.png align="center")

When you create a Database Trigger in MongoDB Atlas App Services, Atlas:

*   Creates a hidden Change Stream behind the scenes
    
*   Keeps it open continuously
    
*   Listens for the event you configured
    
*   When an event matches, your function is executed
    

Your cluster’s change stream capacity determines how many triggers you can create.

## **Types of Triggers**

*   Database Trigger
    
*   Scheduled Trigger
    

## **Database Trigger & Use Cases**

![](https://cdn.hashnode.com/uploads/covers/63577a91dcf7c3f71621c3aa/3b35dd80-f1b0-4b0e-9839-42ecdc47d310.png align="center")

A **Database Trigger** runs when data in a collection changes, such as during an insert, update, or delete operation.

Database triggers also support a **Match Expression**, which allows you to filter which change events should execute the trigger. The trigger runs only if the change event satisfies the match expression. If the expression is left empty, the trigger will match all events and execute for every change.

**Use Cases:**

*   Send an email when a user signs up
    
*   Process orders when they are inserted
    
*   Log activity
    
*   Update other collections
    
*   Sync data to external APIs
    

## **Scheduled Trigger & Use Cases**

![](https://cdn.hashnode.com/uploads/covers/63577a91dcf7c3f71621c3aa/f0266ea0-a282-4c6f-8538-7c520f81a1ec.png align="center")

A **Scheduled Trigger** runs at specific time intervals using **cron expressions**. It does not depend on **database changes** and does **not use Change Streams**.

**Use Cases:**

*   Cleanup old data every night
    
*   Send daily reports
    
*   Send scheduled notifications
    

## **Advantages**

*   No server setup required
    
*   Automatically reacts to data changes
    
*   Great for automation tasks
    
*   Integrates with HTTP endpoints and other Atlas services
    
*   Scales automatically
    
*   Supports scheduled jobs (cron-like tasks)
    

## Disadvantages:

*   Limited debugging compared to traditional backend code
    
*   Risk of infinite loops if the trigger updates the same collection
    
*   Execution time limits for trigger functions
    
*   Can become costly at a very large scale
    
*   Slight latency since triggers run asynchronously
    

## Screenshots:

![](https://cdn.hashnode.com/uploads/covers/63577a91dcf7c3f71621c3aa/bb4f87ff-1b07-4fce-866c-9123a1f45eda.png align="center")

![](https://cdn.hashnode.com/uploads/covers/63577a91dcf7c3f71621c3aa/e6868555-bb30-4326-ae6f-386cfd9cca6b.png align="center")