> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ensync.cloud/llms.txt
> Use this file to discover all available pages before exploring further.

# Overview

> Understanding events in EnSync

## What Are Events?

Events are messages published and delivered through EnSync at runtime. They represent actions or state changes in your system (e.g., `order/created`, `payment/completed`, `inventory/updated`).

**Event Definitions** are optional schemas that define the expected structure of events. **Events** are the actual messages published with real data. If an event definition exists, EnSync validates the payload before publishing.

## Event Structure

Every event in EnSync contains:

* **eventName**: The event path (e.g., `order/created`)
* **payload**: Your custom data
* **idem**: Unique event ID for idempotency
* **block**: Block ID for ordering
* **timestamp**: When the event was created
* **sender**: The appId of the publisher
* **metadata**: System-generated metadata including `$internal` fields

## Event Flow

1. **Publish**: Service provider publishes an event to specific recipients
2. **Queue**: EnSync queues the event per recipient's appId and eventId
3. **Deliver**: EnSync pushes events one at a time to subscribers
4. **Process**: Subscriber processes and acknowledges/defers/discards

## Key Features

* **Ordered delivery**: Events are delivered in order per event type
* **At-least-once delivery**: Events are guaranteed to be delivered
* **Event retention**: Events are stored for replay and debugging (up to 30 days based on plan)
* **End-to-end encryption**: All events are encrypted in transit and at rest