> For the complete documentation index, see [llms.txt](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/event-driven-architecture/event-governance-the-foundation-of-successful-event-driven-architecture/day-3-14-event-schema.md).

# Day 3/14 Event Schema

Day 3 of 14: Understand the event schema as a strict data contract, breaking down message headers, payload design, and the use of AsyncAPI for event-driven systems.

In EDA, **the event schema plays a critical role in ensuring consistency and data synchronization between different components**. It serves as a contract between these components, defining the structure and format of events. Having a well-structured schema is important to minimize dependencies between publishers and subscribers, and to adhere to the decoupling principle in EDA design.\
\
When transmitting events between components, they are packaged as messages. The **message header** contains additional information for pre-processing, such as filtering and routing, before the event is decoded. This information may include the message type, publish time, and other relevant labels.\
\
The event itself is the **payload** of the message, providing detailed information in a modularized structure. This includes elements, attributes, descriptions, patterns, and other relevant components of the event.\
\
Unlike traditional remote procedure call (RPC) and user datagram protocol (UDP) calls that often use ASCII code in their payload, EDA typically utilizes schemas tailored for event-driven systems. One popular choice for defining event schemas is AsyncAPI. AsyncAPI provides a specification and tooling for designing, documenting, and communicating event-driven APIs. You can find more information about AsyncAPI on their official website at [https://www.asyncapi.com](https://www.asyncapi.com/).

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