The Umari Book

Status: Draft.

This book is the official guide to building event-sourced systems with Umari. It covers concepts, patterns, API reference, and operations: everything you need to go from zero to production.

Who this book is for

You should be comfortable with either Rust or TypeScript. Umari modules can be written in either, and every code example in this book is shown in both languages. Prior event sourcing experience helps but is not required; the opening chapters explain the concepts from scratch.

How to read this book

• Concepts: Read straight through. These chapters establish the mental model.
• Building Blocks: Read straight through. You’ll use every concept here.
• Module Types: Reference each chapter as you build that module type.
• Working with Umari: Practical patterns, project structure, and API reference.
• Runtime & Operations: Read when deploying or debugging. (These chapters are still in draft.)

Code examples follow current API conventions and appear in language tabs: pick Rust or TypeScript once and every snippet in the book follows your choice.

Conventions

• SDKs: umari refers to the Rust SDK crate (crates/umari); @umari/js is the TypeScript SDK. umari-runtime refers to the runtime. Module crates/packages use kebab-case names.
• Language tabs: Snippets that differ by language are shown in Rust / TypeScript tabs. The two SDKs share one WIT contract and produce interchangeable WASM modules.