Feature flag evaluator tutorial

This tutorial walks through the first hands-on UMA example: a deterministic feature flag evaluator with one contract, one portable Rust core, a WASI executable, and a TypeScript parity implementation.

By Enrico Piovesan Updated June 22, 2026

Tutorial route

Keep the navigation close to the tutorial itself. Use the links below to move between the source folder, the examples index, and the next chapter without hunting through the footer.

Open source folder All UMA examples Next: Post Fetcher Runtime

What you will learn

what belongs in the evaluator contract instead of the host adapter
how first-match rule evaluation and sticky rollout decisions stay deterministic
how Rust and TypeScript parity is proven from observable output

Prerequisites

Rust with the wasm32-wasip1 target
Wasmtime on your PATH
Node.js 20 or newer for parity checks
npm for TypeScript parity and optional adapters

Run this setup command before the lab if your machine does not already have the target installed.

rustup target add wasm32-wasip1

Full tutorial

Enter the example
```
cd chapter-04-feature-flag-evaluator
```
List the guided labs
```
./scripts/list_labs.sh
```
Run the country match lab
```
./scripts/run_lab.sh lab1-country-match
```
Run the sticky rollout lab
```
./scripts/run_lab.sh lab2-rollout-match
```

Run the default fallback lab

./scripts/run_lab.sh lab3-default-fallback

Run the rule language lab
```
./scripts/run_lab.sh lab4-rule-language
```
Compare Rust and TypeScript behavior
```
./scripts/compare_impls.sh
```
Run the chapter smoke path
```
./scripts/smoke_flag_labs.sh
```

What to inspect

After each command, look for these proof points. They are the signals that connect the code example back to the UMA architecture claim.

enabled
matchedRule
the same lab returning the same decision in Rust and TypeScript

Acceptance check

The chapter-level validation path is:

./scripts/smoke_flag_labs.sh

Return to the repository root for the final acceptance gate:

cd ..
./scripts/smoke_reader_paths.sh

Where to go next

Chapter 5 introduces the runtime layer around a pure service.

Open source folder All UMA examples Read Chapter 5: Building Portable Microservices → Next: Post Fetcher Runtime