Build a service¶

Learn lcl-fastapi by building three complete services. Each chapter starts with a concrete application and explains its configuration, resource ownership, API, verification and limitations. Start with the catalog to learn the framework, or open either browser-based example independently.

Series contents¶

1. Build a product catalog service — typed routes, configuration, logging, a console entrance, and deployment preparation.
2. Monitor a directory with a live browser UI — recursive background work, file APIs, static assets and server-sent events.
3. Build an interactive LCL playground — worker-local sessions, native syntax/dependency inspection and evaluation replay.

Before you start¶

Use CPython 3.14 on Windows or Linux and install lcl-fastapi==0.5.0 from PyPI. This version requires lclang >=1.1.0. Run one service at a time; the framework supports one service per machine. All examples bind to loopback, use local resources, and need no external account. Installation requires package-index access or an operator-prepared wheelhouse.

Every chapter starts in an empty directory and prints the complete files to create and extend. No repository checkout is needed. Build and verify one capability at a time: configuration, lifespan resources, routes, background work or sessions, then the browser and operations. The complete example code is linked at each chapter's start as a reference. Chapters 2 and 3 include all static assets in their own wheels. Documentation tests reconstruct those projects from the exact printed source; their verify.py programs exercise installed native services in temporary directories. The framework wheel does not contain downstream projects. See engineering checks for the Windows/Linux installation matrix.

These examples teach local trusted workflows. Read each chapter's filesystem or expression-execution boundary before using it with other people's data or serving it outside your machine.