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A Uniac package is either a Service or a System. Both subclass marker classes from the uniac SDK and declare their dependencies as class-level type annotations. The kind field in uniac.json selects which one applies.

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Declaration shape

A Service or System is a plain Python class. Class-level annotations of type Node declare what it depends on; __init__ initializes them.
load() constructs the instance, validates that every annotated Node slot is filled, and hydrates each Node’s .url from the deploy-time wiring. See load.

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Service vs System: when to use which

  • Use Service (kind=lib) for code you want other packages to consume — a database client, a logger, a feature flag reader. Publish it with uniac build.
  • Use System (kind=app) for the unit you deploy. A System pulls Services together and exposes the public surface (HTTP, jobs, scheduled work).
A Service can depend on other Services. A System can depend on Services. Services never depend on Systems.

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No magic

The framework does not auto-instantiate annotated Nodes. The annotation is a declaration; the value comes from your __init__. load() validates after construction and raises UniacValidationError if any slot is unset or holds the wrong type. There is no base-class __init__ to call. If you don’t need init args, omit __init__ entirely.