Resonate Basic Durable World Usage — Python

Overview

Durable functions are the payload of Resonate. They are async def coroutines that the SDK treats as a sequence of durable checkpoints. Every await ctx.run(...) or await ctx.rpc(...) is a save point: if the process crashes, the function resumes from the last successful checkpoint on another process or a restart.

This skill covers the Context API (ctx.*) that you use inside those functions. The ephemeral-world counterpart (Resonate class, registration, top-level invocation) lives in resonate-basic-ephemeral-world-usage-python.

The contract

Every durable function:

1. Is registered with r.register(fn) in the ephemeral world.
2. Takes ctx (the Context) as its first parameter; any other parameters after that are the invocation args.
3. Uses await to interact with ctx. Each awaited ctx.* call is a durable checkpoint.
4. Is deterministic when replayed: no direct time.time(), no random.random(), no raw network I/O outside ctx.run.

Minimal shape: