What belongs in a FastAPI lifespan context?

Uses lifespan to acquire and release process-level resources while making readiness, workers, failures, and tests explicit.

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TL;DR

A FastAPI lifespan context owns resources that should be created once for an application process and released when that process stops. Typical examples are shared HTTP clients, connection pools, loaded models, or managed background loops. Code before the lifespan yield performs startup, and code after it performs shutdown. Requests should not be accepted as ready until required startup succeeds, and cleanup should close partially acquired resources if startup or shutdown fails.

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Correct ownership of shared clients, pools, models, tasks, startup failure, and graceful shutdown.

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Core answer
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Overview
Lifespan is the process-level context around serving requests, not a place for arbitrary work before the first request.
A FastAPI lifespan context owns resources that should be created once for an application process and released when that process stops. Typical examples are shared HTTP clients, connection pools, loaded models, or managed background loops. Code before the lifespan yield performs startup, and code after it performs shutdown. Requests should not be accepted as ready until required startup succeeds, and cleanup should close partially acquired resources if startup or shutdown fails.
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I keep request-scoped sessions and user state out of lifespan; the shared object is usually a pool or client from which request work obtains bounded resources. Startup should validate critical configuration and establish only what the service truly needs to become ready. Long migrations, bulk warming, or business jobs normally need separate operational ownership. Tests must enter the lifespan context, otherwise they may exercise an app whose shared resources were never initialized.

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Details

Putting request-scoped sessions in global lifespan state.
Running database migrations independently in every worker startup.
Assuming lifespan executes only once for the whole deployment.
Creating background tasks without cancellation and shutdown ownership.

What belongs in a FastAPI lifespan context?

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