Invocation Flow

Verified status as of March 28, 2026. Runtime note: FastFN auto-installs function-local dependencies from requirements.txt / package.json; host runtimes are required in fastfn dev --native, while fastfn dev depends on a running Docker daemon.

Public path (/<name>)

1. Nginx routes request to fn_gateway.lua.
2. Gateway parses the route (for example /hello) and optional @<version> (for example /hello@v2).
3. Discovery resolves runtime (python, node, php, or rust) and effective policy.
4. Gateway validates method/body/concurrency.
5. Gateway sends framed JSON to runtime socket.
6. Runtime executes handler and returns {status, headers, body}.
7. Gateway returns HTTP response.

/_fn/invoke internal helper

/_fn/invoke does not call runtimes directly.

It builds an internal request and routes it through the same gateway routing/policy layer as external clients. That means method policy, limits, and error mapping are consistent.

Context forwarding

/_fn/invoke can include context in payload. It is forwarded via an internal query marker and decoded by gateway into event.context.user.

Problem

What operational or developer pain this topic solves.

Mental Model

How to reason about this feature in production-like environments.

Design Decisions

• Why this behavior exists
• Tradeoffs accepted
• When to choose alternatives

See also

• Function Specification
• HTTP API Reference
• Run and Test Checklist

Last reviewed: March 28, 2026 · Docs on fastfn.dev