Versioning and Rollout¶

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. FastFN supports side-by-side versions (for example v2) so you can ship changes without breaking existing clients.

1) Folder structure¶

Versioning is represented by a version subfolder under the function directory:

functions/
  hello/
    handler.js          # default version
    v2/
      handler.js        # version v2

2) Example code¶

Default version (functions/hello/handler.js):

exports.handler = async (event) => {
  const name = (event.query || {}).name || "World";
  return { message: `Hello from V1, ${name}` };
};

Version v2 (functions/hello/v2/handler.js):

exports.handler = async (event) => {
  const name = (event.query || {}).name || "World";
  return { status: "success", data: { greeting: `Hello from V2, ${name}` } };
};

3) Calling default vs version¶

Default (if handler.js exists at the root of the function folder):

curl -sS 'http://127.0.0.1:8080/hello?name=World'

Versioned:

curl -sS 'http://127.0.0.1:8080/hello@v2?name=World'

4) Rollout pattern¶

Deploy the v2/ folder.
Test with GET /hello@v2.
Migrate clients gradually.
Remove the old default version when traffic reaches zero.

Next: Authentication and Secrets

Flow Diagram¶

flowchart LR
  A["Client request"] --> B["Route discovery"]
  B --> C["Policy and method validation"]
  C --> D["Runtime handler execution"]
  D --> E["HTTP response + OpenAPI parity"]

Objective¶

Clear scope, expected outcome, and who should use this page.

Prerequisites¶

FastFN CLI available
Runtime dependencies by mode verified (Docker for fastfn dev, OpenResty+runtimes for fastfn dev --native)

Validation Checklist¶

Command examples execute with expected status codes
Routes appear in OpenAPI where applicable
References at the end are reachable

Troubleshooting¶

If runtime is down, verify host dependencies and health endpoint
If routes are missing, re-run discovery and check folder layout