Quick Start

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.

Quick View

• Complexity: Beginner
• Typical time: 10-15 minutes
• Scope: create one function, run locally, call it, and confirm OpenAPI visibility
• Expected outcome: a working GET /hello endpoint and docs at /docs

Prerequisites

• FastFN CLI installed and available in PATH
• One execution mode ready:
• Portable mode: Docker daemon running
• Native mode: openresty and runtime binaries available

1. Create your first function (neutral path)

mkdir -p functions/hello

Choose one runtime implementation inside functions/hello/:

Node.jsPythonRustPHP

File: functions/hello/handler.js

exports.handler = async (event) => ({
  status: 200,
  body: { hello: event.query?.name || "World", runtime: "node" }
});

File: functions/hello/handler.py

def handler(event):
    name = (event.get("query") or {}).get("name", "World")
    return {"status": 200, "body": {"hello": name, "runtime": "python"}}

File: functions/hello/handler.rs

use serde_json::{json, Value};

pub fn handler(event: Value) -> Value {
    let name = event
        .get("query")
        .and_then(|q| q.get("name"))
        .and_then(|n| n.as_str())
        .unwrap_or("World");

    json!({
        "status": 200,
        "body": {
            "hello": name,
            "runtime": "rust"
        }
    })
}

File: functions/hello/handler.php

<?php

function handler(array $event): array {
    $query = $event['query'] ?? [];
    $name = $query['name'] ?? 'World';

    return [
        'status' => 200,
        'body' => [
            'hello' => $name,
            'runtime' => 'php',
        ],
    ];
}

2. Start the local server

fastfn dev functions

3. Validate with curl (per runtime)

Node.jsPythonRustPHP

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

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

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

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

Expected response shape:

{
  "hello": "World",
  "runtime": "<selected-runtime>"
}

4. Verify generated API docs

• Swagger UI: http://127.0.0.1:8080/docs
• OpenAPI JSON: http://127.0.0.1:8080/openapi.json

curl -sS 'http://127.0.0.1:8080/openapi.json' | jq '.paths | has("/hello")'

Expected output:

true

Validation checklist

• GET /hello returns HTTP 200
• /openapi.json contains /hello
• /docs loads and shows the route

Troubleshooting

• Runtime down or 503: check /_fn/health and missing host dependencies
• Route missing: confirm folder layout and rerun discovery (/_fn/reload)
• /docs empty: verify docs/OpenAPI toggles were not disabled

Next links

• Part 1: Setup and first route
• Routing and parameters
• HTTP API reference

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