Fastify

This is how your API handler would typically look like:

src/main.ts

import Fastify from "fastify";
import * as fs from "node:fs/promises";
import { bootstrapAuth, getUser } from "./auth"; // See below

async function startFastifyServer() {

    bootstrapAuth({
        implementation: "real", // or "mock", see: https://docs.oidc-spa.dev/v/v8/integration-guides/backend-token-validation/mock-modes
        issuerUri: process.env.OIDC_ISSUER_URI!,
        expectedAudience: process.env.OIDC_AUDIENCE
    });

    const fastify = Fastify({
        // If you run behind a reverse proxy, you almost always want this enabled.
        // It affects things like the computed request origin.
        trustProxy: true
    });

    fastify.get("/api/todos", async (req, reply) => {

        const user = await getUser({ req, reply });

        const json = await fs.readFile(
            `todos_${user.id}.json`,
            "utf8"
        );

        reply.code(200).type("application/json").send(json);

    });

    fastify.get("/api/todos-for-support/:userId", async (req, reply) => {

        // Will reject the request if user making the request
        // doesn't have "support-staff" role
        await getUser({ req, reply, requiredRole: "support-staff" });

        const { userId } = req.params as { userId: string };

        const json = await fs.readFile(
            `todos_${userId}.json`,
            "utf8"
        );

        reply.code(200).type("application/json").send(json);

    });

    // ...

    await fastify.listen({
        port: parseInt(process.env.PORT ?? "3000"),
        host: "0.0.0.0"
    });
}

Let's see how to export the utils to make it happen:

src/auth.ts

import { oidcSpa, extractRequestAuthContext } from "oidc-spa/server";
import { z } from "zod";
import type { FastifyReply, FastifyRequest } from "fastify";

const { bootstrapAuth, validateAndDecodeAccessToken } = oidcSpa
    .withExpectedDecodedAccessTokenShape({
        // This is purely declarative. Here you'll specify
        // the claim that you expect to be present in the access token payload.
        decodedAccessTokenSchema: z.object({
            sub: z.string(),
            // Keycloak specific, convention to manage authorization.
            realm_access: z
                .object({
                    roles: z.array(z.string())
                })
                .optional()
        })
    })
    .createUtils();

export { bootstrapAuth };

// Your local representation of a user.
export type User = {
    id: string;
};

export async function getUser(params: {
    req: FastifyRequest;
    reply: FastifyReply;
    requiredRole?: "realm-admin" | "support-staff";
}): Promise<User | never> {

    const { req, reply, requiredRole } = params;

    const bail = (statusCode: 400 | 401 | 403) => {
        reply.code(statusCode).send();
        return new Promise<never>(() => {});
    };

    const requestAuthContext = extractRequestAuthContext({
        request: req,
        // Set this to false only if you don't have a reverse HTTP proxy in front of your
        // server. (Almost never the case in modern deployments).
        trustProxy: true
    });

    if (!requestAuthContext) {
        // Demo shortcut: we return 401 on missing Authorization, but a mixed
        // public/private endpoint could instead return undefined here and let
        // the caller decide whether to process an anonymous request.
        console.warn("Anonymous request");
        return bail(401); // Unauthorized
    }

    if (!requestAuthContext.isWellFormed) {
        console.warn(requestAuthContext.debugErrorMessage);
        return bail(400); // Bad Request
    }

    const { isSuccess, debugErrorMessage, decodedAccessToken } =
        await validateAndDecodeAccessToken(
            requestAuthContext.accessTokenAndMetadata
        );

    if (!isSuccess) {
        console.warn(debugErrorMessage);
        return bail(401); // Unauthorized
    }

    // Your custom Authorization logic: Grant per request access depending
    // on the access token claim.
    if (requiredRole) {
        if (!decodedAccessToken.realm_access?.roles.includes(requiredRole)) {
            console.warn(`User missing role: ${requiredRole}`);
            return bail(403); // Forbidden
        }
    }

    // Here you can potentially enrich the user object with additional
    // data that you would retrieve from your database if the access token
    // claim does not contain everything you need.

    const user: User = {
        id: decodedAccessToken.sub
    };

    return user;
}