Feature flags at the edge with Progressively and Vercel (recipe)

There is a glossary at the end of the article if you are not used to these terms.

If you don't want to set up the whole thing, a Nextjs application showcasing this is available on https://github.com/mfrachet/progressively-edge-middleware

---

Summary

• 🧀 Ingredients
• 👨‍🍳 Directions
  • 1. Create a feature flag in Progressively
  • 2. Create feature flag variants
  • 3. Create a Nextjs project
  • 4. Create pages matching the previously created variants
  • 5. Install Progressively sdk
  • 6. Create the middleware file
  • 7. Push the code on GitHub
  • 8. Run the project on Vercel
• 📚 Glossary
  • Feature flags
  • Progressively
  • Vercel Edge Middleware

---

🧀 Ingredients

• A running instance of Progressively
• A personal Github account on Vercel
• Node.js > 16

---

👨‍🍳 Directions

1. Create a feature flag in Progressively

• Create a new project in the dashboard of Progressively (/dashboard/projects/create)
• Click on the Production environment that has been created
• Keep the environment key close, we'll need it in the next step.
• Click the Create a feature flag button and fill out the form

2. Create feature flag variants

• Once the flag is created, click on its name in the table
• On the feature flags page, Click on Variants
• Add 3 variants named A, B, and C, and adjust the percentage to your taste

3. Create a Nextjs project

Run the following command somewhere on your computer:

$ npx create-next-app@latest

4. Create pages matching the previously created variants

In the ./pages folder, create three files called index.ts, b.ts, and c.ts with different content inside.

5. Install Progressively sdk

Run the following command at the root of your Nextjs project:

$ npm install @progressively/server-side

6. Create the middleware file

At the root of your Nextjs project, create a middleware.ts (or .js, depending on your preferences) and copy the following code in it:

import { NextResponse } from "next/server";
import { getProgressivelyData } from "@progressively/server-side";

const PROGRESSIVELY_URL = "PUT YOUR SERVER URL HERE";
const PROGRESSIVELY_ENV = "PUT THE ENVIRONMENT KEY HERE";
const ExperimentRoutes = {
  A: "/",
  B: "/b",
  C: "/c",
};

type ExperimentRoutesKeys = keyof typeof ExperimentRoutes;

export default async function middleware(request) {
  // Get the user ID from cookies in order to always show them the same variant
  // If showing the same variant to the same user every time is not a concern for you, you can remove this line
  const id = request.cookies.get("progressively-id");

  // Get the feature flags from Progressively
  const { data, response } = await getProgressivelyData(PROGRESSIVELY_ENV, {
    apiUrl: PROGRESSIVELY_URL,
    fields: {
      id: id?.value || "",
    },
  });

  // Checking that the progressively servers brings us a valid first user id
  // If showing the same variant to the same user every time is not a concern for you, you can remove this line
  const progressivelyId = response.headers.get("x-progressively-id");
  if (!progressivelyId) return;

  let nextRawUrl =
    ExperimentRoutes[data.initialFlags.newHero] || ExperimentRoutes.A;

  const nextUrl = NextResponse.rewrite(new URL(nextRawUrl, request.url));

  // Stick the user ID to the cookies in order to always show them the same variant
  // If showing the same variant to the same user every time is not a concern for you, you can remove this line
  nextUrl.cookies.set("progressively-id", progressivelyId);

  return nextUrl;
}

export const config = {
  matcher: ["/"],
};

7. Push the code on GitHub

Create a repository on GitHub and push the project.

8. Run the project on Vercel

• Connect to your Vercel account using GitHub
• Select Add a new... and then Project
• Click on Continue with Github
• Select your repository and click Import at the end of the line
• Fill out the form and click Deploy

You now have a Nextjs application, working on the edge using feature flags in Progressively.

NB: if you want to see the different variants when refreshing, make sure to clean your cookies 😉

---

📚 Glossary

Feature flags

Feature flags are a technic allowing to progressively rollout features without modifying code at the moment of the release.
They generally take the form of a switch button that you can toggle on and off.

For instance, you may have created a new fancy UI for your login page, but you want to make sure you don't have errors on it that could block your users from accessing your platform. With a feature flagging tool, you can show this new page to N% of your audience, and augment this percentage while you acquire confidence until everybody sees the new page.

Progressively

Progressively is a free, simple, accessible, lightweight, self-hosted and Open Source feature flagging tool that you can use to rollout features to your users.

Vercel Edge Middleware

Vercel Edge Middleware is code that executes before a request is processed on a site, and before the CDN cache is hit.

They are a very conveniant place to handle A/B testing and feature flagging using page redirections, without content flashes.