🔁 How to Build a Lightweight Round-Robin HTTP Redirector Using OpenResty

Sometimes, you only need simple HTTP redirection to multiple backend URLs in a round-robin fashion — without load balancers or proxying content.
Here’s how you can achieve it using OpenResty (NGINX + Lua).

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🛠 Prerequisites

• Ubuntu system (x86_64 / amd64)

• Root or sudo privileges

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1️⃣ Install OpenResty

# Install basic dependencies
sudo apt-get -y install --no-install-recommends wget gnupg ca-certificates

# Add OpenResty GPG key
wget -O - https://openresty.org/package/pubkey.gpg | sudo apt-key add -

# Add OpenResty repository
echo "deb http://openresty.org/package/ubuntu $(lsb_release -sc) main" > openresty.list
sudo cp openresty.list /etc/apt/sources.list.d/

# Update and install OpenResty
sudo apt-get update
sudo apt-get -y install --no-install-recommends openresty

# Verify installation
openresty -V

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2️⃣ Configure NGINX for Round-Robin Redirects

Edit the main NGINX configuration file:

sudo vim /etc/openresty/nginx.conf

Add the following inside the http { ... } block:

# Shared memory zone to track counter
lua_shared_dict rr_counter 1m;

# Redirector server
server {
    listen 80;
    server_name www.example.com;

    # Health check endpoint
    location /health {
        return 200 "OK";
    }

    # Round-robin redirect
    location / {
        content_by_lua_block {
            local urls = {
                "https://www.example.com/page1",
                "https://www.example.com/page2",
                "https://www.example.com/page3",
                "https://www.example.com/page4",
                "https://www.example.com/page5"
            }

            local dict = ngx.shared.rr_counter
            local count = dict:get("count") or 0
            count = (count % #urls) + 1
            dict:set("count", count)

            local target = urls[count]
            return ngx.redirect(target, 302)
        }
    }
}

How it works:

• lua_shared_dict rr_counter 1m; creates a small shared memory segment to track the current URL index.

• Each request increments the counter modulo the number of URLs.

• NGINX issues a 302 redirect to the next URL in sequence.

• /health endpoint can be used for load balancer or CloudFront health checks.

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3️⃣ Reload OpenResty

sudo /usr/local/openresty/nginx/sbin/nginx -t   # Test configuration
sudo systemctl reload openresty                 # Reload OpenResty

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4️⃣ Test the Redirect

for i in {1..10}; do curl -I http://www.example.com | grep Location; done

Expected output:

Location: https://www.example.com/page1
Location: https://www.example.com/page2
Location: https://www.example.com/page3
Location: https://www.example.com/page4
Location: https://www.example.com/page5
Location: https://www.example.com/page1
...

Requests cycle through the URLs in order — true round-robin redirection.

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✅ Advantages

• Minimal server load (no content proxying)

• True round-robin distribution

• Works perfectly behind ELB or CloudFront

• Easily extendable to multiple domains or URL lists