When you visit an e-commerce website like Jumia or Temu, have you ever wondered how your request is processed and directed to the right page? Behind the scenes, services like AWS Load Balancers **use Listener Rules** to determine how to handle user traffic based on conditions such as URL paths, query strings, and host headers.

In this article, we'll break down listener rules in AWS, explain how query strings work, and use Jumia and Temu as real-world examples to make the concept easier to understand.

1. What is a Load Balancer?
In the last Article, I explained that a **Load Balancer **is a service that distributes incoming traffic across multiple servers to ensure:

  • High availability (prevents server overload)
  • Scalability (handles increasing traffic)
  • Fault tolerance (reroutes requests if a server fails)

However, to efficiently route traffic, a **Load Balancer uses Listener Rules **to determine how requests should be processed.

2. What Are Listener Rules?
A** listener** is a component of a load balancer that checks for connection requests using rules. These rules define conditions that determine how traffic should be forwarded.

Types of Listener Rule Conditions in AWS:
Condition Type Example from Jumia/Temu Use Case
Path-Based Routing: jumia.com.ng/cards or temu.com/fashion Routes based on specific URL paths
Host Header: app.jumia.com.ng vs deals.temu.com Routes based on subdomains
Query String: jumia.com.ng?category=electronics Routes based on key-value pairs in the URL
Source IP: Only allow users from 192.168.x.x Restricts access based on IP addresses
HTTP Header: User-Agent: Mobile Directs mobile users differently
Request Method: GET, POST, PUT, DELETE Routes requests based on HTTP methods

3.Practical Examples with Jumia & Temu
Let’s take two real-world scenarios to illustrate how listener rules work:

Example 1: Path-Based Routing on Jumia
Jumia has different sections for users. If you visit:

  • jumia.com.ng/cards **→ The request is forwarded to **Jumia’s shopping cart service.
  • jumia.com.ng/electronics → The request is forwarded to Jumia’s electronics page.

How AWS Listener Rules Handle This:

  • The load balancer detects the path (/cards) in the request.
  • If the rule states "Forward to Target Group A", AWS routes the request to a specific set of servers handling the cart system.
  • If the rule states "Redirect to another URL", AWS may send users to another page.

Why This Matters?

  • Jumia can manage different sections of its site more efficiently.
  • Users get a faster and more optimized experience.

Example 2: Query String Routing on Temu
Some websites personalize content based on Query Strings. If you visit:

  • temu.com?region=US → You see US-specific deals.
  • temu.com?region=UK → You see UK-based deals.

How AWS Listener Rules Handle This:

  • The Query String (?region=US) is detected in the request.
  • The rule checks the value (US or UK).
  • The system routes the user to the appropriate localized content.

Why This Matters?

  • Temu can personalize experiences for users based on their region.
  • Helps in localized marketing and promotions.

4.Hands-On Guide: Setting Up Listener Rules in AWS
Now, let’s create listener rules in AWS using the AWS Console.

Step 1: Create a Load Balancer

  • Go to AWS Console → EC2 → Load Balancers.
  • Click Create Load Balancer → Choose Application Load Balancer.
  • Configure the listener on port 80 (HTTP).
  • Add at least two EC2 instances **as **target groups.

Step 2: Add Listener Rules

  • Navigate to** Listener Rules** → Click Add Rule.
  • Click Add Condition → Choose one:
    • Path-Based Routing → /cards → Forward to Target Group A.
    • ** Query String-Based Routing** → ?region=US → Forward to Target Group B.
  • Click** Save**.

Step 3: Test Your Rules

5.Best Practices for Listener Rules

  • Use Priorities – Rules execute in order, so set priority levels carefully.
  • *Avoid Overlapping Rules *– Ensure each rule has unique conditions.
  • Use Redirects Wisely – Avoid unnecessary redirections that slow down performance.
  • Optimize Security **– Restrict access using **Source IP rules where necessary.

Advanced Hands-On Scenarios for Listener Rules in AWS
Now that we’ve covered the basics of listener rules, path-based routing, and query strings, let’s explore more advanced hands-on scenarios to deepen your understanding.

  1. Using HTTPS Listeners for Secure Traffic By default, most websites use HTTPS (port 443) to encrypt traffic. AWS Load Balancers allow us to set up an HTTPS listener to handle secure requests.

Steps to Configure an HTTPS Listener

  • Navigate to the AWS Console → Load Balancers.
  • Click on your Application Load Balancer (ALB).
  • Under** Listeners*, **click Add Listener* → Choose HTTPS (443).
  • Attach an SSL/TLS certificate (use AWS Certificate Manager if you don’t have one).
  • Click Create and save your listener.

How This Works

  • When a user visits https://yourwebsite.com, traffic is encrypted.
  • The ALB terminates SSL and forwards the request securely.
  • You can create rules to allow only HTTPS traffic and redirect HTTP to HTTPS.

Bonus: Redirect HTTP to HTTPS
Add a rule under Listener Rules:

  • Condition: If a request comes through HTTP (80)
  • Action: Redirect to HTTPS (443) - Status Code: 301 (Permanent Redirect) Now all traffic will be securely redirected to HTTPS!

2.** Implementing Custom Error Responses & Redirects**
Instead of displaying a generic "404 Not Found" error when a page doesn’t exist, you can customize error responses using listener rules.

Example: Redirecting Users When a Page is Not Found
Let’s say a user visits jumia.com.ng/deals but the page is down. Instead of showing an error, we can redirect them to another page.

Steps to Configure a Custom Redirect in AWS ALB

  • Go to Load Balancer Rules → Click Add Rule.
  • Add a Condition → Choose a Path-based condition (e.g., /deals).
  • Set the Action **→ **Choose Redirect.
  • Enter the new URL (e.g., https://jumia.com.ng/promotions).
  • Save the rule. Now, when users visit /deals, they are automatically redirected to the promotions page instead of seeing an error!

Example: Returning a Custom Error Response
Sometimes, you may want to return a fixed response instead of redirecting users.

  • Condition: Path /deals do not exist.
  • Action: Return Fixed Response → "Sorry, this page is temporarily unavailable. Check back later!"
  • HTTP Status Code: 503 (Service Unavailable). Now, instead of a default 404 error, users will see a custom message!

3. Configuring API Gateway & Lambda with Load Balancers
AWS API Gateway and Lambda can be integrated with ALB to create serverless applications that process user requests dynamically.

Scenario: Using API Gateway to Route Requests via Load Balancer
Imagine Jumia wants to handle customer orders using an API. Instead of directing users to different EC2 instances, we can use API Gateway and Lambda with the Load Balancer.

Step-by-Step Guide

  1. Create an API in API Gateway
  2. Go to API Gateway → Click Create API **→ Choose **HTTP API.
  3. Set the Endpoint as your** Load Balancer’s DNS name**.

  4. Define the paths (e.g., /orders).

  5. Create a Lambda Function for Handling Orders

  • Go to AWS Lambda → Click Create Function.
  • Write a function that processes orders: 
def lambda_handler(event, context):
    return {
        "statusCode": 200,
        "body": "Order received successfully!"
    }
  • Deploy the function and integrate it with API Gateway.

3. Set Up a Listener Rule in ALB
Condition: If the path is /orders.
Action: Forward the request to API Gateway instead of EC2.
Now, when users visit https://jumia.com.ng/orders, the API Gateway will invoke the Lambda function, process the request, and return a response!

  1. Using Priority to Manage Multiple Rules In AWS Load Balancers, rules are evaluated based on priority, meaning:
  2. Lower priority numbers execute first.
  3. If multiple rules match, the one with the lowest number is applied.

Example Scenario: Handling Multiple Conditions
Let's say:

  • Users visiting jumia.com.ng/orders should be routed to API Gateway.
  • Users visiting jumia.com.ng/deals should be redirected to /promotions.
  • Users visiting jumia.com.ng?region=US should be sent to US servers.

Rule Setup in AWS:

Priority Condition Action
1 /orders Forward to API Gateway
2 /deals Redirect to /promotions
3 ?region=US Forward to US-based EC2 servers

The Load Balancer will process the rules in order, ensuring correct traffic routing!

Final Thoughts
We’ve now explored multiple hands-on scenarios for AWS Listener Rules, including:

  • Using HTTPS listeners for secure traffic.
  • Implementing custom redirects & error responses.
  • Integrating API Gateway & Lambda for dynamic routing.
  • Managing priority for multiple listener rules.

Using real-world concepts used in companies like Jumia, Temu, and Amazon to optimize traffic flow and improve user experience.

Difference Between the Beginner and Advanced Hands-On Scenarios for Listener Rules in this article

The beginner-level explanation of listener rules covered the basic concepts and simple configurations using practical examples like Jumia and Temu. The focus was on:

  1. What listener rules are and why they are important in a Load Balancer.
  2. Basic conditions like Path-based routing and Query Strings (e.g., /cart → directs traffic to a specific target group).
  3. Simple actions such as forwarding traffic, redirecting users, and returning error responses.
  4. Understanding **query strings **and how they affect URL-based routing (?region=US). This level is good for understanding how requests are processed and routed in a load balancer using basic rule-based conditions.

The advanced hands-on scenarios take it further by introducing:

  1. Security Enhancements – Implementing HTTPS listeners instead of just HTTP for secure traffic encryption
  2. Custom Error Responses & Redirects – Handling 404 errors dynamically with custom messages or redirects (e.g., if /deals is unavailable, redirect to /promotions).
  3. Integrating API Gateway & Lambda – Instead of routing traffic only to EC2 instances, we invoke serverless APIs dynamically. 4.** Priority-Based Rule Evaluation** – Learning how **AWS processes multiple listener rules **based on priority values.

This level simulates real-world scenarios used in cloud architectures at companies like Jumia, Amazon, and Temu, where security, automation, and flexibility are essential.

6. Conclusion
AWS Listener Rules are powerful tools for controlling traffic flow in a Load Balancer. By using Path-Based Routing, Query Strings, and Host Headers, businesses like Jumia and Temu can efficiently route users to the right pages.

If you're practicing with AWS Console, try setting up **custom listener rules **and experiment with different conditions. The more hands-on experience you get, the better you'll understand how AWS Load Balancers work!