AWS Application Load Balancer: Path-Based Routing Made Easy
Introduction
The AWS Application Load Balancer (ALB) is a Layer 7 load balancer that offers advanced routing capabilities, including content-based routing. Among its most powerful features is path-based routing, which allows you to forward requests to different target groups based on the URL path of the request. This is particularly useful for microservices, containerized applications, or any app that requires routing logic based on specific endpoints.
What is Path-Based Routing?
Path-based routing enables the ALB to inspect the URL path of incoming HTTP/HTTPS requests and direct traffic accordingly. For example:
/api/* can be routed to an EC2 instance or ECS service running your API.
/app/* can be routed to a React app hosted on another backend.
/admin/* can be routed to a completely different set of instances.
Setting Up Path-Based Routing on ALB
1. Create an Application Load Balancer
Go to EC2 Dashboard > Load Balancers.
Choose Application Load Balancer.
Set listeners (HTTP or HTTPS), define availability zones, and configure security groups.
2. Create Target Groups
One target group for each path (e.g., /api, /app, /admin).
Register appropriate EC2 instances or ECS services to each target group.
3. Configure Listener Rules
Go to the Listeners tab on your ALB.
Edit the listener (typically port 80 or 443).
Add rules with conditions such as:
IF path is /api/* → forward to API target group
IF path is /app/* → forward to App target group
IF path is /admin/* → forward to Admin target group
4. Set a Default Rule
Ensure the last/default rule catches all unmatched paths and forwards them to a fallback service or returns a 404.
Example Scenario
Let’s say you're running a SaaS application with three components:
Frontend App at /app/*
REST API at /api/*
Admin Dashboard at /admin/*
You would:
Create three target groups: tg-app, tg-api, and tg-admin.
Deploy your services on separate EC2 instances or containers.
Set path-based rules to forward requests based on URL patterns.
Use Cases
Microservices architecture: Easily direct traffic to different services.
Single domain for multiple apps: Consolidate routing under one domain.
A/B testing: Forward to different backends based on experimental paths.
Benefits of Path-Based Routing
Granular traffic control at Layer 7.
Improved resource utilization by routing only relevant requests to each service.
Simplified DNS and certificate management by using one ALB instead of many.
Cost savings by consolidating infrastructure.
Security Considerations
Use HTTPS listeners with valid TLS certificates.
Implement WAF (Web Application Firewall) rules for each path if needed.
Configure authorization at the application level based on paths.
Monitoring and Logging
Use AWS tools for observability:
CloudWatch Metrics: Track target group health, request counts, etc.
Access Logs: Enable logging for detailed request information.
X-Ray: Trace requests through your application for deeper insights.
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