2.4 Advanced Load Balancing Techniques for Scalable Architectures

Level Up Your Load Balancing: Advanced Techniques for Scalable AWS Architectures

Load balancing is the unsung hero of scalable cloud architectures. It ensures your applications remain performant and available, even under heavy traffic. While basic load balancing distributes incoming requests across multiple instances, advanced techniques offer finer-grained control, enhanced resilience, and optimized user experiences. This post dives into some key advanced load balancing techniques on AWS, geared towards those looking to build truly scalable systems.

1. Content-Based Routing: Smart Traffic Steering

Imagine a busy airport with different terminals for domestic and international flights. Content-based routing in load balancers works similarly. Instead of blindly forwarding all traffic, it inspects the content of each request (like headers, URL paths, or even custom data) and routes it to the appropriate backend service.

Analogy: Think of a restaurant with different sections: one for the bar, one for the main dining area, and another for private events. The host (our load balancer) looks at where the customer wants to go (determined by their request – perhaps they’re asking for a drink, wanting dinner, or have a reservation for a party) and directs them to the correct section.

Practical Example:

• Microservices: You might have different microservices handling user profiles (/api/users), product catalog (/api/products), and order processing (/api/orders). An Application Load Balancer (ALB) can route requests based on the URL path to the respective microservice.
• Mobile vs. Web: You can route requests from mobile applications (identified by a specific header or user-agent) to a different set of backend instances optimized for mobile traffic.
• A/B Testing: Direct a percentage of users to a new version of your application based on a specific cookie or header, allowing you to perform A/B testing without impacting all users.

AWS Service: Application Load Balancer (ALB) is your go-to service for content-based routing using listener rules.

2. Weighted Round Robin and Least Outstanding Requests: Intelligent Distribution

While basic round robin distributes requests evenly, it doesn’t account for the varying capacity or current load of your backend instances. Advanced algorithms like weighted round robin and least outstanding requests provide more intelligent distribution.

• Weighted Round Robin: You assign a weight to each instance, indicating its relative capacity. Instances with higher weights receive proportionally more traffic. This is useful when you have instances with different sizes or performance capabilities.

  Analogy: Imagine a taxi stand with different sized vehicles. Larger vans (higher weight) can carry more passengers, so the dispatcher (load balancer) sends more groups to the vans compared to smaller cars (lower weight).

• Least Outstanding Requests: The load balancer sends new requests to the instance with the fewest currently active (outstanding) requests. This helps to ensure that no single instance gets overwhelmed.

  Analogy: Think of checkout lines at a supermarket. A customer (request) will naturally choose the line with the fewest people currently waiting (outstanding requests) to minimize their wait time.

AWS Service: Both Application Load Balancers (ALB) and Network Load Balancers (NLB) support weighted target groups. ALBs also implicitly consider the health of targets when routing.

Practical Example:

• During peak hours, you might temporarily increase the weight of your larger, more powerful instances to handle the increased load.
• Using least outstanding requests helps to distribute load more dynamically, especially in scenarios with varying request processing times.

3. Health Checks: Ensuring Backend Availability

Regular health checks are crucial for maintaining application availability. Advanced health checks go beyond simple TCP connection checks and can verify the actual health and responsiveness of your application.

Practical Examples:

• HTTP(S) Health Checks: ALB and NLB can send HTTP(S) requests to specified paths on your instances and expect a specific response code (e.g., 200 OK) to consider the instance healthy. You can check dependencies like databases or other services within your application’s health check endpoint.
• Custom Health Checks: For more complex scenarios, you can implement custom health check logic within your application and have the load balancer query an endpoint that reflects the overall health of the application and its dependencies.

AWS Service: Application Load Balancer (ALB) and Network Load Balancer (NLB) offer robust health check configurations.

Step-by-step Instruction (ALB HTTP Health Check):

1. Navigate to the EC2 console in the AWS Management Console.
2. Select “Load Balancers” from the left-hand menu.
3. Choose your Application Load Balancer.
4. Go to the “Target Groups” tab and select your target group.
5. Click on the “Health checks” tab.
6. Click “Edit health checks”.
7. Configure the protocol (HTTP or HTTPS), port, and path (e.g., /health).
8. Optionally, configure advanced settings like healthy/unhealthy thresholds, timeout, and interval.
9. Click “Save”.

 

4. Sticky Sessions (Session Affinity): Maintaining User Context

In some applications, it’s necessary to route all requests from a specific user to the same backend instance for the duration of their session. This is often required for applications that store session-specific data locally on the instance.

Analogy: Think of a personalized concierge service at a hotel. Once a guest interacts with a specific concierge, they might prefer to continue working with the same person for all their requests during their stay to maintain context.

Practical Example:

• E-commerce applications might store shopping cart information in the local session of an application instance. Sticky sessions ensure that a user’s subsequent requests during the same browsing session are directed to the same instance, preventing the loss of their cart data.

AWS Service: Application Load Balancer (ALB) supports sticky sessions based on cookies.

Important Considerations:

• Sticky sessions can limit the even distribution of traffic and might lead to some instances being overloaded while others are underutilized.
• Relying heavily on sticky sessions can make your application stateful, which can complicate scaling and resilience. Consider using a distributed session management solution (like AWS ElastiCache or DynamoDB) as a more scalable alternative.

Step-by-step Instruction (Enabling Sticky Sessions on ALB):

1. Navigate to the EC2 console in the AWS Management Console.
2. Select “Load Balancers” from the left-hand menu.
3. Choose your Application Load Balancer.
4. Go to the “Target Groups” tab and select your target group.
5. Click “Edit attributes”.
6. Under “Stickiness”, check the “Enable load balancer generated cookie stickiness” box.
7. Optionally, configure the “Duration” of the cookie.
8. Click “Save changes”.

Key Takeaways

• Content-based routing allows you to intelligently direct traffic based on request content, enabling flexible microservice architectures and A/B testing.
• Weighted round robin and least outstanding requests optimize traffic distribution by considering instance capacity and current load.
• Advanced health checks ensure that only healthy and responsive instances receive traffic, improving application availability.
• Sticky sessions can be useful for maintaining user context but should be used judiciously due to potential limitations on scalability and load distribution.

Mastering these advanced load balancing techniques on AWS will empower you to build highly scalable, resilient, and performant applications that can handle even the most demanding workloads. Remember to always choose the right load balancer type (ALB, NLB, or Gateway Load Balancer) and configuration based on your specific application requirements.