Configuration options for load balancers (for example, proxy protocol, cross-zone load balancing, session affinity [sticky sessions], routing algorithms)

Can you explain what the Proxy Protocol is and why it’s useful in a load balancing context?

The Proxy Protocol is used to pass the original client’s IP address and port as well as the destination IP address and port to an EC2 instance behind a load balancer. This is useful because it allows the instance to obtain the original client’s IP, which can be necessary for client IP-based access control, logging, or other security purposes. Without it, the instance only sees the load balancer’s IP.

What is cross-zone load balancing in AWS, and how does it benefit a multi-AZ application deployment?

Cross-zone load balancing evenly distributes traffic across all registered instances in all Availability Zones, rather than just within the same zone. This ensures that no single zone becomes a bottleneck, which enhances fault tolerance and performance for applications deployed across multiple zones.

What are sticky sessions and when would you use them in a load balancing scenario?

Sticky sessions, also known as session affinity, allow requests from the same client to be directed to the same target instance. This is useful for applications that need to maintain state between requests, such as shopping carts in e-commerce sites or user sessions in web applications.

Describe at least two routing algorithms that are commonly used by load balancers, and explain the scenarios where you would prefer one over the other.

The two common routing algorithms are round robin and least outstanding requests. Round robin distributes incoming requests uniformly across all available instances, which may be appropriate for instances with similar performance characteristics. Least outstanding requests, on the other hand, route traffic to the instance with the fewest active connections, which is better for when instances have varied performance profiles or request handling times.

In what scenarios would you disable cross-zone load balancing?

You might disable cross-zone load balancing to optimize for cost when data transfer costs between zones are a concern or when deploying a tightly controlled environment where you want to ensure requests are handled within the same zone to minimize inter-zone latencies.

How does AWS’s Elastic Load Balancing handle SSL/TLS termination, and what are the implications for this configuration with respect to backend instances?

AWS Elastic Load Balancing supports SSL/TLS termination, which means the load balancer handles the SSL/TLS connection from the client. Back-end instances can then focus on serving requests without the overhead of encryption and decryption, which can optimize performance. However, this also means that traffic between the load balancer and the instances is not encrypted unless backend encryption is also configured.

How does enabling the Proxy Protocol impact health checks performed by a load balancer?

Enabling the Proxy Protocol does not directly impact the way health checks are performed; health checks will continue to operate as normal, evaluating the health of your instances and routing traffic accordingly. The main effect of enabling Proxy Protocol is that additional information about the client’s connection, such as the client’s IP address and port, becomes available to the application.

What are the benefits of using a Layer 7 (application layer) load balancer over a Layer 4 (transport layer) load balancer?

A Layer 7 load balancer can make routing decisions based on content at the application layer, such as HTTP headers, cookies, or data within the messages. This enables more advanced routing rules, content-based routing, and application-specific load balancing. Layer 4 load balancers operate at the transport layer and route traffic based on IP address and port, which is less flexible but potentially faster due to less computation.

How can session affinity impact the scalability of a web application when using a load balancer?

Session affinity might negatively impact the scalability of a web application as it can create an uneven distribution of traffic, causing some instances to carry a heavier load than others. This can lead to potential hotspots and might not fully take advantage of the distributed nature of the system. However, for certain applications that rely on stateful information, session affinity is necessary despite the potential impact on scalability.

Can you apply different routing algorithms to specific types of traffic or content in AWS ELB?

AWS Elastic Load Balancer, particularly Application Load Balancer (ALB), allows you to create routing rules based on content, such as path-based routing or host-based routing. However, the choice of routing algorithm is generally applied to the load balancer as a whole rather than to specific content types. For fine-grained control over routing, you would use content-based routing rules along with target groups to direct traffic according to your application needs.

What mechanism would you use to ensure that a newly deployed instance behind a load balancer does not start receiving traffic until it is fully configured and ready to handle requests?

You would use health checks to ensure that a new instance is not placed into service until it passes the configured health check criteria. The load balancer will only route traffic to instances that have passed the health checks, ensuring that new instances are fully operational before they start receiving client requests.

When configuring an Application Load Balancer, how can you ensure that clients connecting over IPv6 are managed correctly?

To manage clients connecting over IPv6, you can enable dual-stack support on your Application Load Balancer. This allows the load balancer to accept both IPv4 and IPv6 connections, ensuring compatibility with clients using either IP version. Furthermore, you can configure your listener rules and target groups to handle IPv6 traffic appropriately.