Load balancing (for example, layer 4 compared with layer 7, reverse proxies, layer 3)

What is the difference between Layer 4 and Layer 7 load balancing?

Layer 4 load balancing operates at the transport layer, distributing traffic based on data from network and transport layer protocols, such as IP addresses and TCP/UDP ports. It makes routing decisions based on the source and destination without considering the content of the packets. Layer 7 load balancing, on the other hand, operates at the application layer and makes routing decisions based on content-specific data, such as HTTP headers, cookies, or data within the application message.

How does a reverse proxy differ from a load balancer?

A reverse proxy sits in front of web servers and forwards client requests to them, potentially providing benefits like caching, SSL termination, or security. While a load balancer also distributes traffic across multiple servers, it primarily focuses on optimizing resource use, minimizing response times, and increasing the reliability of applications by ensuring even distribution and failover if necessary.

Can you explain the term “connection draining” in the context of load balancing?

Connection draining is a feature that enables a load balancer to stop sending new requests to a server that is deregistering or unhealthy while keeping existing connections until a specified timeout. This ensures that ongoing sessions are not abruptly terminated, providing a graceful shutdown of services.

What are the benefits of using AWS Elastic Load Balancing (ELB) over managing your load balancers?

Using AWS ELB provides automatic scaling of load balancing capacity as traffic fluctuates, integration with AWS security and monitoring services, ease of management with AWS console or APIs, and high availability across zones. Additionally, AWS handles the maintenance and updates of ELBs, which reduces the operational overhead for users.

What is the SYN flood attack, and how can a Layer 4 load balancer mitigate such an attack?

A SYN flood attack is a type of Denial of Service attack in which an attacker rapidly sends SYN requests to a target’s system in an attempt to overwhelm it and make the system unresponsive. A Layer 4 load balancer can mitigate this by rate-limiting incoming SYN requests, distributing them across multiple servers, and by performing SYN cookies or similar techniques to validate legitimate traffic from attack traffic.

In AWS, how does Cross-zone load balancing improve the efficiency of your load balancing strategy?

Cross-zone load balancing distributes traffic evenly across all registered instances in all enabled Availability Zones, which can lead to better utilization of resources and higher fault tolerance as it ensures that all zones are utilized, even if one zone receives less traffic than another.

What is sticky session and when should it be used with load balancers?

Sticky sessions, also known as session affinity, enable the load balancer to bind a user’s session to a specific instance so that all requests from the user during the session are sent to the same instance. This is particularly useful for applications that maintain state information locally on the server between client requests.

How do layer 3 load balancers differ from layer 4 and layer 7 load balancers?

Layer 3 load balancing operates at the network layer, which primarily deals with routing packets based on their IP addresses. Unlike layer 4, which also considers the transport layer protocols and ports, and layer 7 that inspects the content of the payload, layer 3 focuses solely on the network layer without involving transport or application layer information.

Explain the purpose of Health Checks in the context of load balancing.

Health Checks are used by load balancers to determine if backend resources (like EC2 instances) are healthy and available to receive traffic. The load balancer periodically sends requests defined by the health check configuration to each registered resource and, depending on the response, it decides whether to keep the resource in service or to route traffic away from the unhealthy resource.

Can you describe SSL termination and why it might be implemented at the load balancer level?

SSL termination refers to the process of decrypting SSL/TLS encrypted traffic at the load balancer, rather than at the web server itself. This offloads the computational overhead of encryption and decryption from the web server, thereby improving its performance. It also simplifies SSL certificate management by centralizing it on the load balancer.

What is “Source IP affinity” in load balancing, and can it be achieved in AWS Elastic Load Balancing?

Source IP affinity, also known as IP hash load balancing, uses the source IP address of the client to determine which server should handle the request, ensuring that requests from the same client are sent to the same server. In AWS Elastic Load Balancing, this is achieved with the “sticky sessions” feature of the Classic Load Balancer and Application Load Balancer.

How does AWS’s Application Load Balancer differ from its Classic Load Balancer, specifically in the context of handling HTTP/HTTPS traffic?

AWS’s Application Load Balancer is designed specifically for HTTP/HTTPS traffic and offers advanced routing features based on the content of the request, such as host-based or path-based routing. It also supports WebSocket and HTTP/2 protocols natively. On the other hand, the Classic Load Balancer is a previous generation solution that provides basic load balancing across multiple EC2 instances, without the same level of application-specific understanding.