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Part 4: Understanding System Design - Load Balancers

You will learn everything you need about load balancers, their Benefits, Load Balancing Algorithms, Load balancing Balancers vs. Reverse Proxy, and more.

Hello “👋”

Welcome to another week, another opportunity to become a Great Backend Engineer.

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Before we get down to the business of today. Part 4 of Understanding System Design.

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Now, back to the business of today.

In the previous edition, I will discuss one of the system's components, starting with Proxies. I explored Proxies, forward and reverse proxies. I shared comprehensive differences and benefits of each type of proxy. Check it out here if you haven’t.

In this episode, I will further elucidate another system component to help you understand System Design.

We will look at Load Balancers.

You will learn everything you need about load balancers, their Benefits, Load Balancing Algorithms, Load balancing Balancers vs. Reverse Proxy, and more.

What is a Load Balancer?

When the number of requests in an application increases, it can overload a server, affecting system performance.

A single server has limited throughput and resources.

For example, an online marketplace like Amazon. During Black Fridays or the Christmas season, it experiences an unusual surge in traffic. It’s only a matter of seconds before the server gets overloaded. Therefore, scale is needed to handle the increased demand effectively.

Scaling can be done vertically or horizontally. A load balancer is needed to scale horizontally.

A load balancer is a device used to distribute application traffic across several servers. It improves the overall performance of a system by distributing the traffic to different servers, decreasing the burden on a single server.

A load balancer sits between clients and servers. It routes clients’ requests between servers, ensuring that no single server is overworked, which could make an application unavailable and unreliable.

Load Balancers

Hardware vs Software Load Balancing

Load balancers typically come in two flavors: software-based or hardware-based. Hardware-based load balancers are physical devices, often with specialized processors and proprietary software customized for load balancing.

On the other hand, software-based load balancers run on commodity hardware, and you can install any type of software load balancer, which makes them less expensive and more flexible.

Benefits of load balancers

  1. Scalability—It enables an application to handle a traffic spike effectively, maintain smooth operation, and respond quickly to clients. This makes the application highly available and reliable.

  2. Fault Tolerance—Having multiple servers in your infrastructure can eliminate a single point of failure. A load balancer will route traffic to an available server when one server fails, achieving redundancy.

  3. Avoiding Downtime—A load balancer will enable you to perform server maintenance without downtime by automatically routing traffic to other available servers. This will reduce application downtime and improve system availability.

  4. Improved Security—A load balancer can mitigate DDoS attacks. It routes traffic across the servers in case of a traffic surge. This will protect your application availability, giving the load balancer time to determine whether a spike in traffic is legitimate or not. It also offers a traffic scrubbing effect by blocking malicious requests.

  5. SSL Decryption - A load balancer can handle incoming HTTPS connections, decrypting and passing the unencrypted requests onto the web servers. This eliminates the need to install SSL certificates in different back-end web servers by providing a single configuration point. It also takes the processing load of encryption and decryption away from the web servers.

Load Balancers vs Reverse Proxy

Both components sit between clients and servers, accepting requests and delivering responses from the former. The two are mostly similar. However, a load balancer is commonly deployed when an application needs multiple servers for scalability. A reverse proxy can also be used even in cases where a single server is in place.

Load Balancers are key components in improving a system's performance. They ensure high application availability and reliability by distributing traffic so that no single server gets overloaded, giving users a good user experience.

Load Balancing Algorithms

Load Balancing Algorithms

Effective load balancers will intelligently determine which server should process a user request in a server pool using different algorithms. A load-balancing algorithm is a load balancer's logic that distributes incoming traffic between servers.

The following are examples of load-balancing algorithms:

Round Robin

The load balancers queue and direct client requests in a round-robin fashion. The first request goes to the first server, the second to the second server, and so on.

When the load balancer reaches the end of the list, it directs a request back to the first server. The round-robin approach is easy to implement and evenly distributes the traffic across all servers.

However, there is a risk that a server with low capacity receives many requests and becomes overloaded because the algorithm does not consider the server’s capacity. The algorithms work well in a server pool with the same processing power.

Weighted Round Robin

This algorithm is an advanced version of the round-robin Algorithm. It distributes the traffic based on the servers' weight scores. For instance, if server one is twice as powerful as servers two and three, it is provisioned with a higher weight than servers two and three.

When there are 5 sequential client requests, the load balancer routes 2 requests to server A, 1 to server B, and 3 each, and the last request is routed to server A again.

The bigger the server, the more requests there are to handle.

IP Hash Algorithm

The client and destination IP addresses generate a unique hash key to allocate users to a specific server. If a session is broken, the key can be regenerated, and the user will be redirected back to the server. It has the advantage of caching, as the server caches data for that specific user.

This algorithm is appropriate in scenarios where a client needs to return to the same server for each successive connection.

Least Connection

This algorithm checks which server has the fewest current connections opened and sends traffic to that server. Like the round-robin approach, it assumes all the servers have equal processing power.

Weighted Least Connection

This algorithm is an advanced least connection method that assigns different server weights depending on the processing power. The algorithm will decide where to route the traffic depending on active connections and the weights of servers. If two servers have the least number of connections, the server with the highest weight is chosen.

Weighted Response Time

It averages the response times for all the servers with the number of active connections each server has to determine where to route the request. The algorithm ensures faster service for the user by determining the server with the quickest response time.

Random Algorithm

This algorithm uses a random number generator to distribute client requests to the servers randomly. The algorithms assume the servers have similar configurations.

That’s all

Today, I discussed Load Balancers. I introduced you to everything you need to know about them, including their Benefits, Load Balancing Algorithms, Load balancing Balancers vs. Reverse Proxy, and more.

Next week, I will cover a very important topic in System Design: Distributed Caching.


I hope this guide gives you perspective on Load Balancers.

That will be all for this one. See you on Saturday.

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