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Posted on Jul 28

System Design Basics - Load Balancing Algorithms

#programming #systemdesign #development #softwaredevelopment

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image_credit - ByteByteGo

Hello friends, if you are preparing for System deign interviews then one concept you must prepare is load balancing algorithms. Along with API Gateway, Caching, Rate Limiter, and Distributed Messaging Queues, this is one of the System Design Basics which you must know well.

In modern world of cloud computing, distributed systems, and network architectures, load balancing plays an important role to ensure optimal performance, reliability, and scalability.

Whether it's handling web traffic, distributing computational tasks, or managing resource utilization, the efficacy of load balancing algorithms directly impacts the efficiency and stability of the entire system.

In the past, I have shared 10 System Design Topics for interviews and Problems and load balancing was one of them.

It's also one of the per-requisite for horizontal scalability because in order to achieve scale-out or horizontal scaling you must have a load balancer to distribute traffic to different nodes or servers.

While most of us are familiar load balancer not many of us know how load balancer work or what is load balancing algorithm? Few programmers know about round robin algorithm where one message goes to one server and next message goes to another server, but there are many advanced load balancing algorithms which we don't know and this article will explain those to you.

This is also one of the most important topic for System Design interview. You will either asked about concepts or you need to show your knowledge of load balancer and load balancing algorithms while designing a system they ask like YouTube or Netflix.

By the way, if you are preparing for System design interviews and want to learn System Design in depth then you can also checkout sites like ByteByteGo, Design Guru, Exponent, Educative, Codemia.io and Udemy which have many great System design courses

And, here is also a nice System design template which highlights essential Software architecture components like API Gateway and Load Balancer and can be used to design any system.

P.S. Keep reading until the end. I have a free bonus for you.

8 Must Know Load Balancing Algorithms for System Design Interviews

Here, are 8 essential load balancing algorithms that engineers must know if they are preparing for system design interviews.

1. Round Robin

The Round Robin algorithm operates on a cyclic basis, evenly distributing incoming requests across a pool of servers. It sequentially assigns requests starting from the first server and loops back once reaching the last, ensuring a fair distribution of workload.

The biggest benefit of this algorithms is that it is simple to implement, ensures fair distribution of requests, and works well with servers of equal capacity. It's predictable and easy to understand.

Though, it also have drawback as it doesn't account for server load or request complexity, which can lead to uneven distribution in practice.

Round Robin algorithms works best when you have a homogeneous server environment with similar hardware and the requests are generally uniform in complexity and resource requirements.

Here is a nice diagram which shows how Round Robin algorithms works:

2. Least Connections

Operating on the principle of minimizing server load, the Least Connections algorithm directs incoming requests to the server with the fewest active connections.

This strategy aims to evenly distribute the workload among servers, preventing any single node from becoming overloaded.

Here are pros and cons of this algorithm:

Pros: Allows for distribution based on server capacities, more flexible than simple Round Robin.

Cons: Still doesn't account for real-time server load or request complexity.

When to use: Use this algorithm when your server infrastructure is heterogeneous, with varying capacities, but request complexity is relatively uniform.

Here is a nice diagram which shows how Least Connection algorithms distribute load:

3. Weighted Round Robin

Recognizing the varying capacities of individual servers, the Weighted Round Robin algorithm assigns weights to each server based on their capabilities.

Requests are then distributed proportionally to these weights, ensuring that more powerful servers handle a larger share of the workload.

Here is a nice diagram which shows how Weighted Round Robin algorithms works:

4. Weighted Least Connections

Combining the principles of Least Connections and Weighted Round Robin, this algorithm directs requests to the server with the lowest ratio of active connections to its assigned weight.

By considering both server capacity and current load, it optimizes resource utilization across the infrastructure.

Here is a nice diagram which explains how weighted least connection algorithms will distribute load:

5. IP Hash

Focusing on session persistence and maintaining client-server affinity, the IP Hash algorithm determines the target server for a request based on the source and/or destination IP address.

This approach ensures that requests from the same client are consistently routed to the same server, facilitating stateful communication.

Here are pros and cons of this load balancing algorithm:

Pros: Ensures session persistence, which is crucial for stateful applications. Good for scenarios where client IP doesn't change frequently.

Cons: Can lead to uneven distribution if many users come from the same IP range.

Use when: You need session stickiness and your application isn't designed to be stateless.

Here is a nice diagram which shows how IP has load balancing algorithms works:

6. Least Response Time

Efficiency and responsiveness are paramount in distributed systems, and the Least Response Time algorithm prioritizes servers with the lowest response time and minimal active connections.

By directing requests to the fastest-performing nodes, it enhances user experience and optimizes resource utilization.

Here are the advantages and disadvantages of this LB algorithm:

Benefit: Takes into account both the number of active connections and the response time, providing a good balance of load and performance.

Drawback: More complex to implement and can be more resource-intensive on the load balancer.

Use when: You have a diverse set of applications with varying performance characteristics and want to optimize for user experience.

Here is a nice diagram which shows how Least Response time algorithms works:

7. Random

Simplicity meets unpredictability in the Random algorithm, where incoming requests are randomly assigned to servers within the pool.

While lacking the sophistication of other algorithms, Random load balancing can still offer a basic level of distribution in certain scenarios.

This diagram provides a nice visual explanation of how random algorithm distribute traffic among multiple servers:

8. Least Bandwidth

In bandwidth-sensitive environments, the Least Bandwidth algorithm dynamically directs requests to the server currently utilizing the least amount of network bandwidth.

By preventing network congestion and ensuring efficient data transmission, it promotes smooth operation and stability.

Here is a nice diagram which shows how Least Bandwidth load balancing algorithms works:

Load Balancing System Design Interview Questions

Now, let's see some System Design Interview Questions related to Load balancing algorithms

1. Which Load Balancing Algorithm will you use to handle requests at Scale in large infrastructure??

In large-scale infrastructures, the most widely used and efficient load balancing algorithm is often the Weighted Round Robin.

This algorithm distributes incoming requests across multiple servers in a circular order, but with the added sophistication of assigning different weights to servers based on their capacities.

It's popular due to its balance of simplicity and effectiveness, especially in heterogeneous environments where server capabilities vary.

Here is how it works?

However, the choice of load balancing algorithm often depends on specific infrastructure needs.

Other highly efficient options include the Least Connections method, which directs traffic to the server with the fewest active connections, and more ** advanced adaptive algorithms** that use real-time performance metrics to make routing decisions.

Many large-scale operations actually employ a combination of methods, often using a tiered approach with global load balancing for geographic distribution, followed by more specific algorithms at the network and application levels.

2. What is difference between Least Connection Method and Round Robin? When would you choose one over the other?

The Least Connection Method and Round Robin are both popular load balancing algorithms, but they operate on different principles.

Round Robin distributes requests sequentially across all servers, assuming equal capacity and load.

It's simple and works well when servers have similar specifications and the requests are fairly uniform.

The Least Connection Method, on the other hand, directs new requests to the server with the fewest active connections. This makes it more adaptive to varying server loads and request complexities.

You'd choose Least Connection when you have long-lived connections or when the processing time for requests varies significantly.

It's particularly effective in scenarios where some requests take much longer to process than others, as it helps prevent any single server from becoming overwhelmed.

Round Robin is preferable in environments where requests and server capacities are relatively uniform, or when simplicity and predictability are paramount.

3. What is sticky sessions in load balancing and discuss its advantages and potential drawbacks?

Sticky sessions, also known as session affinity, is a load balancing technique where a series of requests from a specific client are always routed to the same server that handled the initial request.

This is typically achieved by creating a unique session ID for the client and associating it with a particular server.

The main advantage of sticky sessions is maintaining state for applications that aren't designed to be stateless, ensuring a consistent user experience for sessions that rely on server-side data.

It's particularly useful for applications that store session data in server memory rather than in a shared database.

However, sticky sessions come with drawbacks. They can lead to uneven load distribution if some users have particularly long or resource-intensive sessions.

They also complicate scaling and failover processes, as the failure of a server can disrupt all active sessions assigned to it.

In modern system design, the trend is towards stateless applications and storing session data in distributed caches or databases, which reduces the need for sticky sessions and allows for more flexible, resilient architectures.

Best System Design Interviews Resources

And, here are curated list of best system design interview books, online courses, and practice websites which you can check to better prepare for System design interviews.

DesignGuru's Grokking System Design Course: An interactive learning platform with hands-on exercises and real-world scenarios to strengthen your system design skills.
Codemia.io : This is another great platform to practice System design problems for interviews. It got more than 120+ System design problems, many of them are free and also a proper structure to solve them.
"System Design Interview" by Alex Xu: This book provides an in-depth exploration of system design concepts, strategies, and interview preparation tips.
"Designing Data-Intensive Applications" by Martin Kleppmann: A comprehensive guide that covers the principles and practices for designing scalable and reliable systems.
LeetCode System Design Tag: LeetCode is a popular platform for technical interview preparation. The System Design tag on LeetCode includes a variety of questions to practice.
"System Design Primer" on GitHub: A curated list of resources, including articles, books, and videos, to help you prepare for system design interviews.
Educative's System Design Course: An interactive learning platform with hands-on exercises and real-world scenarios to strengthen your system design skills.
High Scalability Blog: A blog that features articles and case studies on the architecture of high-traffic websites and scalable systems.
YouTube Channels: Check out channels like "Gaurav Sen" and "Tech Dummies" for insightful videos on system design concepts and interview preparation.
ByteByteGo: A live book and course by Alex Xu for System design interview preparation. It contains all the content of System Design Interview book volume 1 and 2 and will be updated with volume 3 which is coming soon.
Exponent: A specialized site for interview prep especially for FAANG companies like Amazon and Google, They also have a great system design course and many other material which can help you crack FAANG interviews.

image_credit - ByteByteGo

You should also remember to combine theoretical knowledge with practical application by working on real-world projects and participating in mock interviews. Continuous practice and learning will undoubtedly enhance your proficiency in system design interviews.

Conclusion

That's all about the 8 essential load balancing algorithms Software developers should know for System Design interviews. Mastering these load balancing algorithms equips engineers with the tools needed to design resilient and high-performing distributed systems.

By understanding these algorithms you can optimize resource utilization, enhance scalability, and ensure seamless user experiences across diverse network environments.

And, when it comes to choosing the right load balancing algorithm, it depends on your specific infrastructure, application requirements, and traffic patterns.

In practice, many large-scale systems use a combination of these methods, sometimes employing different algorithms at different levels of their infrastructure (e.g., global load balancing vs. application-level balancing) to achieve optimal performance and reliability.

Bonus

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