Top Load Balancing Algorithms Every Engineer Must Know in 2025
Learn load balancing the easy way. This beginner-friendly guide explains Round Robin, Weighted algorithms, Least Connections, and Sticky Sessions for system design interviews.
This blog explains popular load balancing strategies – like Round Robin, Least Connections, and more – in simple terms, discussing how they work, their pros/cons, and when to use each.
Imagine you run a busy website with thousands of users.
How do you make sure no single server gets swamped while others sit idle?
Enter the load balancer – a traffic cop for your servers.
But a load balancer is only as smart as the rules it follows.
Load balancing algorithms determine how incoming requests are distributed across multiple servers to prevent any single server from being overwhelmed.
And in this post, we'll explore common load balancing algorithms (the rules that decide which server handles each request) and see how they keep your applications running smoothly.
By the end, you'll know exactly why and when to use each approach to keep your system scalable and reliable.
Round Robin
Round Robin is the most straightforward load balancing method.
The load balancer simply cycles through the server list, sending each new request to the next server in line.
Round Robin alternates traffic evenly among servers.
For servers with equal capacity, Round Robin spreads requests evenly and works great.
However, it does not consider any server load or capability – a slower server still gets the same number of requests, which can lead to overload on weaker machines.
Weighted Round Robin
If servers aren't identical, you can assign weights so that more powerful servers receive a larger share of traffic.
For example, if Server A has weight 5 and Server B has weight 1, Server A will get about five times more requests than Server B.
This prevents overloading weaker servers.
The trade-off is that weighting is static – it doesn't automatically adapt if a server's performance changes.
Least Connections
Least Connections is a dynamic strategy: the load balancer checks which server currently has the fewest active connections and sends the next request to that server.
Least Connections directs each new request to the server with the fewest ongoing connections.
This approach adapts to each server's load in real time – a server handling many long-lived connections will temporarily get fewer new ones.
It prevents any single node from becoming a hotspot.
The trade-off is that the balancer must track active connection counts (a bit of overhead), and if all requests are very short, Least Connections might not show much advantage over simpler methods.
Note: Many load balancers also offer Weighted Least Connections, which factors in server capacity (preassigned weights for bigger vs. smaller servers) on top of connection counts. This yields an even more balanced distribution when your servers have different capabilities.
IP Hash (Sticky Sessions)
Sometimes you want the same client to consistently reach the same server.
The IP Hash algorithm achieves this by using a hash of the client's IP address (often combined with the destination server IP) to pick a server.
The result is that a given client IP will always be mapped to the same backend server, providing simple session persistence for that client.
Pros
This method keeps a user bound to one server without needing special cookies or centralized session storage.
It's useful for scenarios like an e-commerce shopping cart, where a user's cart data is stored in-memory on one server – IP Hash ensures all their requests go to that same server.
Cons
It can lead to uneven load distribution.
For example, if many users share one IP (say, behind a NAT or proxy), that one server mapped to that IP could get disproportionate traffic.
Also, if a server goes down, the users tied to it will be routed to a different server and might lose their in-memory session data.
Conclusion
Load balancing algorithms are fundamental tools for achieving scalability and high availability in modern systems.
From the straightforward Round Robin to the adaptive Least Connections strategy, each algorithm has its sweet spot.
In fact, Round Robin is the most common algorithm in simple deployments.
As traffic grows and gets less predictable, teams often switch to more dynamic algorithms (for example, Least Connections or other adaptive strategies) to maintain performance under heavier loads.
Understanding their differences will help you make informed decisions on how to distribute traffic in your architecture.
For beginners and those prepping for system design interviews, mastering these concepts is a big step toward designing robust systems.
If you want to learn more about system design (including load balancing and beyond), consider exploring Grokking System Design Fundamentals and Grokking the System Design Interview by DesignGurus.io.
Happy load balancing!
FAQs
Q1: Which load balancing algorithm is the best?
There is no single "best" load balancing algorithm – the optimal choice depends on your system's needs. For example, Round Robin works well if all servers have equal capacity, Least Connections is better when some servers get more traffic or have heavier loads, and IP Hash is ideal if you need to keep a user's sessions on one server. Always choose the algorithm that matches your scenario, and start simple before moving to more complex strategies.
Q2: What is the difference between Round Robin and Least Connections?
Round Robin cycles through servers in order, giving each server an equal share of requests without looking at any load information. Least Connections actively monitors how many connections each server is handling and always sends new requests to the server with the fewest active connections at that moment.
Q3: When should I use IP Hash load balancing (sticky sessions)?
Use IP Hash when you need each user to stick to the same server (session persistence). For example, if a user's shopping cart or session data is stored only on one server, IP Hash will route all of that user's requests to that specific server so they don't lose their session data.