## What is a Load Balancer?

When dealing with the scalability of a software system, we may often come across the challenge of dealing with an influx of requests. For example, imagine running a popular web app that sells dog apparel. We just released a new product (a fashionable tie-dye dog sweater) and have noticed a significant increase of requests hitting our single server. Right now, our web app architecture looks something like this:

** Art Request for architecture without a load balancer. A single server is being overloaded **

We want to make sure our single server doesn't overload, so we decide to scale our web app horizontally and purchase a few additional servers that will host a replica of our app. However, with multiple servers, we have the following issue:

** Art Request showing a request being confused where to go**

We need a way to direct the traffic! Our app won't know where to send the requests unless we provide guidance on which server to send the request to. This is where a _load balancer_ comes into play.

A load balancer is a piece of hardware or software (and sometimes both) that helps distribute requests between different system resources. Load balancers are not just an essential aspect when scaling a system horizontally; they also help prevent specific system resources from getting overloaded and possibly going offline. In addition, load balancers are flexible enough to be placed in various places in a software systems architecture. In our web app example, since we are primarily trying to distribute the load between our servers, here is what our new web app architecture will look like with a load balancer:

** Art Request with load balancer showing a distribution of requests**

When we examine the above image, the way requests route to individual servers for our web app may seem a bit like magic. How exactly is the load balancer deciding which server is best fit to handle the incoming request? How does the load balancer make sure one server doesn't end up taking all the requests by accident? These questions are all decided based on the _load balancing algorithm_ that the load balancer uses. Let's explore what these algorithms are and how they work to better understand the inner workings of load balancers.

## Load Balancing Algorithms

A load-balancing algorithm is the programmatic logic that a load balancer uses to decide how to distribute requests between a software system's resources. While not an exhaustive list, we will take a look at the following five algorithms:

- [Least Connection](#Least-Connection)
- [Least Response Time](#Least-Response-Time)
- [Least Bandwidth](#Least-Bandwidth)
- [Round Robin](#Round-Robin)
- [Weighted Round Robin](#Weighted-Round-Robin)

### Least Connection

The least connection (LC) load-balancing algorithm is where requests are distributed to the server with the least number of active connections at the time the request is received.

### Least Response Time

The least response time (LRT) load balancing algorithm is a more sophisticated version of the least connection algorithm. This algorithm provides two balancing layers by checking both the resource with the least number of active connections and the least average response time.

### Least Bandwidth

The least bandwidth (LB) load-balancing algorithm is where requests are distributed to the server serving the least amount of traffic (usually measured in Mbps).

### Round Robin

The round-robin (RR) load-balancing algorithm is considered a circular algorithm because requests are distributed to servers one at a time. Once the last server is reached, the algorithm tells the load balancer to start at the first server it sent a request to and continue the process again.

### Weighted Round Robin

The weighted round-robin (WRR) load balancing algorithm is a more advanced version of the round-robin algorithm. This algorithm allows us to assign weights to specific servers and sends requests to the servers with the higher weights.

### Wrap Up

Like most programming tools, load balancers have a specific use case, and not every system will require one. Load balancers are one more tool in our proverbial software engineering toolbelt. They can play a crucial role when we are looking to help increase an application's overall reliability and performance. To understand if using a load balancer is appropriate for your next application, think deeply about what the application requires and what load balancing algorithm would be the most appropriate.