+994512060920
A big campaign launches. The ads are working, posts are going out on social media, and traffic suddenly jumps. For the first half hour everything looks great. Then pages slow down, and soon the site won't open at all. The team restarts the server, it works for a while, then crashes again. Meanwhile the campaign budget keeps burning.
This scenario is all too familiar. The cause is often not the code but the fact that every request is routed to a single server. A load balancer is one of the main tools that solves this problem. In this article we'll explain what it is, how it works, and how even a small business can apply it in a simple way.
What Is a Load Balancer?
A load balancer is a system that distributes incoming requests across several servers. When a user visits your site, the request doesn't go straight to a server. It reaches the load balancer first, and the load balancer sends it to whichever server is best suited at that moment.
Think of the checkout lines in a supermarket. If only one register is open, everyone queues up in front of it and the line grows. If five registers are open and someone at the entrance directs customers to a free one, the lines stay short. That person is the load balancer.
Why Does a Site Crash When Traffic Increases?
In a single-server setup, everything rests on one machine. That has several consequences:
- Resource limits: A server can only process a certain number of requests at once. When the limit is exceeded, new requests start waiting and are then rejected.
- Single point of failure: If the server goes down for any reason, the site becomes completely unreachable. There is no backup.
- Snowball effect: A slowing server holds requests longer, which grows the queue even more and makes things worse.
- Update problems: Restarting or updating a single server means taking the site offline.
So the problem isn't simply "not enough power." The system is tied to one point, and that point has to carry both the load and the risk of failure on its own.
How Does a Load Balancer Work?
- The user goes to the site's address.
- The request reaches the load balancer.
- The load balancer looks at the list of running servers.
- It picks the most suitable server according to a set rule.
- It forwards the request to that server.
- The server prepares the response and returns it to the user.
In addition, the load balancer regularly checks the health of the servers (a health check). If a server doesn't respond, it is temporarily taken out of the list and requests go only to healthy servers. The user never notices.
Traffic Distribution Methods
A load balancer can choose which server gets a request based on a few different rules:
| Method | How it works | When it fits |
|---|---|---|
| Round Robin | Requests go to servers in turn: first, second, third, then back to the first | Servers have equal power and requests are about equally heavy |
| Least Connections | The request goes to the server with the fewest active connections at that moment | Requests vary in length |
| Weighted | A stronger server gets more requests, a weaker one gets fewer | Servers differ in power |
| IP Hash | A user is always routed to the same server based on their IP address | A user needs to stay tied to the same server |
To start, Round Robin or Least Connections is often enough. More complex rules become necessary as the system grows.
The Main Benefits of a Load Balancer
- The load is shared: No single server carries the whole weight.
- Availability improves: If one server goes down, the others keep working.
- Scaling gets easier: When traffic grows, you just add a new server without taking the site offline.
- Updates without downtime: You can update servers one by one and users don't notice.
- An extra layer of security: A load balancer keeps servers from being exposed directly to the internet, and some types can filter suspicious traffic.
Layer 4 and Layer 7 Load Balancers
A load balancer can work on two levels.
Layer 4 looks only at network information: the IP address and port. It doesn't know what's inside the request, but it is very fast and simple.
Layer 7 looks at the content of the request: the address, headers and cookies. That lets it make smarter decisions. For example, it can send requests starting with /api to one group of servers and those starting with /images to another. In a microservices architecture this is very useful, because each service gets routed to its own group of servers.
The Session Problem: When a User Lands on a Different Server
A load balancer can send one user's requests to different servers. If session data (such as the logged-in state or the cart) lives only in one server's memory, the user is effectively "logged out" the moment the next request lands on another server.
There are two main fixes:
- Sticky sessions: The load balancer always routes the same user to the same server. It's simple, but if that server goes down the session is lost, and the load may not be spread evenly.
- Shared session storage: Session data is kept in a common place instead of in each server's memory. Redis is often used for this. It's the more reliable route, because any server can serve any user.
Can the Load Balancer Itself Become a Single Point of Failure?
Yes, and this is an important nuance. If all traffic passes through one load balancer and it goes down, the site becomes unreachable even if the servers are perfectly healthy. That's why in serious systems the load balancer itself is built with redundancy: two or more run together, and when one fails the other takes over. The ready-made load balancer services from cloud providers usually handle this for you.
Which Options Can You Use?
| Option | What it's known for |
|---|---|
| Nginx | A widely used open source solution that works both as a web server and as a load balancer |
| HAProxy | Popular for high performance and heavy traffic, focused only on this job |
| Cloud load balancers | Ready-made services from AWS, Azure and Google Cloud, with redundancy and scaling built in |
| Kubernetes Ingress and Service | Distribute traffic across Pods in a container environment |
| Cloudflare and similar services | Receive traffic at the edge, distribute it and protect it |
In container-based systems, traffic distribution is already part of the platform. You can read more in our Kubernetes article.
Does a Load Balancer Solve Everything?
No. A load balancer spreads the load across servers, but if the main problem is somewhere else, it won't remove it. For example:
- A database bottleneck: If all servers connect to the same weak database, adding servers doesn't help. You need caching and query optimization.
- Slow code: If each request is slow on its own, multiplying servers raises the cost without fixing the problem.
- Shared resources: The file system or an external service can also be the bottleneck.
That's why a scaling strategy has to be thought through as a whole: the load balancer, cache, database and monitoring all need to be considered together.
A Load Balancer Isn't Enough Without Monitoring
As the number of servers grows, it gets harder to see which one is loaded and which one has a problem. It's important to watch the load balancer's own indicators (request count, error rate, server health). The steps in our monitoring and logging article are a good place to start. For those who want to go further, proactive monitoring helps you see traffic growth ahead of time.
Load Testing: Check Before It Crashes
The riskiest approach is to find out whether the system can carry the load during a real campaign. It's smarter to run a load test beforehand: you simulate a large number of users artificially and see when the system slows down and when it breaks. That way you can find and fix weak points before the campaign starts.
The Most Common Mistakes
- Not preparing for traffic growth: Campaigns, sale days and news effects should be planned for in advance.
- Setting up a load balancer and forgetting the health check: Requests keep going to a faulty server.
- Keeping sessions in server memory: Users get logged out at random.
- Leaving the load balancer as a single point: When it goes down, the whole system goes down.
- Only adding more servers: If the bottleneck is the database, nothing improves.
- Not testing: Trying it under real load for the first time is the most expensive way to learn.
- Ignoring traffic during a Deployment: When a new version is released, servers should be updated one by one, and the load balancer should take them out and bring them back at the right time. This is more reliable when it's automated with a CI/CD pipeline.
How to Get Started
- Measure the current situation: how loaded is the server at peak hours?
- Find the bottleneck: is the problem the server or the database?
- Separate the session from the server (for example, with shared storage).
- Start with two servers and put a load balancer in front of them.
- Configure the health check properly.
- Use a load test to learn the system's limit.
- As traffic grows, add servers and keep watching with monitoring.
If the project is large or traffic is expected to jump sharply, making these decisions at the architecture stage is far more cost-effective than fixing things later. Work like this is usually part of the initial planning of custom software projects.
Frequently Asked Questions
Does a small website need a load balancer?
Most of the time, no. If traffic is low, a single server and proper caching are enough. A load balancer makes sense when traffic grows or availability becomes critical.
Are a load balancer and a CDN the same thing?
No. A CDN serves static files from a server close to the user. A load balancer distributes requests among your own servers. They are often used together.
Does a load balancer make a site faster?
Indirectly, yes. Because the load is shared, servers get less tired and response times stay stable. But if the code is slow, a load balancer won't speed it up.
Will users notice when a server goes down?
If it's set up properly, no. The load balancer removes the faulty server from the list and sends requests to healthy ones. Only the requests in flight to that server at that moment may fail.
Should I use a cloud service or build my own load balancer?
For small and mid-sized projects, a ready-made cloud service is usually simpler and more reliable. If you have special requirements or want to optimize costs, you can consider building your own with Nginx or HAProxy.
Conclusion
When a site crashes as traffic grows, the cause is often not a weak server but the whole load landing on a single point. A load balancer distributes requests among several servers, takes a failing server out of rotation and makes scaling easier. But it's not a magic fix on its own. It delivers real results when it's considered together with caching, the database, session management, monitoring and load testing.
If you want to check whether your site or system is ready for heavy traffic, get in touch with the Crocusoft team. We'll first find where the bottleneck is, then work out the right architecture together.
