The Essential Guide to Load Balancing Strategies and Techniques

What are Load Balancers?

Load balancers distribute incoming network traffic across multiple servers to ensure no single server bears too much load. By spreading the requests efficiently, they increase the capacity and reliability of applications. Here are some common strategies and algorithms used in load balancing:

1. Round Robin

Round Robin is the simplest form of load balancing, where each server in the pool gets a request in a sequential, rotating order. When the last server is reached, it loops back to the first.

This works well for servers with similar specifications and when the load is uniformly distributable.

2. Least Connections

Least Connections algorithm directs traffic to the server with the fewest active connections. This is particularly useful when there are sessions of variable lengths and demands.

It is ideal for longer tasks or when the server load is not evenly distributed.

3. Least Response Time

This responsiveness-focused algorithm chooses the server with the lowest response time and with the fewest active connections.

Effective when the goal is to provide the fastest response to requests.

4. IP Hash

IP Hash determines which server receives the request based on the hash of the client’s IP address. This ensures a client consistently connects to the same server.

Useful for session persistence in applications where it’s important that a client consistently connects to the same server.

5. Weighted Algorithms

There are also variants of the above methods that can be weighted. For example, in Weighted Round Robin or Weighted Least Connections, servers are assigned weights typically based on their capacity or performance metrics.

Weighted algorithms are effective when servers in the pool have different capabilities (e.g., CPU, RAM).

6. Geographical Algorithms

These are location-based algorithms that direct requests to the server geographically closest to the user or based on specific regional requirements.

Useful for global services where latency reduction and local regulatory compliance are priorities.

7. Consistent Hashing

Consistent hashing uses a hash function to distribute data across various nodes.

Imagine a hash space that forms a circle where the end wraps around to the beginning, often referred to as a “hash ring”. Both the nodes (servers) and the data (like keys of stored values) are hashed onto this ring.

This also ensures that the same client consistently connects to the same server.

Health Checks

An essential feature of load balancers is continuous health checking of servers to ensure traffic is only directed to servers that are online and responsive. If a server fails, the load balancer will stop sending traffic to it until it is back online.

Load Balancer Examples

Load balancers come in various forms, including hardware appliances, software solutions, and cloud-based services. Here are some examples:

Hardware Load Balancers

1. F5 BIG-IP: A widely used hardware load balancer known for its high performance and extensive feature set, offering local traffic management, global server load balancing, and application security.
2. Citrix ADC: Formerly known as NetScaler, it provides load balancing, content switching, and application acceleration.

Software Load Balancers

1. HAProxy: A popular open-source software load balancer and proxy server for TCP and HTTP-based applications. It’s known for its efficiency, reliability, and low memory footprint.
2. NGINX: Often used as a web server, NGINX also functions as a load balancer and reverse proxy for HTTP and other network protocols.

Cloud-Based Load Balancers

1. AWS Elastic Load Balancing (ELB): Offers several types of load balancers as part of the AWS cloud services, including the Application Load Balancer, Network Load Balancer, and Classic Load Balancer.
2. Microsoft Azure Load Balancer: Provides high availability and network performance to applications running in Azure. It supports inbound and outbound scenarios.
3. Google Cloud Load Balancing: A fully distributed, software-defined, managed service for all your traffic. It offers various types, including HTTP(S), TCP/SSL, and UDP load balancing.

Virtual Load Balancers

1. VMware NSX Advanced Load Balancer (Avi Networks): Offers a software-defined application delivery controller that can be deployed on-premises or in the cloud.

What happens when the load balancer goes down?

The load balancer is basically a single point of failure, and in case it goes down, all of the servers become unavailable for the clients.

To avoid or minimize the impact of a load balancer failure, several strategies are typically employed:

Redundancy

Implementing redundant load balancing by using more than one load balancer, often in pairs, is a common approach. If one fails, the other takes over, a method known as failover.

Health Checks and Monitoring

Continuous monitoring and health checks of the load balancer itself can ensure that any issues are detected early and can be addressed before causing significant disruption.

Auto-scaling and Self-Healing Systems

Some modern infrastructures are designed to automatically detect the failure of a load balancer and replace it with a new instance without manual intervention.

DNS Failover

In some configurations, DNS failover can reroute traffic away from an IP address that’s no longer accepting connections (like a failed load balancer) to a preconfigured standby IP.

If you’re new here, I’m Hayk. I help web developers secure their first tech jobs or advance to senior roles at the Web Dev Mastery community.

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Hayk Simonyan | Substack