What Is a Load Balancer?
A load balancer is a device or software function that distributes network or application traffic across a cluster of servers. Its primary goal is to optimize resource use, maximize throughput, minimize response time, and avoid overload of any single server.
By balancing client requests across multiple servers, a load balancer reduces individual server load and prevents one server from becoming a bottleneck, increasing overall service reliability and availability.
Load balancers can be either hardware-based or software-based. While hardware-based solutions are known for their performance, they can be expensive and lack flexibility. On the other hand, software-based solutions are more cost-effective and offer greater flexibility in terms of deployment and scalability.
A load balancer operates at various layers of the Open Systems Interconnection (OSI) model, a conceptual framework that standardizes the functions of a communication system into abstract layers. Load balancers that function at Layer 4 (Transport Layer) distribute traffic based on network information such as IP address and TCP port, while those operating at Layer 7 (Application Layer) distribute requests based on content type, such as distributing video and image requests to different servers.
The two main types of load balancing are:
- Distributive Load Balancing. Here, the load balancer disseminates incoming requests uniformly across all servers in a pool. The decision is typically based on a round-robin system, where each server is selected in rotation.
- Dynamic Load Balancing. In this method, the load balancer makes decisions based on the system's current state, such as the existing load on each server. This requires real-time communication between the load balancer and the server pool.
Load balancers use a variety of algorithms to determine which server should handle a given request. Some of these include round-robin, least connections, and IP hash.
A load balancer also performs health checks on the servers in its pool to ensure they are responsive and capable of handling requests. If a server fails to respond to a health check, the load balancer will stop sending it new traffic until it becomes responsive.