Big Tech Coach

What is a Load Balancer?

May 23, 2024
Discover the essential role of load balancers in system design, ensuring optimal performance and reliability by efficiently distributing traffic across multiple servers.
In the world of system design, load balancers play a crucial role in ensuring that a system's infrastructure delivers optimal performance and reliability. A load balancer effectively distributes incoming network or application traffic across multiple servers. This not only maximizes speed and capacity utilization but also ensures no single server bears too much demand. By spreading the load, a load balancer helps to prevent any application server from becoming a bottleneck, which can degrade performance and lead to system failures.
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What is a Load Balancer?

A load balancer is a device or software that acts as a reverse proxy and distributes network or application traffic across a number of servers. Load balancers are used to increase the capacity (concurrent users) and reliability of applications. They improve the overall performance of applications by decreasing the burden on servers associated with managing and maintaining application and network sessions, as well as by performing application-specific tasks.

Functions of Load Balancers

  1. Distribution of Client Requests: The core functionality of load balancing is to distribute incoming requests evenly across multiple servers. This distribution can be based on various criteria such as round-robin, least connections, or hash of some part of the request, ensuring efficient utilization of resources.
  1. Health Checks: Load balancers continuously monitor the health of connected servers to ensure traffic is only directed to servers that are currently healthy and can fulfill requests. If a server fails a health check, it is temporarily removed from the pool until it is deemed healthy again.
  1. Session Persistence: Sometimes it is necessary to maintain a user's session by directing all requests from that user to the same server. This is known as session persistence, or sticky sessions, which are crucial for user experience in applications where the user’s state is saved locally on the server.
4ax. SSL Termination: Load balancers can also terminate SSL connections, offloading the CPU-intensive encryption and decryption work from the application server onto the load balancer. This also simplifies SSL certificate management as certificates only need to be installed on the load balancer.
  1. Scalability and Flexibility: Load balancers enable systems to easily scale out to accommodate increased traffic; new servers can be added to the pool without significant changes to application architecture. They also allow for maintenance or upgrades to be performed on individual servers without affecting service availability.

Types of Load Balancers

Load balancers can operate at different layers of the OSI model, primarily at the data link layer (Layer 2), transport layer (Layer 4), and application layer (Layer 7). Depending on the layer, load balancers read different information from the incoming packet, and this affects the distribution strategy:
  • Layer 4 Load Balancing: Operates at the transport layer, using IP address and port number to distribute requests. It makes routing decisions based on data from network and transport layer protocols such as IP, TCP, or UDP.
  • Layer 7 Load Balancing: Operates at the application layer, distributing requests based on data from the application layer. Layer 7 load balancers can make more complex decisions by inspecting content within the request payload, headers, or even cookies.

Load Balancing Algorithms

Different algorithms are used for traffic distribution, each suitable for specific scenarios:
  • Round Robin: Distributes client requests sequentially across the server pool.
  • Least Connections: Sends requests to the server with the fewest active connections.
  • IP Hash: Selects a server based on a hash of the client’s IP address, ensuring a client consistently reaches the same server.


Load balancers are an essential component in designing scalable, reliable, and efficient systems. They not only manage the distribution and routing of traffic to various servers but also ensure system stability by managing server load to prevent overloading, thereby enhancing the user experience. In an age where downtime can cost companies significantly, integrating a robust load balancing solution is indispensable for maintaining an edge in the competitive market.