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What is Load Balancing?

Summary

Load balancing is the distribution of workloads across computing resources. It allows a single system to access a large pool of resources. This gives it a greater capacity, and the ability to serve clients with better performance. It also eliminates single points of failure, and introduces redundancy (and thus reliability) for the resources in question.

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Summary

Load balancing is the distribution of workloads across computing resources. It allows a single system to access a large pool of resources. This gives it a greater capacity, and the ability to serve clients with better performance. It also eliminates single points of failure, and introduces redundancy (and thus reliability) for the resources in question.

What is Load Balancing?

Load balancing is the distribution of workloads across computing resources. It allows a single system to access a large pool of resources. This gives it a greater capacity, and the ability to serve clients with better performance. It also eliminates single points of failure, and introduces redundancy (and thus reliability) for the resources in question.

A load balancer (LB) can be used in a variety of contexts within IT. The discussion below will focus on load balancing of Internet traffic.

It is common today for a web application to have thousands (or even hundreds of thousands) of simultaneous users. Each user must be served the requested resources quickly and efficiently. This workload often requires large numbers of servers (in a “server farm”). In turn, this requires a method of distributing incoming client requests across the servers that are currently available.

When a LB is used, clients do not (and should not) know the details of the backend server farm. They do not need to know the total resources that are available, or which specific servers they will use. To the clients, the system should appear as a monolithic entity: a single system to which they submit requests, and from which they receive responses. The LB makes this possible. It is the interface between the frontend clients and the backend resources.

There are a variety of approaches to load balancing. The simplest forms use DNS to route clients to different servers. Although this avoids the expense and complication of having dedicated LB hardware or software, this approach has a number of disadvantages. (For more information, see How Load Balancing Works.)

Some vendors offer LB appliances. These devices run specialized software, and usually contain some proprietary hardware as well. These can work well, but they have some limitations: for example, they tend to be expensive, and they do not scale well. Other LBs are software products, which run on commodity hardware.

The most recent development is cloud load balancing: software LB products that run in the cloud. They have a number of advantages: low cost, ease of use, inherent scalability, and others. (For more information, see What is Cloud Load Balancing?)

Load balancing can be configured to distribute workloads according to different criteria. Here are some common choices.

  • Round Robin: The LB maintains a list of available servers, and distributes incoming requests to them sequentially.
  • Least Connections: The LB chooses the server with the fewest current client connections.
  • Available Capacity: The LB chooses the server with the highest available capacity (using metrics such as current CPU usage).
  • Response Time: The LB chooses the server with the lowest response time to health monitoring requests, since this implies that it has the most available capacity.
  • Geolocation: The LB chooses the server with the shortest physical distance from the client.

Strictly speaking, load balancing is only about the distribution of workloads. However, modern LBs typically include other features as well. The most important of these is autoscaling. (For more information, see What is Autoscaling?)

LB systems are Internet-facing. Therefore, when adding one to a network, potential security issues must be considered. Indeed, load balancing does introduce a few security risks; fortunately, they are straightforward to mitigate. For more information, see Load Balancing and Security.

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