Load Balancing Algorithms and Techniques

How does a load balancer distribute client traffic across servers?

There are numerous techniques and algorithms that can be used to intelligently load balance client access requests across server pools. The technique chosen will depend on the type of service or application being served and the status of the network and servers at the time of the request. The methods outlined below will be used in combination to determine the best server to service new requests. The current level of requests to the load balancers often determines which method is used. When the load is low then one of the simple load balancing methods will suffice. In times of high load, the more complex methods are used to ensure an even distribution of requests.

Load Balancing Techniques:

Round Robin

Round-robin load balancing is one of the simplest and most used load balancing algorithms. Client requests are distributed to application servers in rotation. For example, if you have three application servers: the first client request to the first application server in the list, the second client request to the second application server, the third client request to the third application server, the fourth to the first application server and so on.

This load balancing algorithm does not take into consideration the characteristics of the application servers i.e. it assumes that all application servers are the same with the same availability, computing and load handling characteristics.

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Weighted Round Robin

Weighted Round Robin builds on the simple Round-robin load balancing algorithm to account for differing application server characteristics. The administrator assigns a weight to each application server based on criteria of their choosing to demonstrate the application servers traffic-handling capability. If application server #1 is twice as powerful as application server #2 (and application server #3), application server #1 is provisioned with a higher weight and application server #2 and #3 get the same weight. If there five (5) sequential client requests, the first two (2) go to application server #1, the third (3) goes to application server #2, the fourth (4) to application server #3 and the fifth (5) to application server #1.

Least Connection

Least Connection load balancing is a dynamic load balancing algorithm where client requests are distributed to the application server with the least number of active connections at the time the client request is received. In cases where application servers have similar specifications, an application server may be overloaded due to longer lived connections; this algorithm takes the active connection load into consideration.

Weighted Least Connection

Weighted Least Connection builds on the Least Connection load balancing algorithm to account for differing application server characteristics. The administrator assigns a weight to each application server based on criteria of their choosing to demonstrate the application servers traffic-handling capability. The LoadMaster is making the load balancing criteria based on active connections and application server weighting.

Resource Based (Adaptive)

Resource Based (Adaptive) is a load balancing algorithm requires an agent to be installed on the application server that reports on its current load to the load balancer. The installed agent monitors the application servers availability status and resources. The load balancer queries the output from the agent to aid in load balancing decisions.

Resource Based (SDN Adaptive)

SDN Adaptive is a load balancing algorithm that combines knowledge from Layers 2, 3, 4 and 7 and input from an SDN Controller to make more optimized traffic distribution decisions. This allows information about the status of the servers, the status of the applications running on them, the health of the network infrastructure, and the level of congestion on the network to all play a part in the load balancing decision making.

Fixed Weighting

Fixed Weighting is a load balancing algorithm where the administrator assigns a weight to each application server based on criteria of their choosing to demonstrate the application servers traffic-handling capability. The application server with the highest weigh will receive all of the traffic. If the application server with the highest weight fails, all traffic will be directed to the next highest weight application server.

Weighted Response Time

Weighted Response Time is a load balancing algorithm where the response times of the application servers determines which application server receives the next request. The application server response time to a health check is used to calculate the application server weights. The application server that is responding the fastest receives the next request.

Source IP Hash

Source IP hash load balancing algorithm that combines source and destination IP addresses of the client and server to generate a unique hash key. The key is used to allocate the client to a particular server. As the key can be regenerated if the session is broken, the client request is directed to the same server it was using previously. This is useful if it’s important that a client should connect to a session that is still active after a disconnection.

URL Hash

URL Hash is a load balancing algorithm to distribute writes evenly across multiple sites and sends all reads to the site owning the object.

Kemp LoadMaster web user interface (WUI) demo

Take a quick guided tour of Kemp LoadMaster web user interface (WUI) for set-up and configuration of a Kemp load balancer.