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Server Load Balancing: Algorithms
Published: Monday, May 17, 2004
Types of load balancing
Load balancing of servers by an IP sprayer can be implemented in different ways. These methods of load balancing can be set up in the load balancer based on available load balancing types. There are various algorithms used to distribute the load among the available servers. - Random Allocation
In a random allocation, the HTTP requests are assigned to any server picked randomly among the group of servers. In such a case, one of the servers may be assigned many more requests to process, while the other servers are sitting idle. However, on average, each server gets its share of the load due to the random selection.
Pros: Simple to implement.
Cons: Can lead to overloading of one server while under-utilization of others.
- Round-Robin Allocation
In a round-robin algorithm, the IP sprayer assigns the requests to a list of the servers on a rotating basis. The first request is allocated to a server picked randomly from the group, so that if more than one IP sprayer is involved, not all the first requests go to the same server. For the subsequent requests, the IP sprayer follows the circular order to redirect the request. Once a server is assigned a request, the server is moved to the end of the list. This keeps the servers equally assigned.
Pros: Better than random allocation because the requests are equally divided among the available servers in an orderly fashion.
Cons: Round robin algorithm is not enough for load balancing based on processing overhead required and if the server specifications are not identical to each other in the server group.
- Weighted Round-Robin Allocation
Weighted Round-Robin is an advanced version of the round-robin that eliminates the deficiencies of the plain round robin algorithm. In case of a weighted round-robin, one can assign a weight to each server in the group so that if one server is capable of handling twice as much load as the other, the powerful server gets a weight of 2. In such cases, the IP sprayer will assign two requests to the powerful server for each request assigned to the weaker one.
Pros: Takes care of the capacity of the servers in the group.
Cons: Does not consider the advanced load balancing requirements such as processing times for each individual request.
The configuration of a load balancing software or hardware should be decided on the particular requirement. For example, if the website wants to load balance servers for static HTML pages or light database driven dynamic webpages, round robin will be sufficient. However, if some of the requests take longer than the others to process, then advanced load balancing algorithms
are used. The load balancer should be able to provide intelligent monitoring to distribute the load, directing them to the servers that are capable of handling them better than the others in the cluster of server.
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Server Load Balancing: Introduction
