Algorithms behind load-balancers?

Question

I need to study about load-balancers, such as Network Load Balancing, Linux Virtual Server, HAProxy,...There're somethings under-the-hood I need to know:

• What algorithms/technologies are used in these load-balancers? Which is the most popular? most effective?

I expect that these algorithms/technologies will not be too complicated. Are there some resources written about them?

Thank you very much for your help.

Answer 1

Not sure if this belongs on serverfault or not, but some load balancing techniques are:

• Round Robin
• Least Connections

I used least connections. It just made the most sense to send the person to the machine which had the least amount of load.

Answer 2

Load balancing in Apache, for example, is taken care of by the module called mod_proxy_balancer. This module supports 3 load balancing algorithms:

• Request counting
• Weighted traffic counting
• Pending request counting

For more details, take a look here: mod_proxy_balancer

Answer 3

In addition to those already mentioned, a simple random assignment can be a good enough algorithm for load balancing, especially with a large number of servers.

Here's one link from Oracle: http://download-llnw.oracle.com/docs/cd/E11035_01/wls100/cluster/load_balancing.html

Answer 4

In general, load balancing is all about sending new client requests to servers which are the least busy. Based on the application running, assign a 'busy factor' to each server: basically a number reflecting one/several points of interest for your load balancing algorithm (connected clients, cpu/mem usage, etc.) and then, at runtime, choose the server with the lowest such score. Basically ANY load balancing technique is based on something like this:

1. Round robin does not implement a 'busy score' per se, but assigns each consecutive request to the next server in a circular queue.
2. Least connections has its score = number_of_open_connections to the server. Obviously, a server with fewer connections is a better choice.
3. Random assignment is a special case - you make an uninformed decision about the server's load, but assume that the function has a statistically even distribution.