How the Round Robin balancer algorithm works 31.05.23

Round Robin

A round robin algorithm is a cyclical scheduling function: the first request is sent to one server, the second to another, and so on. Once the last server has been sent a request, the cycle starts anew.

The most popular implementation of this algorithm is of course the round robin DNS balancing method. As you know, every DNS server saves “host name – IP address” pairs for every machine under a specific domain. This list may look like the following:

example.com	xxx.xxx.xxx.2
www.example.com	xxx.xxx.xxx.3

Every domain on the list may be associated with several IP addresses:

www.example.com	xxx.xxx.xxx.2
www.example.com	xxx.xxx.xxx.3
www.example.com	xxx.xxx.xxx.4
www.example.com	xxx.xxx.xxx.5
www.example.com	xxx.xxx.xxx.6

The DNS server scans all of these records and returns an IP address for every new request: xxx.xxx.xxx.2 to the first request, xxx.xx.xxx.3 to the second, and so on. This way, all of the servers in the cluster receive the same number of requests.

Out of all the obvious advantages of this algorithm, we should first mention that it works independent of a top-level protocol. Round robin algorithms can be used by any protocol that refers servers by name.
Round robin balancing in no way depends on the server’s workload: caching DNS servers help manage client inflow.

Since round robin algorithms can work even when servers are not connected to each other, they can be used for both local and global balancing.
Finally, solutions built on round robin algorithms are often low cost: adding a few records to the DNS server is usually all it takes to get going.

We should mention that round robin algorithms have their drawbacks as well. In order for the workload distribution to meet the aforementioned criteria of fairness and efficiency, each server must have the same set of resources available. When performing operations, the same number of resources should also be activated. In practice, this condition is not often met.

Additionally, the workloads of individual servers in the cluster are not considered when balancing with a round robin algorithm. Let’s say, hypothetically, that one server is under 100% load, and at the same time, others are only under 10-15%. The round robin algorithm doesn’t consider this, so the overloaded server will still receive requests. This lacks any fairness, efficiency, or predictability.

Because of these setbacks, actual red robin applications are extremely limited.
Weighted Round Robin

This is the improved version of the round robin algorithm. In this case, every server is assigned a weighted coefficient according to its capacity. This makes workload distribution more flexible since heavier servers process more requests. However, load balancing problems still remain. More effective balancing can be achieved using other methods that take more factors into consideration when planning and distributing workloads.

https://selectel.ru/blog/en/2015/02/05/load-balancing-basic-algorithms-and-methods/

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How the Round Robin balancer algorithm works 31.05.23

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