A High-Level Overview of Load Balancing Algorithms

Introduction

Load balancing is the process of evenly distributing your network load across several servers. It helps in scaling the demand during peak traffic hours by helping spread the work uniformly. The server can be present in a cloud or a data center or on-premises. It can be either a physical server or a virtual one. Some of the main functions of a load balancer (LB) are:

Routes data efficiently
Prevents server overloading
Performs health checks for the servers
Provisions new server instances in the face of large traffic

Types of Load Balancing Algorithms

In the seven-layer OSI model, load balancing occurs from layers 4 (transport layer) to 7 (application layer).

diagram of the layers in the OSI model with arrows indicating where the application load and network load balancers are used

The different types of LB algorithms are effective in distributing the network traffic based on how the distribution of traffic looks, i.e., whether it’s network layer traffic or application layer traffic.

The network layer traffic is routed by LB based on TCP port, IP addresses, etc.
The application layer traffic is routed based on various additional attributes like HTTP header, SSL, and it even provides content switching capabilities to LBs.

Network Layer Algorithms

1. Round-robin algorithm

The traffic load is distributed to the first available server, and then that server is pushed down into the queue. If the servers are identical and there are no persistent connections, this algorithm can prove effective. There are two major types of round-robin algorithms:

**Weighted round-robin: **If the servers are not of identical capacity, then this algorithm can be used to distribute load. Some weights or efficiency parameters can be assigned to all the servers in a pool and based on that, in a similar cyclic fashion, load is distributed.
**Dynamic round-robin: **The weights that are assigned to a server to identify its capacity can also be calculated on runtime. Dynamic round-robin helps in sending the requests to a server based on runtime weight.

2. Least-connections algorithm

This algorithm calculates the number of active connections per server during a certain time and directs the incoming traffic to the server with the least connections. This is super helpful in the scenarios where a persistent connection is required.

3. Weighted least-connections algorithm

This is similar to the least-connections algorithm above but apart from the number of active connections to a server, it also keeps in mind the server capacity.

4. Least-response-time algorithm

This is again similar to the least-connections algorithm, but it also considers the response time of servers. The request is sent to the server with the least response time.

5. Hashing algorithm

The different request parameters are used to determine where the request will be sent. The different types of algorithms based on this are:

Source/destination IP hash: The source and destination IP addresses are hashed together to determine the server that will serve the request. In case of a dropped connection, the same request can be redirected to the same server upon retry.
URL hash: The request URL is used for performing hashing, and this method helps in reducing duplication of server caches by avoiding storing the same request object in many caches.

6. Miscellaneous algorithms

There are a few other algorithms as well, which are as follows:

Least-bandwidth algorithm: The server with the least consumption of bandwidth in the last 14 minutes is selected by the load balancer.
Least-packets algorithm: Similar to above, the server that is transmitting the least number of packets is chosen by the load balancer to redirect traffic.
Custom-load algorithm: The load balancer selects the server based on the current load on it, which can be determined by memory, processing unit usage, response time, number of requests, etc.

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