Load balancer

Types

Network Model
BalancerDescProsCons
Round-Robin DNS load balancerA DNS server with round-robin enabled will respond different IP addresses to requests.Easy to implement.1. Not evenly-distributed load balancing caused by client-side and DNS caching.
2. Cannot be depended upon for site reliability.if one of the servers goes down, traffic will still be routed to this down server.
3. Does not account for server size, server load, transaction time, geographical distance, and other factors.
L3/L4 load balancerTraffic is routed by IP address and port. L3 is network layer (IP). L4 is session layer (TCP).Better granularity, simple, responsive
L7 load balancerTraffic is routed by what is inside the HTTP protocol.Better granularity

Algorithms

AlgorithmDescProsCons
Round RobinClient requests are distributed to application servers in rotation.simpleDoes 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.
Weighted Round RobinWeighted Round Robin builds on the simple Round-robin load balancing algorithm to account for differing application server characteristics. 
Least ConnectionClient requests are distributed to the application server with the least number of active connections at the time the client request is received.
Weighted Least ConnectionAccount for differing application server characteristics. 
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. 
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.
Fixed WeightingAdministrator assigns a weight to each application server based on criteria of their choosing to demonstrate the application servers traffic-handling capability. 
Weighted Response TimeThe response times of the application servers determines which application server receives the next request. If a server fails and makes quick error response, it will attract more traffic and fails more.
Source IP Hashcombines 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.
URL HashURL Hash is a load balancing algorithm to distribute writes evenly across multiple sites and sends all reads to the site owning the object.

References

Author: huadonghu

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