It is important to take into account two loss of service scenarios, in order to do this analysis it is important to spend a moment talking about the OSI seven layer model, basically each layer defines a function of an element in the network, starting from the most basic and finishing with the top level or most complex elements.
On this basis we find that the physical cabling of your network is categorized as Layer 1 networking and the applications that provide the programs that users access as part of their daily work are categorized at Layer 7, with 6 other layers in between. Now let’s focus on the 2 Layers that are important for high availability, these are:
Layer 4 load balancers check the performance of the servers themselves, each server could be virtualized and running many applications over 300 or 400. Layer 4 load balancers are able to monitor the health of the server and decide whether to take it out of the network or to continue to use it, these load balancers cannot monitor the health of the applications. The result is that the server could be performing perfectly but the application, for example Microsoft Lync, has hung but the load balancer continues sending access requests to it. A good example of a popular Layer 4 load balancer is Windows Network Load Balancer or WNLB.
Layer 7 load balancers health check the performance of the applications on the servers, these servers that come from the hardware and/or virtual load balancer manufacturers for example KEMP Technologies are able to monitor the performance of each individual application and should it fall below the KPIs that have been defined by the network management switch the service to back up servers. This form of load balancing is naturally more precise and effective than only using Layer 4 load balancing.