Figure 3: Model of an Email system [5] 

However, this model fails to take into account the interdependencies a service may have on 

other hosts and services. If one was to consider an E mail service, it has many components 

on which it depends including DNS, IP connectivity and SMTP servers. Failure of any of 

these components often means that the whole service fails to operate. 

Two separate, but similar models that address this focus on actually determining the total 

availability of the service defined taking into account all the components the service depends 

on [6, 7]. The latter is more focused on actually determining the total availability of the 

service defined taking into account all the components the service depends on. The first 

targets the issues of how to compose a service model automatically. It approaches the 

problem by defining templates for typical ISP services and then combining this with some 

auto discovery tools which are then passed through an engine that can generate the service 

model instance.