This is because internal monitoring systems are modeled after network-performance tools, which are more sensitive to network details and can offer data on a packet level if necessary. Internal monitors, however, provide only an internal view of response times and do not accurately represent your customer's experience when doing business with you. Many hosted services provide more detailed data in the area of response times, giving both DNS and server times. Some services can provide network time as well. Some internal monitors, such as HP's Web Transaction Observer, can provide the customer view, but they require that applications be modified to include special code to provide this data.
Most hosted monitors offer traditional URL-based monitoring services.
They are not able to monitor back-end systems at all. These types of services must be monitored internally. Most enterprises monitor database and middleware services routinely since they are used in most mission-critical applications. You may not need to implement a new system to monitor these services. Instead, you may want to work with the existing monitors to collect the data you need.