Sun Java Enterprise System Deployment Planning Guide

Prioritizing Service Availability

From a user perspective, availability often applies more on a service-by-service basis rather than on the availability of the entire system. For example, the unavailability of instant messaging services usually has little or no impact on the availability of other services. However, the unavailability of services upon which many other services depend (such as Directory Server) has a much wider impact. Higher availability specifications should clearly reference specific use cases and usage analysis that require the increased availability.

It is helpful to list availability needs according to an ordered set of priorities. The following table prioritizes the availability of different types of services.

Table 3–4 Availability of Services by Priority


Service Type 


Mission critical 

Services that must be available at all times. For example, database services (such as LDAP directories) to applications. 

Must be available 

Services that must be available, but can be available at reduced performance. For example, messaging service availability might not be critical in some business environments. 

Can be postponed 

Services that must be available within a given time period. For example, calendar services availability might not be essential in some business environments. 


Services that can be postponed indefinitely. For example, in some environments instant messaging services can be considered useful but not necessary.