Availability implies an agreed minimum up time and level of performance for your directory service. Failure, in this context, is defined as anything that prevents the directory service from providing this minimum level of service.
In assessing availability requirements, consider the following:
Is your directory service accessed only at particular times of the day?
Do you have different availability requirements for read and write operations?
Does the service span multiple geographical sites, and if so, do these sites have different access time requirements?
Can your system be shut down for maintenance?
If so, what is the maximum acceptable downtime?
Can the system be shut down during migration?
What is the cost of downtime to your organization?
For suggestions on deploying a highly available directory service, see Chapter 12, Designing a Highly Available Deployment.