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## 1.1 General high availability and disaster recovery overview 

This section describes HA and DR topics as a high-level overview. Before providing details, the terminology that is used throughout this publication is defined. These definitions might not match the information that you find on the internet because sometimes different meanings exist for the same phrase. 

Availability The ability of a service component to perform its required function at a stated instant or over a stated period. It is expressed as the availability ratio, for example, the proportion of time that the service is available for use by the customers within the agreed service hours. 

Continuous availability (CA) The attribute of a system to deliver nondisruptive service to the user 365 days a year, 24 hours a day (assuming no planned or unplanned outages exist). Figure 1-1 shows this relationship. 

Note:  In most cases when people talk about HA, they mean CA. 

<img src='content_image/978962.jpg'>

Continuous operations (CO) The ability of a system to continuously operate and mask planned outages from users. It uses redundant hardware and software components (often clustering) along with nondisruptive maintenance and change management procedures. 

