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Table of Contents:

Introduction

1. Defining Availability

2. Redundancy: The Key to Carrier-Grade Availability

3. Preserving Application State: Two Techniques

4. Highly Available Database Managers

5. A Sidebar on How an RDBMS Works

6. Understanding Safeness Levels

7. Data Loss Considerations

8. MTTR Considerations

9. Application Throughput Considerations

10. Three-Way Tradeoff Summary

Self-Test

Test Answers

Glossary

PDF of this tutorial

The simplest definition of availability is that it refers to the amount of time a system is usable compared to the amount of time we wish it to be usable, as shown in Figure 1:

A more precise definition relates the Mean Time Between Failures (MTBF), Mean Time To Repair (MTTR) to obtain an availability measure, as shown in Figure 2 :

Availability is often defined in terms of "nines". A "two-nines" system is available 99% of the time, a "three-nines" system is available 99.9% of the time, and so on. See Figure 3.

Different classes of applications make different availability demands, and different hardware/software combinations offer different levels of availability. Figure 4 shows some MTBF and MTTR estimates.