Add availability in numbers section (#237)

README.md

@@ -112,6 +112,7 @@ Review the [Contributing Guidelines](CONTRIBUTING.md).
 * [Availability patterns](#availability-patterns)
     * [Fail-over](#fail-over)
     * [Replication](#replication)
+    * [Availability in numbers](#availability-in-numbers)
 * [Domain name system](#domain-name-system)
 * [Content delivery network](#content-delivery-network)
     * [Push CDNs](#push-cdns)
@@ -527,6 +528,52 @@ This topic is further discussed in the [Database](#database) section:
 * [Master-slave replication](#master-slave-replication)
 * [Master-master replication](#master-master-replication)
 
+### Availability in numbers
+
+Availability is often quantified by uptime (or downtime) as a percentage of time the service is available.  Availability is generally measured in number of 9s--a service with 99.99% availability is described as having four 9s.
+
+#### 99.9% availability - three 9s
+
+| Duration            | Acceptable downtime|
+|---------------------|--------------------|
+| Downtime per year   | 8h 45min 57s       |
+| Downtime per month  | 43m 49.7s          |
+| Downtime per week   | 10m 4.8s           |
+| Downtime per day    | 1m 26.4s           |
+
+#### 99.99% availability - four 9s
+
+| Duration            | Acceptable downtime|
+|---------------------|--------------------|
+| Downtime per year   | 52min 35.7s        |
+| Downtime per month  | 4m 23s             |
+| Downtime per week   | 1m 5s              |
+| Downtime per day    | 8.6s               |
+
+#### Availability in parallel vs in sequence
+
+If a service consists of multiple components prone to failure, the service's overall availability depends on whether the components are in sequence or in parallel.
+
+###### In sequence
+
+Overall availability decreases when two components with availability < 100% are in sequence:
+
+```
+Availability (Total) = Availability (Foo) * Availability (Bar)
+```
+
+If both `Foo` and `Bar` each had 99.9% availability, their total availability in sequence would be 99.8%.
+
+###### In parallel
+
+Overall availability increases when two components with availability < 100% are in parallel:
+
+```
+Availability (Total) = 1 - (1 - Availability (Foo)) * (1 - Availability (Bar))
+```
+
+If both `Foo` and `Bar` each had 99.9% availability, their total availability in parallel would be 99.9999%.
+
 ## Domain name system
 
 <p align="center">