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