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Donne Martin
2019-05-12 07:12:33 -04:00
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@@ -400,9 +400,7 @@ Next, we'll look at high-level trade-offs:
* **Performance** vs **scalability** * **Performance** vs **scalability**
* **Latency** vs **throughput** * **Latency** vs **throughput**
* ** * **Availability** vs **consistency**
bility** vs **consistency**
Keep in mind that **everything is a trade-off**. Keep in mind that **everything is a trade-off**.
@@ -532,7 +530,7 @@ This topic is further discussed in the [Database](#database) section:
### Availability in numbers ### Availability in numbers
Availability is generally quantified by uptime (or downtime) as a percentage of time the service is guaranteed to be available. Availability is often measured in number of 9s. A service with 99.99% availability is described as having four 9s. Availability is generally quantified by uptime (or downtime) as a percentage of time the service is available. Availability is often measured in number of 9s--a service with 99.99% availability is described as having four 9s.
#### 99.9% availability - three 9s #### 99.9% availability - three 9s
@@ -552,13 +550,11 @@ Availability is generally quantified by uptime (or downtime) as a percentage of
| Downtime per week | 1m 5s | | Downtime per week | 1m 5s |
| Downtime per day | 8.6s | | Downtime per day | 8.6s |
### Availability in parallel vs sequence #### 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 connected in sequence or in parallel. If a service consists of multiple components prone to failure, the service's overall availability depends on whether the components are connected in sequence or in parallel.
![Sequence vs Parallel](https://i.imgur.com/ZHNo49M.png) ###### Connected in Sequence
##### Connected in Sequence
Overall availability decreases when two components with availability < 100% are connected in sequence: Overall availability decreases when two components with availability < 100% are connected in sequence:
@@ -568,7 +564,7 @@ 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%. If both `Foo` and `Bar` each had 99.9% availability, their total availability in sequence would be 99.8%.
##### Connected in Parallel ###### Connected in Parallel
Overall availability increases when two components with availability < 100% are connected in parallel: Overall availability increases when two components with availability < 100% are connected in parallel: