您是否经历过 AppEngine 中的 DataStore 停机？几率有多大？
Google 开始使用高复制数据存储 (HRD) 作为新应用程序的默认设置。
来自文档的人力资源：
HRD 是一个高度可用、高度 可靠的存储解决方案。它仍然存在 可用于读取和写入期间 计划内停机并且非常 面对灾难有韧性 失败——但它的代价比 主/从选项。
来自文档的 M/S：
您的数据可能是暂时的 数据中心出现问题时不可用 或计划内停机
现在，您是否经历过停机？如果这种“停机免责声明”只是理论上的事情并且不会经常发生，我会使用 M/S，因为它更便宜。
Google 处理“停机”的数字是多少？也许他们的停机时间一年只有几秒钟，这对于某些类型的应用程序来说是完全可以接受的。
希望有经验的 AppEngine 开发人员给出答案。
Google start to use The High Replication datastore (HRD) as the default for new applications.
HR from the docs:
The HRD is a highly available, highly
reliable storage solution. It remains
available for reads and writes during
planned downtime and is extremely
resilient in the face of catastrophic
failure—but it costs more than the
master/slave option.
M/S from the docs:
your data may be temporarily
unavailable during data center issues
or planned downtime
Now, have you ever expirienced downtime? If this "downtime disclaimer" is just something theorical and doesn't happen frecuently I would use the M/S becouse it's cheaper.
What are the numbers that Google handle to say "downtime"? maybe their downtime is just a few seconds in a year, something totaly acceptable for some kind of apps.
Would love answers from experienced AppEngine developers.
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我建议你使用 HRD，因为谷歌表示他们将使 M/S 比 HRD 更贵，直到今年年底，甚至删除 M/S 选项，因为他们希望“强迫”企业和客户。开发人员可以利用所有人力资源开发的好处。真正的原因是维护单一类型的基础设施比同时维护 HRD 和 M/S 更便宜，因此 Google 选择了 HRD。
来源：Google I/O 2011
I would recommend you to use HRD as Google said they will make M/S more expensive than HRD until the end of the year and even remove the M/S option as they are looking to "force" the businesses & developers to take advantage of all HRD goodies. The real reason is that maintaining a single type of infrastructure is cheaper than to maintain the both HRD and M/S so Google picks HRD.
Source : Google I/O 2011
停机不是理论上的——它发生在任何分布式系统中。粗略地说，有两种类型：局部型和全局型。当特定机器出现故障并且无法处理请求时，就会出现局部问题；当整个服务发生问题时，就会发生全局停机。
两种情况都可能发生在 App Engine 上：前者是由于局部硬件故障造成的，后者通常仅是由于计划维护（需要将主从数据存储区设置为只读）而造成的。 HR 数据存储的处理能力比 MS 数据存储更强大，并且在维护时段不需要只读期。
一旦新的定价方案生效，两个数据存储将按相同的费率收费。
出于这些以及许多其他原因，您应该始终在新应用程序中使用 HR 数据存储。
Downtime isn't theoretical - it happens in any distributed system. There are two types, roughly speaking: localized and global. Localized issues occur when a particular machine has trouble and can't serve requests; global downtime happens when something happens to the service as a whole.
Both can occur on App Engine: the former due to localized hardware failure, and the latter generally only due to planned maintenance that requires setting the master-slave datastore read-only for a brief period. The HR datastore handles both more robustly than the MS datastore, and doesn't require a read-only period during maintenance windows.
Once the new pricing scheme comes into effect, both datastores will be charged at the same rate.
For these and many other reasons, you should always use the HR datastore in new apps.