配置Apache Cassandra进行灾难恢复 [英] Configuring Apache Cassandra for Disaster Recovery

问题描述

如何配置Apache Cassandra以允许灾难恢复，从而允许两个数据中心之一出现故障？

How do you configure Apache Cassandra to allow for disaster recovery, to allow for one of two data-centres to fail?

DataStax文档介绍了使用复制策略确保至少一个复制写入每个数据 - 中心。但我不认为灾难发生后如何帮助。如果切换到其余的数据中心，所有的写入都将失败，因为这些写入将无法复制到其他数据中心。

The DataStax documentation talks about using a replication strategy that ensures at least one replication is written to each of your two data-centres. But I don't see how that helps once the disaster has actually happened. If you switch to the remaining data-centre, all your writes will fail because those writes will not be able to replicate to the other data-centre.

我想你会希望您的软件以两种模式运行：正常模式，写入必须在两个数据中心和灾难模式之间复制，不需要。但是改变复制策略似乎是不可能的。

I guess you would want your software to operate in two modes: normal mode, for which writes must replicate across both data-centres, and disaster mode, for which they need not. But changing replication strategy does not seem possible.

我真正想要的是两个数据中心过度配置，在正常操作期间使用两个数据中心的资源，但是当只有一个数据中心运行时，只使用一个剩余数据中心的资源（性能降低）。

What I really want is two data-centres that are over provisioned, and during normal operations use the resources of both data-centres, but use the resources of only the one remaining data-centre (with reduced performance) when only one data-centre is functioning.

推荐答案

诀窍是改变通过API为写入给出的一致性设置，而不是改变复制因素。在灾难期间，当只有一个数据中心可用时，使用 LOCAL_QUORUM 设置进行写入。在正常操作期间，使用 EACH_QUORUM 可确保两个数据中心都有数据副本。读取可以始终使用 LOCAL_QUORUM 。

The trick is to vary the consistency setting given through the API for writes, instead of varying the replication factor. Use the LOCAL_QUORUM setting for writes during a disaster, when only one data-centre is available. During normal operation use EACH_QUORUM to ensure both data-centres have a copy of the data. Reads can use LOCAL_QUORUM all the time.

这里是多个数据中心，以及概念上相关的灾难恢复（0.7）。

Here is a summary of the Datastax documentation for multiple data centers and the older but still conceptionally relevant disaster recovery (0.7).

做一个配方，满足你的需求一致性 LOCAL_QUORUM 和 EACH_QUORUM 。

Make a recipe to suite your needs with the two consistencies LOCAL_QUORUM and EACH_QUORUM.

这里，本地表示单个数据中心的本地，而每个表示在每个数据中心中严格保持相同级别的一致性。

Here, "local" means local to a single data center, while "each" means consistency is strictly maintained at the same level in each data center.

假设您有2个数据中心，一个严重用于灾难恢复，那么您可以将复制因子设置为...

Suppose you have 2 datacenters, one used strictly for disaster recovery then you could set the replication factor to...

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现在，您的数据有多么关键实际写入灾难恢复节点，则可以使用EACH_QUORUM或LOCAL_QUORUM。假设您正在使用复制放置策略 NetworkTopologyStrategy（NTS），

Now depending how critical it is that your data is actually written to the disaster recovery nodes, you can either use EACH_QUORUM or LOCAL_QUORUM. Assuming you are using a replication placement strategy NetworkTopologyStrategy (NTS),

LOCAL_QUORUM 写入操作只会延迟客户端本地写入DC1并异步写入DC2中的恢复节点。

LOCAL_QUORUM on writes will only delay the client to write locally to the DC1 and asynchronously write to your recovery node(s) in DC2.

EACH_QUORUM 将确保所有数据都被复制，但会延迟写入，直到两个DC都确认成功操作。

EACH_QUORUM will ensure that all data is replicated but will delay writes until both DCs confirm successful operations.

对于读取，最好只是请使用LOCAL_QUORUM避免数据中心间延迟。

For reads it's likely best to just use LOCAL_QUORUM to avoid inter-data center latency.

这种方法有抓获！如果您选择在您的写入中使用EACH_QUORUM，则增加了潜在的故障点（DC2已关闭，DC1-DC2链路已关闭，DC1仲裁将无法满足）。

There are catches to this approach! If you choose to use EACH_QUORUM on your writes you increase the potential failure points (DC2 is down, DC1-DC2 link is down, DC1 quorum can't be met).

奖金是一旦你的DC1下来，你有一个有效的DC2灾难恢复。另外请注意，在第二个链接，它谈到自定义snitch设置正确路由您的IP。

The bonus is once your DC1 goes down, you have a valid DC2 disaster recovery. Also note in the 2nd link it talks about custom snitch settings for routing your IPs properly.

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