如果Cassandra报告失败但做了部分写入，该怎么办? [英] What to do if Cassandra reports failure but did a partial write?

问题描述

Cassandra不保证原子行为，因此一个副本出现故障的可能性很小，而另一个副本确实保留了更改.

Cassandra does not guarantee atomic behavior so there is a slight chance that one replica fails but other replica do persist the change.

是否有任何信息可以防止这种情况发生，以及如何解决该问题(如果发生的话)?卡桑德拉是否可以在这方面自愈?

Are there any information how to defend against this and what to do in order to heal it if it happens? Does Cassandra heal itself in that regard?

[更新]

我特别关注这样一种情况:您发送写请求给所有副本，而只有一个副本因写入错误而失败.写入失败的节点仍然有效并且可以运行.根据Cassandra文档，即使另外两个(如果您的复制因子为3)成功，写入请求也将返回失败.

I specially focus on the case where you send a write request to lets say all replica and only one replica fails with a write error. The node failing with the write is still alive and operational. According to the Cassandra documentation the write request will return a failure even the two other (if you have a replication factor of 3) succeeded.

在这种情况下，根据文档，两个副本已更改，而一个副本仍然是原始副本.有人说，在这种情况下，它的状态是不一致的，因为其他两个将无法回滚任何已写的更改.

According to the documentation in this case two replica has changed and one remains original. There was stated that in this case its a non-consistent state since the other two will not be able to roll back any change written.

因此问题来了，如何才能抵御这一挑战.

So the question goes how can one defend against that.

推荐答案

在cassandra中，这样的超时不被视为失败.请参阅此博客文章描述Cassandra的处理方式写的不同条件:

In cassandra a timeout such as this is not considered a failure. See this blog post describing how Cassandra handles different conditions when it comes to writes:

请记住，对于写入来说，超时并不是失败.

Remember that for writes, a timeout is not a failure.

由于我们不知道副本失败之前发生了什么，我们该怎么说呢?协调器可以将结果强制为更新前或更新后状态.这就是Cassandra进行提示切换的方式.

How can we say that since we don’t know what happened before the replica failed? The coordinator can force the results towards either the pre-update or post-update state. This is what Cassandra does with hinted handoff.

...协调器将更新存储在本地，并在恢复时将其重新发送到失败的副本，从而将其强制为客户端最初想要的更新后状态.

...the coordinator stores the update locally, and will re-send it to the failed replica when it recovers, thus forcing it to the post-update state that the client wanted originally.

所以要回答您的问题，是的，cassandra会通过提示的切换来自我修复，并且当该过程失败时(即在副本上线之前已超过max_hint_window_in_ms)，修复应该使情况保持一致.这是建议定期进行维修的原因之一.

So to answer your question, yes cassandra will heal itself using hinted handoff, and when that process fails (i.e. max_hint_window_in_ms exceeded before replica becomes online), a repair should get things into a consistent state. This is one reason why it is recommended to run repairs regularly.

本文在更多内容中解释了提示的切换细节.

This article explains hinted handoff in more detail.

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