Storm 【最新版 0.9.3】－官方翻译 1： Fault-tolerance

This page explains the design details of Storm that make it a fault-tolerant system.

在本页，我们将了解使Storm成为一个容错性系统的设计细节

What happens when a worker dies?

如果worker死掉以后会发生什么？

When a worker dies, the supervisor will restart it. If it continuously fails on startup and is unable 

每当一个Worker死掉了，那么supervisor将会重启它，如果是在启动的过程之中一直失败，那么它就会丢失掉

to heartbeat to Nimbus, Nimbus will reassign the worker to another machine.

河Numbus的连接，Nimbus将会重新分配一个Worker给其他的机器

What happens when a node dies?

  当一个节点挂掉的时候

The tasks assigned to that machine will time-out and Nimbus will reassign those tasks to other machines.

 一旦一个节点挂掉的时候，分配到这个机器之上的的task任务就会出现超时，并且Numbus会重新分配这些人物到其他的机器

What happens when Nimbus or Supervisor daemons die?

当Nimbus 或者Supervisor的后台线程会挂掉

Numbus和Supervisor的后台线程被设计于 fail-fast机制，以及无状态机制（实时上，我们的状态是保持在Zookeeper之中的），Numbus 和Supervisor的daemons 一定要运通过一定的工具，好比  daemontools 或则是 monit，于是，如果Nimbus 或则Supervisor daemons死掉了，那么就会像没发生一样的去重启。

The Nimbus and Supervisor daemons are designed to be fail-fast (process self-destructs whenever any unexpected situation is encountered) and stateless (all state is kept in Zookeeper or on disk). As described in Setting up a Storm cluster, the Nimbus and Supervisor daemons must be run under supervision using a tool like daemontools or monit. So if the Nimbus or Supervisor daemons die, they restart like nothing happened.

Most notably, no worker processes are affected by the death of Nimbus or the Supervisors. This is in contrast to Hadoop, where if the JobTracker dies, all the running jobs are lost.

更加显著的来说，通常而言，Worker的处理过程不会被Nimbus和Supervisor的死亡所影响。这一点和Hadoop系统是不一眼过得，要知道，一旦Hadoop之中的JobTracker死掉了，所有运行的作业就丢失。

Is Nimbus a single point of failure?

Nimbus是否也就是一个单带你的故障 

如果你的Numbus节点Los了，那么这Worker就会继续这些处理流程，更进一步来讲，Supervisor将继续的重启这些Workers

一旦Workers会死掉，然而，没有了Nimbus，一旦Worker工作之中有需要用到其他的计算硬件，Worker不能够被分配到其他机器。

If you lose the Nimbus node, the workers will still continue to function. Additionally, supervisors will continue to restart workers if they die. However, without Nimbus, workers won’t be reassigned to other machines when necessary (like if you lose a worker machine).

于是乎，真正的正确的答案是，Numbus本身是一系列的 SPOF（SPof 是我们的  single point of failure的缩写），实际上来讲，在Storm之中。这并不是一个相当的大的问题，在将来会有使Nimbus变得可用的可能。

So the answer is that Nimbus is “sort of” a SPOF. In practice, it’s not a big deal since nothing catastrophic happens when the Nimbus daemon dies. There are plans to make Nimbus highly available in the future.

How does Storm guarantee data processing?

Storm 对于消息的处理有保证性的机制，不管是在机器亦或者是消息丢失的情况之下。如果感兴趣的朋友，可以直接参考官方的文档，《Guaranteeing Message 》

Storm provides mechanisms to guarantee data processing even if nodes die or messages are lost. See Guaranteeing message processing for the details.

对于另外的机制，请参考本ID的另外一篇博文

 JDK【Java的一些特性】－ 系列 1：  fail-fast机制