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To understand Byzantine fault tolerance, it is necessary to understand Byzantine generals’ problem.

After much meandering, the five generals, who each had 1,000 soldiers, succeeded in surrounding the Byzantine castle. There were 3,000 soldiers guarding the castle and more than a half of the generals must agree on attacking the castle at the same time. Since the generals were in distant, they needed to use a messenger to communicate. However, there was a possibility that more than one of them could be traitors, and the operation could fail.

In this case, the message was delivered to all five generals correctly, so everyone agreed on 9 o'clock operation, and the castle was occupied successfully. But what will happen if there were traitors?

If few traitors faked the agreement when delivering the message to the next general, the consensus would be misled and failed as shown on the image. In this case, five generals failed to occupy the castle because of the rupture of consensus. In a distributed network, there are various failed nodes in the system such as malicious nodes like traitors, and disable nodes, which can't perform the verification. Even with these failed nodes, the rest of nodes certainly achieve the consensus; this type of consensus algorithm is called BFT.