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Andreas M. Antonopoulos 2017-07-18 10:21:18 +08:00
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@ -773,14 +773,14 @@ image::images/mbc2_1002.png["Before the fork - all nodes have the same perspecti
A "fork" occurs whenever there are two candidate blocks competing to form the longest blockchain. This occurs under normal conditions whenever two miners solve the Proof-of-Work algorithm within a short period of time from each other. As both miners discover a solution for their respective candidate blocks, they immediately broadcast their own "winning" block to their immediate neighbors who begin propagating the block across the network. Each node that receives a valid block will incorporate it into its blockchain, extending the blockchain by one block. If that node later sees another candidate block extending the same parent, it connects the second candidate on a secondary chain. As a result, some nodes will "see" one candidate block first, while other nodes will see the other candidate block and two competing versions of the blockchain will emerge.
In <<fork2>>, we see two miners (Node A and Node B) who mine two different blocks almost simultaneously. Both of these blocks are children of the star block, and extend the chain by building on top of the star block. To help us track it, one is visualized as a triangle block originating from Node A, and the other is shown as an upside-down triangle block originating from Node B.
In <<fork2>>, we see two miners (Node X and Node Y) who mine two different blocks almost simultaneously. Both of these blocks are children of the star block, and extend the chain by building on top of the star block. To help us track it, one is visualized as a triangle block originating from Node X, and the other is shown as an upside-down triangle block originating from Node Y.
[[fork2]]
[role="smallersixty"]
.Visualization of a blockchain fork event: two blocks found simultaneously
image::images/mbc2_1003.png["Visualization of a blockchain fork event: two blocks found simultaneously"]
Let's assume, for example, that a miner Node A finds a Proof-of-Work solution for a block "triangle" that extends the blockchain, building on top of the parent block "star." Almost simultaneously, the miner Node B who was also extending the chain from block "star" finds a solution for block "upside-down triangle," his candidate block. Now, there are two possible blocks; one we call "triangle," originating in Node A; and one we call "upside-down triangle," originating in Node B. Both blocks are valid, both blocks contain a valid solution to the Proof-of-Work, and both blocks extend the same parent (block "star"). Both blocks likely contain most of the same transactions, with only perhaps a few differences in the order of transactions.
Let's assume, for example, that a miner Node X finds a Proof-of-Work solution for a block "triangle" that extends the blockchain, building on top of the parent block "star." Almost simultaneously, the miner Node Y who was also extending the chain from block "star" finds a solution for block "upside-down triangle," his candidate block. Now, there are two possible blocks; one we call "triangle," originating in Node X; and one we call "upside-down triangle," originating in Node Y. Both blocks are valid, both blocks contain a valid solution to the Proof-of-Work, and both blocks extend the same parent (block "star"). Both blocks likely contain most of the same transactions, with only perhaps a few differences in the order of transactions.
As the two blocks propagate, some nodes receive block "triangle" first and some receive block "upside-down triangle" first. As shown in <<fork3>>, the network splits into two different perspectives of the blockchain; one side topped with a triangle block, the other with the upside-down-triangle block.