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@ -730,43 +730,43 @@ In the first diagram (<<fork1>>), the network has a unified perspective of the b
.Visualization of a block chain fork event—before the fork
image::images/msbt_0802.png["globalfork1"]
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.
A "fork" occurs whenever there are two candidate blocks competing to form the longest block chain. 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 block chain, extending the block chain 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 block chain will emerge.
In <<fork2>>, we see two miners who mine two different blocks almost simultaneously. Both of these blocks are children of the blue block, meant to extend the chain by building on top of the blue block. To help us track it, one is visualized as a red block originating from Canada, and the other is marked as a green block originating from Australia.
[[fork2]]
.Visualization of a blockchain fork event—two blocks found simultaneously
.Visualization of a blockchain fork event: two blocks found simultaneously
image::images/msbt_0803.png["globalfork2"]
Let's assume, for example, that a miner in Canada finds a Proof-Of-Work solution for a block "red" that extends the blockchain, building on top of the parent block "blue." Almost simultaneously, an Australian miner who was also extending block "blue" finds a solution for block "green," his candidate block. Now, there are two possible blocks, one we call "red," originating in Canada and one we call "green," originating in Australia. Both blocks are valid, both blocks contain a valid solution to the proof of work, and both blocks extend the same parent. 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 in Canada finds a proof-of-work solution for a block "red" that extends the block chain, building on top of the parent block "blue." Almost simultaneously, an Australian miner who was also extending block "blue" finds a solution for block "green," his candidate block. Now, there are two possible blocks, one we call "red," originating in Canada, and one we call "green," originating in Australia. Both blocks are valid, both blocks contain a valid solution to the proof of work, and both blocks extend the same parent. 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 "red" first and some receive block "green" first. As shown in <<fork3>>, the network splits into two different perspectives of the blockchain, one side topped with a red block, the other with a green block.
As the two blocks propagate, some nodes receive block "red" first and some receive block "green" first. As shown in <<fork3>>, the network splits into two different perspectives of the block chain, one side topped with a red block, the other with a green block.
[[fork3]]
.Visualization of a blockchain fork event—two blocks propagate, splitting the network
.Visualization of a blockchain fork event: two blocks propagate, splitting the network
image::images/msbt_0804.png["globalfork3"]
From that moment, the bitcoin network nodes closest (topologically, not geographically) to the Canadian node will hear about block "red" first and will create a new greatest-cumulative-difficulty blockchain with "red" as the last block in the chain (e.g., blue-red), ignoring the candidate block "green" that arrives a bit later. Meanwhile, nodes closer to the Australian node will take that block as the winner and extend the blockchain with "green" as the last block (e.g., blue-green), ignoring "red" when it arrives a few seconds later. Any miners that saw "red" first will immediately build candidate blocks that reference "red" as the parent and start trying to solve the Proof-Of-Work for these candidate blocks. The miners that accepted "green" instead, will start building on top of "green" and extending that chain.
From that moment, the bitcoin network nodes closest (topologically, not geographically) to the Canadian node will hear about block "red" first and will create a new greatest-cumulative-difficulty block chain with "red" as the last block in the chain (e.g., blue-red), ignoring the candidate block "green" that arrives a bit later. Meanwhile, nodes closer to the Australian node will take that block as the winner and extend the block chain with "green" as the last block (e.g., blue-green), ignoring "red" when it arrives a few seconds later. Any miners that saw "red" first will immediately build candidate blocks that reference "red" as the parent and start trying to solve the proof of work for these candidate blocks. The miners that accepted "green" instead will start building on top of "green" and extending that chain.
Forks are almost always resolved within one block. As part of the network's hashing power is dedicated to building on top of "red" as the parent, another part of the hashing power is focused on building on top of "green." Even if the hashing power is almost evenly split, it is likely that one set of miners will find a solution and propagate it before the other set of miners have found any solutions. Let's say, for example, that the miners building on top of "green" find a new block "pink" that extends the chain (e.g., blue-green-pink). They immediately propagate this new block and the entire network sees it as a valid solution as shown in <<fork4>>.
[[fork4]]
.Visualization of a blockchain fork event—a new block extends one fork
.Visualization of a block chain fork event: a new block extends one fork
image::images/msbt_0805.png["globalfork4"]
All nodes that had chosen "green" as the winner in the previous round will simply extend the chain one more block. The nodes that chose "red" as the winner, however, will now see two chains: blue-green-pink and blue-red. The chain blue-green-pink is now longer (more cumulative difficulty) than the chain blue-red. As a result, those nodes will set the chain blue-green-pink as main chain and change the blue-red chain to being a secondary chain, as shown in <<fork5>>. This is a chain reconvergence, because those nodes are forced to revise their view of the blockchain to incorporate the new evidence of a longer chain. Any miners working on extending the chain blue-red will now stop that work because their candidate block is an "orphan," as its parent "red" is no longer on the longest chain. The transactions within "red" are queued up again for processing in the next block, because that block is no longer in the main chain. The entire network re-converges on a single blockchain blue-green-pink, with "pink" as the last block in the chain. All miners immediately start working on candidate blocks that reference "pink" as their parent to extend the blue-green-pink chain.
All nodes that had chosen "green" as the winner in the previous round will simply extend the chain one more block. The nodes that chose "red" as the winner, however, will now see two chains: blue-green-pink and blue-red. The chain blue-green-pink is now longer (more cumulative difficulty) than the chain blue-red. As a result, those nodes will set the chain blue-green-pink as main chain and change the blue-red chain to being a secondary chain, as shown in <<fork5>>. This is a chain reconvergence, because those nodes are forced to revise their view of the block chain to incorporate the new evidence of a longer chain. Any miners working on extending the chain blue-red will now stop that work because their candidate block is an "orphan," as its parent "red" is no longer on the longest chain. The transactions within "red" are queued up again for processing in the next block, because that block is no longer in the main chain. The entire network re-converges on a single block chain blue-green-pink, with "pink" as the last block in the chain. All miners immediately start working on candidate blocks that reference "pink" as their parent to extend the blue-green-pink chain.
[[fork5]]
.Visualization of a blockchain fork event—the network reconverges on a new longest chain
.Visualization of a block chain fork event: the network reconverges on a new longest chain
image::images/msbt_0806.png["globalfork5"]
It is theoretically possible for a fork to extend to two blocks, if two blocks are found almost simultaneously by miners on opposite "sides" of a previous fork. However, the chance of that happening is very low. Whereas a one-block fork may occur every week, a two-block fork is exceedingly rare.
It is theoretically possible for a fork to extend to two blocks, if two blocks are found almost simultaneously by miners on opposite "sides" of a previous fork. However, the chance of that happening is very low. Whereas a one-block fork might occur every week, a two-block fork is exceedingly rare.
Bitcoin's block interval of 10 minutes is a design compromise between fast confirmation times (settlement of transactions) and the probability of a fork. A faster block time would make transactions clear faster but lead to more frequent blockchain forks, whereas a slower block time would decrease the number of forks but make settlement slower.(((range="endofrange", startref="ix_ch08-asciidoc22")))(((range="endofrange", startref="ix_ch08-asciidoc21")))(((range="endofrange", startref="ix_ch08-asciidoc20")))(((range="endofrange", startref="ix_ch08-asciidoc19")))(((range="endofrange", startref="ix_ch08-asciidoc18")))
Bitcoin's block interval of 10 minutes is a design compromise between fast confirmation times (settlement of transactions) and the probability of a fork. A faster block time would make transactions clear faster but lead to more frequent block chain forks, whereas a slower block time would decrease the number of forks but make settlement slower.(((range="endofrange", startref="ix_ch08-asciidoc22")))(((range="endofrange", startref="ix_ch08-asciidoc21")))(((range="endofrange", startref="ix_ch08-asciidoc20")))(((range="endofrange", startref="ix_ch08-asciidoc19")))(((range="endofrange", startref="ix_ch08-asciidoc18")))
=== Mining and the Hashing Race
((("hashing race", id="ix_ch08-asciidoc23", range="startofrange")))((("mining","hashing race and", id="ix_ch08-asciidoc24", range="startofrange")))((("processing power and hash racing", id="ix_ch08-asciidoc25", range="startofrange")))Bitcoin mining is an extremely competitive industry. The hashing power has increased exponentially every year of bitcoin's existence. Some years the growth has reflected a complete change of technology, such as in 2010 and 2011 when many miners switched from using CPU mining to((("Graphical Processing Units (GPUs)","processing power of"))) Graphical Processing Unit (GPU) mining and((("Field Programmable Gate Array (FPGA)"))) Field Programmable Gate Array (FPGA) mining. In 2013 the introduction of((("Application Specific Integrated Circuit (ASIC)"))) Application Specific Integrated Circuit (ASIC) mining lead to another giant leap in mining power, by placing the SHA256 function directly on silicon chips specialized for the purpose of mining. The first such chips could deliver more mining power in a single box than the entire bitcoin network in 2010.
((("hashing race", id="ix_ch08-asciidoc23", range="startofrange")))((("mining","hashing race and", id="ix_ch08-asciidoc24", range="startofrange")))((("processing power and hash racing", id="ix_ch08-asciidoc25", range="startofrange")))Bitcoin mining is an extremely competitive industry. The hashing power has increased exponentially every year of bitcoin's existence. Some years the growth has reflected a complete change of technology, such as in 2010 and 2011 when many miners switched from using CPU mining to((("graphical processing units (GPUs)","processing power of"))) GPU mining and((("field programmable gate array (FPGA)"))) field programmable gate array (FPGA) mining. In 2013 the introduction of((("application-specific integrated circuit (ASIC)"))) ASIC mining lead to another giant leap in mining power, by placing the SHA256 function directly on silicon chips specialized for the purpose of mining. The first such chips could deliver more mining power in a single box than the entire bitcoin network in 2010.
The following list shows the total hashing power of the bitcoin network, over the first five years of operation: