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Josh Green
67 lines
4.2 KiB
Markdown
67 lines
4.2 KiB
Markdown
# Blockchain
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## What is a Blockchain
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The Bitcoin Cash Blockchain is a linear collection of [Blocks](/protocol/blockchain/block).
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The "state" of the blockchain is calculated by sequentially executing each set of changes (in the form of [Transactions](/protocol/blockchain/transaction)) included in each Block.
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When a Block is created it includes the [Hash](/protocol/blockchain/hash) of the previous block; this cryptographically ensures the Block's parent block cannot be changed without completely rebuilding the new block.
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This chaining structure is similar to [Git](//git-scm.com) commits—where a block is analogous to a commit (a set of file changes), and each commit's parent is tied directly to its parent, forming a chain.
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Unlike Git, in order to create a commit (i.e. a Block) a certain amount of work must be performed.
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This [Proof of Work](/protocol/blockchain/proof-of-work) prevents blocks from being created instantly, and is the mechanic that prevents old history from being rewritten, since modifying a block would change its hash and disconnect it from its previous children.
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Therefore, changing a block requires that each of the child blocks be recreated as well.
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With this property, changing old blocks becomes nearly impossible as long as the Work to create a block is sufficiently time-intensive.
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## Block Height
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Blocks are applied sequentially, and therefore for a given chain, a block can be identified by its distance from the Genesis Block.
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This distance is commonly referred to as `Block Height`.
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There may be multiple blocks at the same `Block Height` if two blocks are built that share a previous Block's Hash.
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Multiple Blocks with the same `Block Height` are often referred to as "sibling" Blocks, or "contentious" blocks.
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Sibling Blocks can happen if a Block is created with sufficient Work before the mining Node has been made aware of the new (Sibling) Block.
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Sibling Blocks are incompatible with one another, and eventually one will become orphaned.
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The [Genesis Block](/protocol/blockchain#genesis-block) is the first Block in a chain, and has a `Block Height` of `0` (as its distance to the Genesis Block is zero).
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As of [BIP-0034](/blockchain/forks/bip-34), the `Block Height` is included within the [Coinbase Transaction](/protocol/blockchain/block#coinbase-transaction).
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## Work
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The longest valid chain with the most [Proof of Work](/protocol/blockchain/proof-of-work) is generally considered to be the main chain.
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Work can be quickly verified by evaluating the Block's [Difficulty](/protocol/blockchain/proof-of-work/difficulty) and validated by checking the Block's hash.
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[Chain Work](/protocol/blockchain/proof-of-work#chain-work) is the summation of all work done on each block up to a point on the Blockchain.
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As of [HF-20171113](/protocol/forks/hf-20171113), Chain Work is used to calculate new Block's [difficulty adjustment](/protocol/blockchain/proof-of-work/difficulty-adjustment-algorithm) as of Block Height.
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## Blockchain Reorganization
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When two valid blocks share the same block height, they are considered contentious blocks.
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Contentious blocks can happen organically due to the random nature of mining—the likeliness being proportional to the block propagation speed and the time between blocks.
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Slow-propagating blocks require a longer block interval in order to prevent contentious blocks.
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When two blocks become contentious, the first-seen block is usually considered the valid block on the main chain; however, different nodes across the network may have seen different blocks first.
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Consensus is formed when a second block is mined on a contentious block.
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This second block may also be contentious, although a rare circumstance.
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In this case, the process repeats and the race continues until consensus is reached.
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```diagramLabel
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1. Blocks 2a and 2b are mined before the either have fully propogated throughout the network.
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2. Some Nodes follow Block 2a, others follow Block 2b, depending on which Block they received first.
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3. Block 3 is mined on top of Block 2b. Nodes originally following Block 2a abandon Block 2A, and begin following Block 2b/3.
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```
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```mermaid
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graph LR
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0 --> 1
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1 --> 2a
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1 --> 2b
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2b --> 3
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style 2a stroke: #AAAAAA, stroke-dasharray: 5, 5
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```
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## Genesis Block |