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Merge branch 'edeykholt-patch-1' into develop

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Minh T. Nguyen 2014-08-10 21:12:21 -07:00
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[[blockchain]]
=== The Blockchain
The blockchain data structure is an ordered back-linked list of blocks of transactions. The blockchain can be stored as a flat file, or in a simple database. The bitcoin core client stores the blockchain metadata using Google's LevelDB database. Blocks are linked "back", each referring to the previous block in the chain. The blockchain is often visualized as a vertical stack, with blocks layered on top of each other and the first block ever serving as the foundation of the stack. The visualization of blocks stacked on top of each other results in the use of terms like "height", to refer to the distance from the first block, and "top" or "tip" to refer to the most recently added block.
The blockchain data structure is an ordered back-linked list of blocks of transactions. The blockchain can be stored as a flat file, or in a simple database. The bitcoin core client stores the blockchain metadata using Google's LevelDB database. Blocks are linked "back", each referring to the previous block in the chain. The blockchain is often visualized as a vertical stack, with blocks layered on top of each other and the first block ever serving as the foundation of the stack. The visualization of blocks stacked on top of each other results in the use of terms like "height" to refer to the distance from the first block, and "top" or "tip" to refer to the most recently added block.
Each block within the blockchain is identified by a hash, generated using the SHA256 cryptographic hash algorithm on the header of the block. Each block also references a previous block, known as the _parent_ block, through the "previous block hash" field in the block header. In other words, each block contains the hash of its parent inside its own header. The sequence of hashes linking each block to its parent, creates a chain going back all the way to the first block ever created, known as the _genesis block_.
@ -56,9 +56,9 @@ The primary identifier of a block is its cryptographic hash, a digital fingerpri
Note that the block hash is not actually included inside the block's data structure, neither when the block is transmitted on the network, nor when it is stored on a node's persistence storage as part of the blockchain. Instead, the block's hash is computed by each node as the block is received from the network. The block hash may be stored in a separate database table as part of the block's metadata, to facilitate indexing and faster retrieval of blocks from disk.
A second way to identify a block is by its position in the blockchain, called the _block height_. The first block ever created is at block height 0 (zero), and is the same block that was referenced by the block hash +000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f+ above. A block can thus be identified two ways, either by referencing the block hash, or by referencing the block height. Each subsequent block added "on top" of that first block is one position "higher" in the blockchain, like boxes stacked one on top of the other. The block height on January 1st 2014 was approximately 278,000, meaning there were 278,000 blocks stacked on top of the first block created in January 2009.
A second way to identify a block is by its position in the blockchain, called the _block height_. The first block ever created is at block height 0 (zero) and is the same block that was referenced by the block hash +000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f+ above. A block can thus be identified two ways, either by referencing the block hash, or by referencing the block height. Each subsequent block added "on top" of that first block is one position "higher" in the blockchain, like boxes stacked one on top of the other. The block height on January 1st 2014 was approximately 278,000, meaning there were 278,000 blocks stacked on top of the first block created in January 2009.
Unlike the block hash, the block height is not a unique identifier. While a single block will always have a specific and invariant block height, the reverse is not true - the block height does not always identify a single block. Two or more blocks may have the same block height, competing for the same position in the blockchain. This scenario is discussed in detail in the section on <<forks>>. The block height is also not a part of the block's data structure, it is not stored within the block. Each node dynamically identifies a block's position (height) in the blockchain when it is received from the bitcoin network. The block height may also be stored as metadata in an indexed database table for faster retrieval.
Unlike the block hash, the block height is not a unique identifier. While a single block will always have a specific and invariant block height, the reverse is not true - the block height does not always identify a single block. Two or more blocks may have the same block height, competing for the same position in the blockchain. This scenario is discussed in detail in the section on <<forks>>. The block height is also not a part of the block's data structure; it is not stored within the block. Each node dynamically identifies a block's position (height) in the blockchain when it is received from the bitcoin network. The block height may also be stored as metadata in an indexed database table for faster retrieval.
[TIP]
====
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Merkle trees are used extensively by Simple Payment Verification nodes. SPV nodes don't have all transactions and do not download full blocks, just block headers. In order to verify that a transaction is included in a block, without having to download all the transactions in the block, they use an _authentication path_, or merkle path.
Consider for example an SPV node that is interested in incoming payments to an address contained in its wallet. The SPV node will establish a bloom filter on its connections to peers to limit the transactions received to only those containing addresses of interest. When a peer sees a transaction that matches the bloom filter, it will send that block using a +merkleblock+ message. The +merkleblock+ message contains the block header as well as a merkle path that links the transaction of interest to the merkle root in the block. The SPV node can use this merkle path to connect the transaction to the block and verify that the transaction is included in the block. The SPV node also uses the block header to link the block to the rest of the blockchain. The combination of these two links, between the transaction and block, and between the block and blockchain, proves that the transaction is recorded in the blockchain. All in all, the SPV node will have received less than a kilobyte of data for the block header and merkle path, an amount of data that is more than a thousand times less than a full block (about 1 megabyte currently)
Consider for example an SPV node that is interested in incoming payments to an address contained in its wallet. The SPV node will establish a bloom filter on its connections to peers to limit the transactions received to only those containing addresses of interest. When a peer sees a transaction that matches the bloom filter, it will send that block using a +merkleblock+ message. The +merkleblock+ message contains the block header as well as a merkle path that links the transaction of interest to the merkle root in the block. The SPV node can use this merkle path to connect the transaction to the block and verify that the transaction is included in the block. The SPV node also uses the block header to link the block to the rest of the blockchain. The combination of these two links, between the transaction and block, and between the block and blockchain, proves that the transaction is recorded in the blockchain. All in all, the SPV node will have received less than a kilobyte of data for the block header and merkle path, an amount of data that is more than a thousand times less than a full block (about 1 megabyte currently).