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myarbrough@oreilly.com 2014-11-18 08:41:12 -08:00
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@ -57,7 +57,7 @@ The nonce, difficulty target, and timestamp are used in the mining process and w
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"))) _block height_. The first block ever created is at block height 0 (zero) and is the same block that was previously referenced by the following block hash +000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f+. 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"))) pass:[<phrase role="keep-together"><emphasis>block height</emphasis>. The first block ever created is at block height 0 (zero) and is the same</phrase>] block that was previously referenced by the following block hash +000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f+. 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 <<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.