Block attributes

If we consider a blockchain as a data structure, then the blocks are aggregated sets of data that are used for formation of the blockchain. Blockchain formation is similar to linked list formation, where each node has a reference to the next node in the sequence. In the case of blockchain, each block has a reference to the previous node, thus forming a link all the way to the initial block (known as the genesis block) of the chain. As we mentioned earlier, a blockchain can be stored in either flat file or database format. Bitcoin uses LevelDB to store metadata about all the blocks that are downloaded to the disk.

Just like a linked list node, each block has a pointer, which is the identifier of the block. These are just hashed values of the block's header data. More detail about hashing will be covered in Chapter 2, A Bit of Cryptography. We can consider the hash as a unique identifier of a fixed size that represents each block; no two blocks will have the same identifier. Since all the blocks are linked together by this hash value, each block will have the identifier of the previous block. The previous block is referred to as the parent block, and each block can have only one parent.

Each block could also be referenced by the height of the blockchain. This height is nothing but the distance of the block, or the block count, from the genesis block. Height is an important attribute of the blockchain, as it's easier to refer to a block with a plain number rather than a lengthy hash value. The block hash is not a hash value of the entire block, but rather the hash value of the block header, which consists only of metadata. In Bitcoin, a SHA256 hashing algorithm is used to hash the block header and to create a unique identifier for the block.