Blockchain is a distributed ledger that consists of a list of records, called blocks, that are linked using a cryptographic hash. Each block contains the cryptographic hash of the previous block, a timestamp, and the transaction data. Everyone has access to the ledger and new records can only be added with the consensus of the network majority.
Blocks store valuable information - bitcoin blocks store transactions - alongside some technical information like its version, current timestamp, and the hash of the previous block.
The way that hashes are calculated is fundamental to securing the blockchain. Calculating a hash is a computationally difficult operation that makes adding new blocks difficult, thus preventing retroactive modification of the block.
In it's base form, a blockchain is just an ordered back-linked list. A key component of a blockchain is that one has to perform some hard work to put data in it; this hard work makes the blockchain secure and consistent.
A hash is a unique representation of the original data input. A hash function takes data of arbitrary size and produces a fixed size hash that cannot be reverse engineered to find the original data input. Since a change in even one byte of input data could result in a completely different hash, hashing functions are used to check the consistency of data. In a blockchain, hashing is used to guarantee the consistency of a block. the input data for the hashing algorithm contains the hash of the previous block to prevent modification of the chain; one would have to recalculate its hash and the hashes of all the blocks after it.
The maintainers (miners) of the network that perform the work must have their work proven through a mechanism called proof-of-work. The high computational power it takes to generate the hash makes it difficult to produce but once completed, the hash is easy for others to verify.
Hashcash is Bitcoin's Proof-of-Work algorithm. It is a brute force algorithm that hashes the combination of publicly known data and a counter until the hash meets a certain requirement; an example could be "the first 20 bits of the hash have to be zeros".
To store data in the DB, this implementation copies Bitcoin Core. Two buckets are used to store data:
- blocks store metadata describing all the blocks in a chain.
- chainstate stores the state of a chain.
Blocks are stored as separate files on the disk so that reading a single block won't require loading all of the other blocks.
In blocks, the key -> value pairs are:
'b' + 32-byte block hash -> block index record 'f' + 4-byte file number -> file information record 'l' -> 4-byte file number: the last block file number used 'R' -> 1-byte boolean: whether we're in the process of reindexing 'F' + 1-byte flag name length + flag name string -> 1 byte boolean: various flags that can be on or off 't' + 32-byte transaction hash -> transaction index record
In chainstate, the key -> value pairs are:
'c' + 32-byte transaction hash -> unspent transaction output record for that transaction 'B' -> 32-byte block hash: the block hash up to which the database represents the unspent transaction outputs