Читать книгу Blockchain Data Analytics For Dummies - Michael G. Solomon - Страница 77

The last main category of data associated with a blockchain is the state data. State data is the data that is most like traditional database data. Each smart contract can define one or more variables or structures to store data values. These values can include things such as highest order number (for an order entry contract) or a list of products (for a supply chain contract). State data make it possible to store data that contracts use each time one of their functions is invoked.

Although transaction data is stored in blocks on the blockchain, Ethereum stores state data not in blockchain blocks but in an external (off-chain) database. Each block stores a hash value that points to the root of that block's state trie in the off-chain database, which stores the block’s state data. Storing data using a trie structure makes it possible to query the trie for a value, and validate the integrity of that value, without having to read the entire trie.

Refer to Figure 3-5, which shows the contents of a block header, including the state root hash that points to the root of the state values for that block (which is stored in the off-chain database).

Unlike blockchain data, state data can change. Each time a function in a smart contract runs, it may change the value of one or more state data items. The transaction that caused the change is stored in a block on the blockchain, and log entries may be created by events, but state data changes are stored in the off-chain database.

Each Ethereum client can select its own database for storing state data. For example, the Geth client uses LevelDB, and the Parity client uses RocksDB. Each database uses different methods for access, so the blockchain client you use for analysis should support a database that is familiar.