Notes

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Blockchain types

Like other public databases, a blockchain can be thought of as a record keeping system for the entire network of nodes. There are four types of blockchains that are fundamentally different in how they store data and in their trust models. They are public, private, consortium and federated blockchains.

Public blockchains

A public blockchain's storage is publicly-verifiable. A public blockchain relies on a distributed network of nodes to keep the blockchain synchronized and to collect new transactions. In order for a new transaction to be accepted it has to be broadcast to all nodes of the network. The nodes on the network verify that the transaction has not already been included in a previous block and then validate the block, signing it as they do so. The more nodes that run the software, the higher the verification power, the more difficult it is for a network to be Sybil attacked and the more difficult it is for fraudulent transactions to be committed.

A blockchain whose data is a single file stored on a single server is a database file. In contrast, a blockchain whose data is stored in an incorruptible ordered list of all transactions is a ledger.

The main use-case for public blockchains is for applications where security and decentralisation are paramount, such as currency exchange, electronic voting, drugs trade, or asset exchange.
In contrast with private blockchains, public blockchains are not limited to specific applications or to a particular data model.
These often struggle with issues of scalability (with nodes adding little or no value as a result) and the ability to validate the integrity of the data. See also: greenblock#Scalability

Private blockchains

In a private blockchain, the data is stored in a database. The database is typically stored in a multi-tier architecture that can be replicated and hosted over multiple servers and which allows for changes to the data to be made, such as by a human administrator. Private blockchains are distributed as there is no single, central point of control that is stored in the blockchain itself, such as a blockchain IP address. The validation of new transactions is done by a central server that receives a private key and broadcasts the transaction to the different parts of the network or local clients.

Private blockchains are used for a wide variety of systems that require specific features, such as confidentiality, in order to function. These systems can be anonymous, such as the cash-transfer service Venmo. Other examples include private messaging platforms such as
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