Читать книгу Introduction to Blockchain Technology - Tiana Laurence - Страница 14

There are three main types of blockchains, as shown in figure 9:

■ public blockchains,

■ private blockchains, and

■ hybrids.

Public blockchains allow anyone to participate in the network as long as they have access to the internet, hardware and electricity. Private blockchains only allow trusted parties to operate their blockchain. Hybrid blockchains control who can participate and at what level of participation each node is allowed to operate. These key differences are important to understand as they affect how distributed a blockchain network can become.

Distribution is characterized by how many independent nodes are operating on a network and keeping a full history of their respective blockchain. For public blockchain this would include all nodes that are mining new blocks and all nodes that are validating transactions.

Figure 9 Hybrid, public and private networks.

A key driver in the distribution of full nodes is economic incentivization. Those blockchains where an individual makes a profit from operating as a miner or processing transactions have more full nodes. Public blockchains offer up their native cryptocurrency as a reward to those who are maintaining the network.

The fair market value of a cryptocurrency will determine how many individuals will compete to maintain the network. The market value is driven by speculation, scarcity and utility. Bitcoin, for example, has fluctuated in price wildly (see figure 10) and so has the number of independent full nodes.

Distribution is a very important consideration when picking a network to work with. The greater the number of full independent nodes, the harder it is to compromise the data that has been written into that blockchain. The greater the number of full nodes, the more difficult it is to censor data from being written into a blockchain.

Figure 10 Price volatility of cryptocurrency.

Public blockchains that do not attract enough nodes are vulnerable to attack. Primarily, the attacker is looking to corrupt the transaction history so that they can spend a token or cryptocurrency twice. This is called a “51% attack“, see figure 11. Given that blockchains have one job, making permanent data, 51% attacks create an existential threat.

Figure 11 51% attack.

Private and hybrid blockchains combat 51% attacks by gating full node access to only known parties. However, they are exposed to just the same problem from within. A few hybrid blockchains have created workarounds where they publish a hash of their network every so often into a highly distributed public blockchain. This hash is called a Merkle tree root and allows a hybrid blockchain to restore itself to its last known valid block in case its network is attacked. See figure 11.

Figure 12 Stabilizing hybrid and private blockchains.