The most basic unit of blockchain infrastructure that stores data.

What Is a Node?

A node is the most basic unit and critical part of a blockchain infrastructure, storing its data and allowing all communication (transaction) to pass through it. It can be run by any personal computing device or server. Nodes are interconnected, and hence, can readily pass data amongst each other. It is essential for them to be always up-to-date in order to function properly. 
There are different kinds of nodes, varying by the extent of data stored and processing capability. Moreover, they are required to verify the legitimacy of blocks by the included signatures in a chain, and have the capability to accept or reject them.
Common nodes are distinguishable from miner nodes since miners actually provide computing power to a network to solve mathematical problems and propose blocks. Miners also run full nodes in order to find valid transactions for accumulation into a block. The same isn’t true for common nodes, which only store, broadcast and verify network activity without proposing blocks. Running a node typically requires an internet connection, a computing device with varying requirements, and different levels of technical expertise.
Proof-of-work (POW) nodes can’t be penalized for being offline or inactive. In proof-of-stake (POS), a node can be penalized if it fails to maintain online status. If a node goes offline, it needs to synchronize with the rest of the blockchain first before proceeding any further when it’s back online. 
The number of nodes is a great measure of security and decentralization, as the network becomes more resilient with the rise in their numbers. This is because a large number of nodes ensure that in the event of network attacks and some of them going offline, a reasonable number would remain to ensure continuity of the network and its functions.

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