Full Node

A full node maintains a full copy of the blockchain ledger, including every block header and transaction from the genesis block to the current tip. This involves storing the UTXO set (for Bitcoin-like chains) or the entire world state (for Ethereum-like chains). This local, authoritative copy allows the node to independently verify all data without trusting third parties.

The core responsibility is to validate all new transactions and blocks against the network's consensus rules. This includes checking:

• Digital signatures for authorization.
• Double-spend attempts.
• Adherence to protocol rules (e.g., block size, gas limits).
• Proof-of-Work difficulty or Proof-of-Stake signatures. Invalid data is rejected, preventing malicious actors from corrupting the ledger.

Full nodes act as network peers, relaying valid transactions and blocks to other nodes. They receive data from connected peers, validate it, and then propagate it across the peer-to-peer (P2P) network. This function is critical for decentralization, as it prevents reliance on a few centralized data sources and ensures robust data availability.

Full nodes provide data to light clients (Simplified Payment Verification clients) and wallets that do not store the full chain. They answer queries for block headers, merkle proofs, and account states via protocols like Ethereum's JSON-RPC. This service enables lightweight applications to securely interact with the blockchain.

By running a full node, a user enforces the consensus rules they choose to run. This provides sovereign verification, meaning the user does not need to trust miners, other nodes, or centralized APIs. It is the only way to have absolute certainty that the rules of the protocol are being followed, making it the backbone of a trustless system.

Running a full node requires significant resources, creating a trade-off with light clients.

• Storage: Hundreds of gigabytes to several terabytes of disk space.
• Bandwidth: Constant data upload/download for syncing and relaying.
• Compute: CPU usage for validating cryptographic proofs and signatures. These requirements ensure network security but limit the number of users who can practically run a node.

A full node's primary role is to validate every transaction and block against the network's consensus rules. It checks cryptographic signatures, verifies no double-spending occurred, and ensures block structure is correct. By rejecting invalid data, it enforces the protocol without trust, making the network Byzantine Fault Tolerant.

Unlike lightweight clients, a full node stores the entire blockchain history—every transaction from the genesis block. This includes:

• The UTXO set (for Bitcoin-like chains) or the full world state (for Ethereum-like chains).
• All block headers and their cryptographic proofs. This complete data allows it to answer historical queries and validate new data independently of any third party.

Full nodes form the peer-to-peer (P2P) network. They:

• Propagate valid transactions and blocks to other peers.
• Relay network information, ensuring data availability.
• Maintain connections to multiple peers, creating a resilient, decentralized mesh network. This prevents any single point of failure and resists censorship.

The security of lightweight clients (SPV wallets) depends entirely on the honest majority of full nodes. By connecting to multiple full nodes, clients can detect inconsistencies. A higher number of independently operated full nodes increases decentralization, making it exponentially harder for an attacker to control the network's view of the ledger.

Running a full node requires significant resources, creating a trade-off with decentralization:

• Storage: Hundreds of GBs to multiple TBs for mature chains.
• Bandwidth: Constant data upload/download for relaying.
• Compute: CPU for validation, especially for complex smart contracts. Unlike miners/validators, full node operators typically receive no direct monetary reward, relying on altruism, operational needs (exchanges, developers), or sybil resistance mechanisms.

Critical Distinction:

• Full Node: Validates and stores. Anyone can run one.
• Mining Node (PoW): A full node that also performs proof-of-work to create new blocks.
• Validator Node (PoS): A full node that stakes cryptocurrency to propose/validate blocks. All miners and validators must run a full node, but not all full nodes are miners/validators. The latter are a specialized subset with additional responsibilities and rewards.