What a full node does

Wonder

1. How does one computer verify every transaction since 2009 without trusting any other party?
2. What specific rules cause a node to reject a block that a miner spent significant energy producing?
3. What does running a node add to holding your own keys?

Explore

1. A full node downloads every block from genesis, which was mined on 3 Jan 2009. It independently checks each block against consensus rules: valid signatures and scripts, no input spent twice, outputs not exceeding inputs, block weight within 4,000,000 weight units, the coinbase claiming no more than the scheduled subsidy plus fees, proof of work meeting the target, and timestamps within limits. This process ensures that no single entity controls the ledger’s validity.
2. Independence is the defining feature. A node accepts no block because a miner, exchange, or developer says so; it only accepts what passes cryptographic verification. This embodies the ‘don’t trust, verify’ principle. The node maintains the current Unspent Transaction Output (UTXO) set, which represents all available coins in the system. It relays valid transactions and blocks to peers while serving data requests from other nodes.
3. The full blockchain passed 600 GB in 2024 and was about 760 GB by mid-2026, growing by roughly 70-80 GB a year. While exact node counts vary due to many running on Tor or being unreachable, many thousands of full nodes operate worldwide. Each node acts as a private source of truth for its owner’s wallet. By connecting a wallet directly to one’s own node, users verify transactions against their local copy of the rules rather than relying on third-party servers that might be compromised or censoring.
4. Validation is rigorous. If a block breaks any consensus rule—such as exceeding the weight limit or containing an invalid signature—it is rejected by every honest node regardless of how much hashing power produced it. Miners propose blocks, but nodes decide whether they count. This separation ensures that economic power cannot override technical rules without coordinated adoption across the network.
5. The UTXO set is a database of all unspent outputs. When a transaction spends an output, that entry is removed from the set and new outputs are added. A full node tracks this state continuously. Because it holds the complete history, it can verify any past transaction’s validity at any time, providing a permanent audit trail that no centralized server can alter or delete.
6. Running a full node costs storage, bandwidth and some maintenance. In return, its owner relies on no one else to say which payments are real or which rules apply: their wallet checks every incoming payment against their own copy of the chain, and their node counts among those that would refuse an invalid block or an unwanted rule change.

Key facts

Exercise nine, six, three, one

Fact check

Dates, figures and claims checked against the published sources listed below.

• Satoshi Nakamoto, 'Bitcoin: A Peer-to-Peer Electronic Cash System' (2008), Section 5
• A. M. Antonopoulos & D. A. Harding, Mastering Bitcoin, 3rd ed. (2023), ch. 10
• bitcoin.org, 'Running a Full Node'
• Bitcoin Core Source Code, validation.cpp