BA-00 · LESSON 04

Full nodes, the blockchain, and reorgs

Connect peer messages, mempool, block index, chainstate, most-work selection, confirmations, and temporary reorganizations into one full-node view.

65 min Difficulty 1/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · node reorgA full node changes selected history through a reproducible reorgSelect any state to inspect its exact role.

RELATIONSHIP MAPCompeting paths and resolution

01
PEERSAnnounce A and B
Messages are untrusted data sources rather than acceptance instructions.
02
BLOCK INDEXA work 6, B work 4
The node records parent, validation status, and accumulated work for known branches.
03
VALIDATEA and B valid
Validity is checked before any work comparison.
04
SELECTA active
The selected chain is the fully valid branch with most accumulated work.
05
CHAINSTATEDisconnect A if B reaches 7
Undo and connect operations update the UTXO set atomically during a reorg.

Rule shown: Only valid branches enter accumulated-work comparison, and chainstate changes by ordered disconnect and connect operations.

STAGE 1 OF 5PEERS: Announce A and B

Messages are untrusted data sources rather than acceptance instructions.

Governing relationship: Only valid branches enter accumulated-work comparison, and chainstate changes by ordered disconnect and connect operations. Complete path: 1. PEERS, Announce A and B. Messages are untrusted data sources rather than acceptance instructions. 2. BLOCK INDEX, A work 6, B work 4. The node records parent, validation status, and accumulated work for known branches. 3. VALIDATE, A and B valid. Validity is checked before any work comparison. 4. SELECT, A active. The selected chain is the fully valid branch with most accumulated work. 5. CHAINSTATE, Disconnect A if B reaches 7. Undo and connect operations update the UTXO set atomically during a reorg.

1. State transitionWhat exists, what changes, and what results

BEFORE

Branch A contains a payment and has work 6. Branch B initially has work 4, then gains another valid block and reaches work 7.

MECHANISM

Peers announce transactions, headers, and blocks. Message arrival proves only that a peer sent bytes. The node parses bounded messages and verifies each object locally.

AFTER

Confirmations measure depth in a selected valid history, while a reorg is the node changing that selection after another valid branch gains more work.

Text equivalent: Starting from Branch A contains a payment and has work 6. Branch B initially has work 4, then gains another valid block and reaches work 7. The mechanism begins when Peers announce transactions, headers, and blocks. Message arrival proves only that a peer sent bytes. The node parses bounded messages and verifies each object locally. The resulting lesson is Confirmations measure depth in a selected valid history, while a reorg is the node changing that selection after another valid branch gains more work.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Receive claims from peers

Peers announce transactions, headers, and blocks. Message arrival proves only that a peer sent bytes. The node parses bounded messages and verifies each object locally.

Complete path: 1. Receive claims from peers: Peers announce transactions, headers, and blocks. Message arrival proves only that a peer sent bytes. The node parses bounded messages and verifies each object locally. 2. Keep tentative and confirmed state separate: The mempool is a local set of unconfirmed candidates and can differ between nodes. Chainstate records outputs from the selected confirmed chain and changes only when blocks connect or disconnect. 3. Index valid competing branches: Two valid blocks can share a parent. The block index records parents, validation status, and accumulated work even when only one branch supplies active chainstate. 4. Select the valid most-work chain: Among fully valid candidates, the node selects the branch with the most accumulated proof of work. Height alone, peer count, and first arrival do not replace this rule. 5. Reorganize atomically: If another valid branch gains more work, the node disconnects old blocks with undo data, connects the winning branch in order, and reconsiders still-valid disconnected transactions for its mempool.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

Only fully valid branches enter work comparison, and every disconnect and connect produces a reproducible chainstate transition.

CHANGED CONDITION

A branch with greater claimed work is ignored if its proof, parent, transactions, or scripts fail. A reorg does not make invalid bytes acceptable.

Important distinction: Every Bitcoin node shares one global mempool and receives a reorg instruction from the network. Mempools are local. Each node validates branches, compares accumulated work, and performs its own reorganization.

Text equivalent: The accepted path is enforced by Only fully valid branches enter work comparison, and every disconnect and connect produces a reproducible chainstate transition. If the required condition changes, A branch with greater claimed work is ignored if its proof, parent, transactions, or scripts fail. A reorg does not make invalid bytes acceptable.

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Learn the idea

A full node is an independently verifying participant. It receives untrusted messages from peers, keeps a local mempool of policy-accepted transactions, stores block relationships in an index, and maintains chainstate as its current set of spendable outputs. The blockchain is the node's selected valid history, not a file broadcast by one authority.

GUIDED EXPLANATION1/5 ideas inspected

Receive claims from peers

Peers announce transactions, headers, and blocks. Message arrival proves only that a peer sent bytes. The node parses bounded messages and verifies each object locally.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Every Bitcoin node shares one global mempool and receives a reorg instruction from the network.”

Mempools are local. Each node validates branches, compares accumulated work, and performs its own reorganization.

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