BA-08 · LESSON 05

Chain selection and reorgs

KEY DEFINITIONS

Terms used in this lesson

UTXO: Unspent transaction output: one discrete chunk of bitcoin that can be used as a transaction input.
Proof-of-work target: The maximum header-hash value accepted for a block; lowering it makes valid hashes harder to find.

Nodes follow the valid chain with the most accumulated work and may disconnect blocks during reorganization.

35 min Difficulty 2/5 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 · reorg selectionA node can replace its selected branchSelect any state to inspect its exact role.

RELATIONSHIP MAPCompeting paths and resolution

01
FORK POINTHeight 840,000
Two valid children of the same parent create temporary competing histories.
02
BRANCH A2 blocks, work 6
Transactions confirmed only on A remain exposed to replacement.
03
BRANCH B1 block, work 4
A still has more accumulated work.
04
SELECTRemain on A
The node validates B before changing its selected chain.
05
MEMPOOLNo reorganization
Still-valid transactions from the old branch may return to the mempool; conflicts do not.

Rule shown: Among fully valid branches, the node selects the one with the most accumulated proof of work, not the most blocks or the first message received.

STAGE 1 OF 5FORK POINT: Height 840,000

Two valid children of the same parent create temporary competing histories.

Governing relationship: Among fully valid branches, the node selects the one with the most accumulated proof of work, not the most blocks or the first message received. Complete path: 1. FORK POINT, Height 840,000. Two valid children of the same parent create temporary competing histories. 2. BRANCH A, 2 blocks, work 6. Transactions confirmed only on A remain exposed to replacement. 3. BRANCH B, 1 block, work 4. A still has more accumulated work. 4. SELECT, Remain on A. The node validates B before changing its selected chain. 5. MEMPOOL, No reorganization. Still-valid transactions from the old branch may return to the mempool; conflicts do not.

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

BEFORE

The active branch has ancestor block 1 followed by blocks 2 and 3, with a payment in block 3. A competing branch from block 1 grows to blocks 4, 5, and 6 with more accumulate…

MECHANISM

A node can know side branches while one valid branch is active. Invalid branches never compete by work.

AFTER

A reorg is deterministic state rollback and replay, not deletion of historical bytes from every disk.

Text equivalent: Starting from The active branch has ancestor block 1 followed by blocks 2 and 3, with a payment in block 3. A competing branch from block 1 grows to blocks 4, 5, and 6 with more accumulated work but omits the payment. The mechanism begins when A node can know side branches while one valid branch is active. Invalid branches never compete by work. The resulting lesson is A reorg is deterministic state rollback and replay, not deletion of historical bytes from every disk.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Maintain competing valid headers

A node can know side branches while one valid branch is active. Invalid branches never compete by work.

Complete path: 1. Maintain competing valid headers: A node can know side branches while one valid branch is active. Invalid branches never compete by work. 2. Calculate chainwork: Each valid block contributes work derived from its target. The node sums that work from the chain origin. 3. Find the common ancestor: When another branch gains more total work, the node identifies where active and candidate branches diverge. 4. Disconnect old blocks: In reverse order, restore their spent outputs and remove outputs they created, while reconsidering displaced transactions where appropriate. 5. Connect the stronger branch: Validate and apply candidate blocks forward from the ancestor. Wallets and applications must update confirmation and conflict state.

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

ACCEPTED PATH

Only valid branches compete by accumulated target-derived work; reorganization then disconnects to the common ancestor and connects the winner forward.

CHANGED CONDITION

An invalid higher-work branch is excluded, while a displaced transaction loses confirmation and may remain conflicted or be reconsidered under local policy.

Important distinction: Nodes always choose the branch with the most blocks. They choose the valid branch with the most accumulated work. A shorter branch can carry more work when targets differ.

Text equivalent: The accepted path is enforced by Only valid branches compete by accumulated target-derived work; reorganization then disconnects to the common ancestor and connects the winner forward. If the required condition changes, An invalid higher-work branch is excluded, while a displaced transaction loses confirmation and may remain conflicted or be reconsidered under local policy.

Loading lesson visuals...

Learn the idea

Chain selection compares valid branches by accumulated proof of work. A reorganization, or reorg, occurs when a node disconnects blocks from its current branch and connects a valid branch with more work. Height alone is insufficient because different targets can represent different work.

GUIDED EXPLANATION1/5 ideas inspected

Maintain competing valid headers

A node can know side branches while one valid branch is active. Invalid branches never compete by work.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Nodes always choose the branch with the most blocks.”

They choose the valid branch with the most accumulated work. A shorter branch can carry more work when targets differ.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.