DA-03 · LESSON 10

CPFP for Ducat transaction pairs

KEY DEFINITIONS

Terms used in this lesson

CPFP: Child Pays for Parent: a high-fee child transaction raises the combined fee rate of an unconfirmed package.
Vault: A set of Bitcoin outputs and spending rules that holds collateral and represents the current loan state.
Proof-of-work target: The maximum header-hash value accepted for a block; lowering it makes valid hashes harder to find.
Transaction ID: The identifier produced by hashing a Bitcoin transaction’s non-witness serialization.
Identifier: Identifier: a value used to distinguish one request, object, or participant from another.

Package feerate explains how a fee-paying vault child can make a zero-fee issue or burn parent mineable.

45 min Difficulty 4/5 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 · ducat vaultA Ducat action moves one Bitcoin-anchored vault state to a successorSelect any state to inspect its exact role.

RELATIONSHIP MAPInputs, rule, and outputs

01
PREDECESSORCurrent vault outpoint
The current Bitcoin output anchors collateral and identifies the state being spent.
02
REQUESTCPFP for Ducat transaction pairs
The CPFP for Ducat transaction pairs workflow proposes one exact action and successor state.
03
POLICYProfile, oracle, signatures
Applicable policy, price evidence, authorization, and transaction shape are checked.
04
BITCOIN SPENDScript-valid transaction
Bitcoin miners and nodes can include only a transaction that satisfies Bitcoin rules.
05
SUCCESSORNew outpoint and state
Ducat validators reconstruct debt, collateral, token, and account changes from the confirmed transaction.

Rule shown: Bitcoin checks the spend; Ducat components independently reconstruct and validate the additional protocol state.

STAGE 1 OF 5PREDECESSOR: Current vault outpoint

The current Bitcoin output anchors collateral and identifies the state being spent.

Governing relationship: Bitcoin checks the spend; Ducat components independently reconstruct and validate the additional protocol state. Complete path: 1. PREDECESSOR, Current vault outpoint. The current Bitcoin output anchors collateral and identifies the state being spent. 2. REQUEST, CPFP for Ducat transaction pairs. The CPFP for Ducat transaction pairs workflow proposes one exact action and successor state. 3. POLICY, Profile, oracle, signatures. Applicable policy, price evidence, authorization, and transaction shape are checked. 4. BITCOIN SPEND, Script-valid transaction. Bitcoin miners and nodes can include only a transaction that satisfies Bitcoin rules. 5. SUCCESSOR, New outpoint and state. Ducat validators reconstruct debt, collateral, token, and account changes from the confirmed transaction.

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

BEFORE

Use a 180-vB parent paying 100 sats, a 320-vB child, and an 8 sat/vB package target. Required practice uses another package.

MECHANISM

Record the issue or burn parent's virtual size and fee, which may be intentionally low.

AFTER

The child still spends the parent, and current node minimums and ancestor policy can reject correct arithmetic. Required practice uses a separate server-executed vector.

Text equivalent: Starting from Use a 180-vB parent paying 100 sats, a 320-vB child, and an 8 sat/vB package target. Required practice uses another package. The mechanism begins when Record the issue or burn parent's virtual size and fee, which may be intentionally low. The resulting lesson is The child still spends the parent, and current node minimums and ancestor policy can reject correct arithmetic. Required practice uses a separate server-executed vector.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 4

Measure the parent

Record the issue or burn parent's virtual size and fee, which may be intentionally low.

Complete path: 1. Measure the parent: Record the issue or burn parent's virtual size and fee, which may be intentionally low. 2. Measure the child: Record the dependent vault child's virtual size and proposed fee. 3. Calculate package feerate: Add both fees and divide by the sum of both virtual sizes. 4. Respect relay and mining policy: A mathematically attractive package can still depend on node version, package relay, ancestor limits, and miner policy.

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

ACCEPTED PATH

The worked case executes the named inputs and checks in the displayed order. A mathematically attractive package can still depend on node version, package relay, ancestor limits, and miner policy.

CHANGED CONDITION

A missing, malformed, stale, unauthorized, or inconsistent required input stops at its first named boundary. The parent is unchanged. Miners consider the combined economic value of including both dependent transactions.

Important distinction: CPFP changes the parent's txid or fee. The parent is unchanged. Miners consider the combined economic value of including both dependent transactions.

Text equivalent: The accepted path is enforced by The worked case executes the named inputs and checks in the displayed order. A mathematically attractive package can still depend on node version, package relay, ancestor limits, and miner policy. If the required condition changes, A missing, malformed, stale, unauthorized, or inconsistent required input stops at its first named boundary. The parent is unchanged. Miners consider the combined economic value of including both dependent transactions.

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Learn the idea

Child Pays for Parent, or CPFP, lets a high-fee child raise the combined fee rate of a low-fee parent it spends. Ducat's two-transaction actions can use the vault child to contribute fees for an issue or burn parent.

GUIDED EXPLANATION1/4 ideas inspected

Measure the parent

Record the issue or burn parent's virtual size and fee, which may be intentionally low.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“CPFP changes the parent's txid or fee.”

The parent is unchanged. Miners consider the combined economic value of including both dependent transactions.

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.