BA-01 · LESSON 04

Data structures and state machines

KEY DEFINITIONS

Terms used in this lesson

UTXO: Unspent transaction output: one discrete chunk of bitcoin that can be used as a transaction input.

Bitcoin tracks spendable transaction outputs instead of account balances. Every valid transaction consumes existing outputs and creates new ones.

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 · state machineOne valid action transforms a known stateSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
BEFOREKnown state
Identify the facts and objects that exist before the action.
02
ACTIONData structures and state machines
The Data structures and state machines lesson isolates the event that proposes a change.
03
RULEDeterministic check
The enforcing rule accepts or rejects the proposed transition.
04
AFTERSuccessor state
Accepted work creates a new state without silently changing the prior facts.
05
FAILUREState unchanged
A failed condition must not produce a partially accepted successor.

Rule shown: A valid transition names the prior state, action, enforcing rule, and successor state.

STAGE 1 OF 5BEFORE: Known state

Identify the facts and objects that exist before the action.

Governing relationship: A valid transition names the prior state, action, enforcing rule, and successor state. Complete path: 1. BEFORE, Known state. Identify the facts and objects that exist before the action. 2. ACTION, Data structures and state machines. The Data structures and state machines lesson isolates the event that proposes a change. 3. RULE, Deterministic check. The enforcing rule accepts or rejects the proposed transition. 4. AFTER, Successor state. Accepted work creates a new state without silently changing the prior facts. 5. FAILURE, State unchanged. A failed condition must not produce a partially accepted successor.

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

BEFORE

The initial state contains output A worth 7 and output B worth 5. A transaction consumes A and creates C worth 4 and D worth 3.

MECHANISM

For a simple bearer ledger, the state can be a set of uniquely identified unspent outputs with values and spending conditions.

AFTER

The transaction does not edit A. It replaces a consumed object with newly identified objects.

Text equivalent: Starting from The initial state contains output A worth 7 and output B worth 5. A transaction consumes A and creates C worth 4 and D worth 3. The mechanism begins when For a simple bearer ledger, the state can be a set of uniquely identified unspent outputs with values and spending conditions. The resulting lesson is The transaction does not edit A. It replaces a consumed object with newly identified objects.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Represent the current state

For a simple bearer ledger, the state can be a set of uniquely identified unspent outputs with values and spending conditions.

Complete path: 1. Represent the current state: For a simple bearer ledger, the state can be a set of uniquely identified unspent outputs with values and spending conditions. 2. Define a transition input: A transaction names outputs to consume and proposes new outputs. It is a candidate change, not an accepted change by itself. 3. Check preconditions: The machine checks that inputs exist, are unspent, have valid authorization, and conserve value under the system's rules. 4. Apply one deterministic update: If every check passes, consumed outputs are removed and new outputs are inserted. If a check fails, the state remains unchanged. 5. Replay to reproduce state: A new node can start from the initial state and apply the same ordered transitions to reconstruct the same current result.

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

ACCEPTED PATH

The transition checks input existence, unique use, authorization, nonnegative amounts, and outputs no greater than inputs before replacing state atomically.

CHANGED CONDITION

A replay of consumed A or a C:4 spend creating E:5 is rejected and leaves B, C, and D unchanged.

Important distinction: State is whatever the latest application database row says. In a replicated protocol, authoritative state is the result of applying agreed valid transitions. Databases may index or cache that result, but a mutable row is not the rule itself.

Text equivalent: The accepted path is enforced by The transition checks input existence, unique use, authorization, nonnegative amounts, and outputs no greater than inputs before replacing state atomically. If the required condition changes, A replay of consumed A or a C:4 spend creating E:5 is rejected and leaves B, C, and D unchanged.

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Learn the idea

A data structure organizes information so rules can find and update it. State is the information a system currently treats as true. Bitcoin tracks unspent transaction outputs, shortened to UTXOs: individually identified outputs that have been created but not yet spent. Their values are commonly counted in satoshis, shortened to sats; one satoshi is one hundred-millionth of a bitcoin. A state machine defines valid states and deterministic transitions, meaning every correct participant given the same starting state and input computes the same next state.

GUIDED EXPLANATION1/5 ideas inspected

Represent the current state

For a simple bearer ledger, the state can be a set of uniquely identified unspent outputs with values and spending conditions.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“State is whatever the latest application database row says.”

In a replicated protocol, authoritative state is the result of applying agreed valid transitions. Databases may index or cache that result, but a mutable row is not the rule itself.

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.