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myarbrough@oreilly.com 2014-11-18 07:49:15 -08:00
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@ -69,11 +69,11 @@ A transaction contains a number of fields, as shown in <<tx_data_structure>>.
[[tx_inputs_outputs]]
=== Transaction Outputs and Inputs
((("transactions","unspent transaction output")))((("unspent transaction output (UTXO)")))The fundamental building block of a bitcoin transaction is an _unspent transaction output_, or UTXO. UTXO are indivisible chunks of bitcoin currency locked to a specific owner, recorded on the blockchain, and recognized as currency units by the entire network. The bitcoin network tracks all available (unspent) UTXO currently numbering in the millions. Whenever a user receives bitcoin, that amount is recorded within the block chain as a UTXO. Thus, a user's bitcoin might be scattered as UTXO amongst hundreds of transactions and hundreds of blocks. In effect, there is no such thing as a stored balance of a bitcoin address or account; there are only scattered UTXO, locked to specific owners. The concept of a user's bitcoin balance is a derived construct created by the wallet application. The wallet calculates the user's balance by scanning the blockchain and aggregating all UTXO belonging to that user.
((("transactions","unspent transaction output")))((("unspent transaction output (UTXO)")))The fundamental building block of a bitcoin transaction is an _unspent transaction output_, or UTXO. UTXO are indivisible chunks of bitcoin currency locked to a specific owner, recorded on the blockchain, and recognized as currency units by the entire network. The bitcoin network tracks all available (unspent) UTXO currently numbering in the millions. Whenever a user receives bitcoin, that amount is recorded within the block chain as a UTXO. Thus, a user's bitcoin might be scattered as UTXO amongst hundreds of transactions and hundreds of blocks. In effect, there is no such thing as a stored balance of a bitcoin address or account; there are only scattered UTXO, locked to specific owners. The concept of a user's bitcoin balance is a derived construct created by the wallet application. The wallet calculates the user's balance by scanning the block chain and aggregating all UTXO belonging to that user.
[TIP]
====
((("accounts")))((("balances")))There are no accounts or balances in bitcoin; there are only _unspent transaction outputs_ (UTXO) scattered in the blockchain.
((("accounts")))((("balances")))There are no accounts or balances in bitcoin; there are only _unspent transaction outputs_ (UTXO) scattered in the block chain.
====
A UTXO can have an arbitrary value denominated as a multiple of((("satoshis"))) satoshis. Just like dollars can be divided down to two decimal places as cents, bitcoins can be divided down to eight decimal places as satoshis. Although UTXO can be any arbitrary value, once created it is indivisible just like a coin that cannot be cut in half. If a UTXO is larger than the desired value of a transaction, it must still be consumed in its entirety and change must be generated in the transaction. ((("change, making")))In other words, if you have a 20 bitcoin UTXO and want to pay 1 bitcoin, your transaction must consume the entire 20 bitcoin UTXO and produce two outputs: one paying 1 bitcoin to your desired recipient and another paying 19 bitcoin in change back to your wallet. As a result, most bitcoin transactions will generate change.