Why undo files are needed in the Bitcoin Core?


The Bitcoin core software uses a data structure known as the Unspent Transaction Output (UTXO) set to keep track of the available bitcoin output that can be spent in future transactions. The UTXO set is updated each time new transactions are added to the blockchain, and when transactions are spent, their outputs are removed from the UTXO set. The UTXO set is critical to the functioning of the Bitcoin network, so it is important that it is kept accurate and up-to-date.

However, there are several scenarios where an incorrect UTXO set state may be recorded, such as

  1. Forks and reorganisations

    When a chain forks, there may be temporary inconsistencies in the UTXO set until a single chain is selected as the valid one.

  2. Orphaned blocks

    If a block is mined and broadcast, but is not incorporated into the blockchain because it conflicts with a longer branch, the transactions in that block must be ‘undone’ and any output consumed by transactions in the orphaned block must be returned to the set of unspent outputs.

  3. Rollbacks and reorgs

    A blockchain reorganisation may cause the UTXO set to change. All issues previously considered spent must be returned to the unspent set.

In each of these scenarios, the Bitcoin core software must keep track of transactions that were previously considered valid but are no longer so. To do this, the software uses undo files to keep a record of the changes made to the UTXO set over time.

Undo files, also known as rev*.dat files, contain the undo data for all the transactions in a block. The undo data records all the changes made to the UTXO set by a particular transaction, in the order in which the transactions were added to the block. If undoing a transaction means that outputs need to be added to the UTXO set, the undo data will also record the data needed to recreate those outputs.

When a new block is added, the Bitcoin Core software uses the undo data from the previous block to undo the changes made by that block. This ensures that the UTXO record is always kept in a consistent state.

In summary, Bitcoin Core uses undo files to ensure that the transaction history is correctly maintained and that the UTXO set remains accurate even in the event of forks, rollbacks, reorgs or orphaned blocks. The use of undo files is critical to the stability and security of the Bitcoin network.


What are undo files in Bitcoin Core?

Undo files are files that store all the changes made to the blockchain by a particular node. They are used as a backup in case a block needs to be undone and the node needs to revert to a previous state.

Why are undo files necessary?

Undo files are necessary to ensure the security and reliability of the Bitcoin network. If a block is found to be invalid or fraudulent, nodes need to be able to quickly undo the changes made by that block to prevent it from being permanently included in the blockchain.

How do undo files work?

Each node keeps track of the changes made to the blockchain by storing the differences between each new block and the previous one. If a block needs to be undone, the node can quickly revert to the previous state by applying the undo file to the current state.

Are undo files unique to the Bitcoin core?

No, undo files are a common feature of many blockchain implementations. They are necessary to ensure the integrity of the blockchain and prevent invalid blocks from being added to the network.Hope this helps! Let me know if you have any further questions.

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