ledger-prototype/utxo/docs/65cdd331-aa64-48bf-9224-0e1e13c68e41.md

Verax

Data model

Verax's data model is heavily inspired by Bitcoin's unspent transaction output (UTXO) model. It is a data where transactions are a set of payments that are spent, and a set of new payments to be created.

Each payment is a data structure with this information

struct Payment {
    created_by_transaction: TxId,
    amount: Amount,
    destination: Address,
    spend_by: Option<TxId>
}

Any payment that has spend_by: None is part of the active balance of each address. Any other payment is not longer part of the active balance, but part of the historial record. These spent payments are read-only from that point forward.

Every payment is created from a transaction, no exception.

Transactions

flowchart LR
    UserA --> |1000 USD| B{Transfer}
    B --> |1000 USD| UserB

After this simple transction, UserA cannot longer spend their 1000 USD and balance, and UserB can spend 1000 USD more.

This data model does not care how many payments are being spend or created, as long as the amounts are the same on both ends.

In the following example UserA transfer 1000 USD to UserB, but 1 USD is deducted from the transfer by the system and that is being transfer to the FeeManager account.

flowchart LR
    UserA --> |1000 USD| B{Transfer}
    B --> |999 USD| UserB
    B --> |1 USD| FeeManager

Multi-step transactions

As mentioned before, the transaction can spend multiple payments and can create multiple as well. As long as the amounts are equal on both ends (in this case 1000 USD, 980 EUR is equal to 999USD + 1 USD, 979EUR + 1EUR), the transaction will be valid.

flowchart LR
    UserA' --> |1000 USD| B{Transfer}
    UserB' --> |980 EUR| B
    B --> |999 USD| UserB
    B --> |979 EUR| UserA
    B --> |1 USD| FeeManager
    B --> |1 EUR| FeeManager

When the transaction will be attempted to be persisted, the storage layer will make sure to flag UserA' and UserB' payments. If that operation fails, the whole transaction creation fails.

Concurrency model

Because Verax is heavily inspired in Bitcoin's model, the concurrency model is quite simple. When a new transaction is commited into the database, each payment in input section is attempted to be spent (altering their spend_by field from None to Some(new_transaction_id)). If any `payment is already spent, or not valid, the whole transaction creation fails and a rollback is issued upper stream. The storage layer ensures that transaction creation and updates are atomic and updates.

sequenceDiagram
    Transaction ->>+ DB:s
    critical Spend each input
        loop Spend Inputs
            Transaction ->>+ DB: Spend input
            DB ->>+ Transaction: OK
        end
        DB ->>+  Transaction: OK
        loop Output
            Transaction ->>+ DB: Creates new output
        end
    option Success
        Transaction ->>+ DB: Commit
        DB ->>+  Transaction: OK
    option Failure
        DB ->>- Transaction: Error
        Transaction ->>+ DB: Rollback
    end

Because of the input and output model, there is no need to check if the account has enough balance, and there is no need to enforce any locking mechanism, as long as each selected payment can be spendable at transaction storing time, the transaction will be created atomically. Each payment in the inputs must spendable, or else the whole operation fails, because every update is atomic, ensured by the storage layer.

The conditions for a payment ot be spendable are:

• It must be unspent: Payments are spendable once.
• It must be finalized: This means that the transaction which created this new payment is settled. Any other state is not acceptable and will render this

No global state knowledge is required to be sure that no asset is being created or destroyed by mistake, as long as the inputs are spendable and the sum of amounts inside inputs and outputs matches.