Once a transaction is seen, it goes through several stages of validation, in this order:
The validity rules below assume sequential transaction validation for side effects (i.e. state changes). However, by construction, transactions with disjoint write access lists can be validated in parallel, including with overlapping read-only access lists. Transactions with overlapping write access lists must be validated and placed in blocks in topological order.
UTXOs and contracts in the read-only and write-destroy access lists must exist (i.e. have been created previously) in order for a transaction to be valid. In other words, for a unique state element ID, the write-create must precede the write-destroy.
Read-only access list:
Write-destroy access list:
InputType.Coin
(txId, outputIndex) InputType.Contract
(txId, outputIndex) InputType.Message
messageID Write-create access list:
OutputType.ContractCreated
contractID Note that block proposers use the contract ID contractID for inputs and outputs of type InputType.Contract and OutputType.Contract rather than the pair of txId and outputIndex.
This section defines VM precondition validity rules for transactions: the bare minimum required to accept an unconfirmed transaction into a mempool, and preconditions that the VM assumes to hold prior to execution. Chains of unconfirmed transactions are omitted.
For a transaction tx, UTXO set state, contract set contracts, and message set messages, the following checks must pass.
Note:
InputMessageswhereinput.dataLength > 0are not dropped from themessagesmessage set until they are included in a transaction of typeTransactionType.Scriptwith aScriptResultreceipt whereresultis equal to0indicating a successful script exit.
Base sanity checks are defined in the transaction format .
for input in tx.inputs:
if input.type == InputType.Contract:
if not input.contractID in contracts:
return False
elif input.type == InputType.Message:
if not input.nonce in messages:
return False
else:
if not (input.txId, input.outputIndex) in state:
return False
return True If this check passes, the UTXO ID (txId, outputIndex) fields of each contract input is set to the UTXO ID of the respective contract. The txPointer of each input is also set to the TX pointer of the UTXO with ID utxoID.
For each asset ID assetId in the input and output set:
def gas_to_fee(gas, gasPrice) -> int:
"""
Converts gas units into a fee amount
"""
return ceil(gas * gasPrice / GAS_PRICE_FACTOR)
def sum_data_messages(tx, assetId) -> int:
"""
Returns the total balance available from messages containing data
"""
total: int = 0
if assetId == 0:
for input in tx.inputs:
if input.type == InputType.Message and input.dataLength > 0:
total += input.amount
return total
def sum_inputs(tx, assetId) -> int:
total: int = 0
for input in tx.inputs:
if input.type == InputType.Coin and input.assetId == assetId:
total += input.amount
elif input.type == InputType.Message and assetId == 0 and input.dataLength == 0:
total += input.amount
return total
def transaction_size_gas_fees(tx) -> int:
"""
Computes the intrinsic gas cost of a transaction based on size in bytes
"""
return size(tx) * GAS_PER_BYTE
def minted(tx, assetId) -> int:
"""
Returns any minted amounts by the transaction
"""
if tx.type != TransactionType.Mint or assetId != tx.mintAssetId:
return 0
return tx.mint_amount
def sum_outputs(tx, assetId) -> int:
total: int = 0
for output in tx.outputs:
if output.type == OutputType.Coin and output.assetId == assetId:
total += output.amount
return total
def input_gas_fees(tx) -> int:
"""
Computes the intrinsic gas cost of verifying input utxos
"""
total: int = 0
witnessIndices = set()
for input in tx.inputs:
if input.type == InputType.Coin or input.type == InputType.Message:
# add fees allocated for predicate execution
if input.predicateLength == 0:
# notate witness index if input is signed
witnessIndices.add(input.witnessIndex)
else:
# add intrinsic gas cost of predicate merkleization based on number of predicate bytes
total += contract_code_root_gas_fee(input.predicateLength)
total += input.predicateGasUsed
# add intrinsic cost of vm initialization
total += vm_initialization_gas_fee()
# add intrinsic cost of verifying witness signatures
total += len(witnessIndices) * eck1_recover_gas_fee()
return total
def metadata_gas_fees(tx) -> int:
"""
Computes the intrinsic gas cost of processing transaction outputs
"""
total: int = 0
if tx.type == TransactionType.Create:
for output in tx.outputs:
if output.type == OutputType.OutputContractCreated:
# add intrinsic cost of calculating the code root based on the size of the contract bytecode
total += contract_code_root_gas_fee(tx.witnesses[tx.bytecodeWitnessIndex].dataLength)
# add intrinsic cost of calculating the state root based on the number of sotrage slots
total += contract_state_root_gas_fee(tx.storageSlotCount)
# add intrinsic cost of calculating the contract id
# size = 4 byte seed + 32 byte salt + 32 byte code root + 32 byte state root
total += sha256_gas_fee(100)
# add intrinsic cost of calculating the transaction id
total += sha256_gas_fee(size(tx))
elif tx.type == TransactionType.Script:
# add intrinsic cost of calculating the transaction id
total += sha256_gas_fee(size(tx))
return total
def intrinsic_gas_fees(tx) -> int:
"""
Computes intrinsic costs for a transaction
"""
fees: int = 0
# add the cost of initializing a vm for the script
if tx.type == TransactionType.Create or tx.type == TransactionType.Script:
fees += vm_initialization_gas_fee()
fees += metadata_gas_fees(tx)
fees += intrinsic_input_gas_fees(tx)
return fees
def min_gas(tx) -> int:
"""
Comutes the minimum amount of gas required for a transaction to begin processing.
"""
gas = transaction_size_gas_fees(tx) + intrinsic_gas_fees(tx)
return gas
def max_gas(tx) -> int:
"""
Computes the amount of gas required to process a transaction.
"""
gas = min_gas(tx)
gas = gas + (tx.witnessBytesLimit - tx.witnessBytes) * GAS_PER_BYTE
if tx.type == TransactionType.Script:
gas = gas + tx.gasLimit
return gas
def reserved_feeBalance(tx, assetId) -> int:
"""
Computes the maximum potential amount of fees that may need to be charged to process a transaction.
"""
maxGas = max_gas(tx)
feeBalance = gas_to_fee(maxGas, tx.gasPrice)
# Only base asset can be used to pay for gas
if assetId == 0:
return feeBalance
else:
return 0
def available_balance(tx, assetId) -> int:
"""
Make the data message balance available to the script
"""
availableBalance = sum_inputs(tx, assetId) + sum_data_messages(tx, assetId) + minted(tx, assetId)
return availableBalance
def unavailable_balance(tx, assetId) -> int:
sentBalance = sum_outputs(tx, assetId)
# Total fee balance
feeBalance = reserved_fee_balance(tx, assetId)
# Only base asset can be used to pay for gas
if assetId == 0:
return sentBalance + feeBalance
return sentBalance
# The sum_data_messages total is not included in the unavailable_balance since it is spendable as long as there
# is enough base asset amount to cover gas costs without using data messages. Messages containing data can't
# cover gas costs since they are retryable.
return available_balance(tx, assetId) >= (unavailable_balance(tx, assetId) + sum_data_messages(tx, assetId))def address_from(pubkey: bytes) -> bytes:
return sha256(pubkey)[0:32]
for input in tx.inputs:
if (input.type == InputType.Coin or input.type == InputType.Message) and input.predicateLength == 0:
# ECDSA signatures must be 64 bytes
if tx.witnesses[input.witnessIndex].dataLength != 64:
return False
# Signature must be from owner
if address_from(ecrecover_k1(txhash(), tx.witnesses[input.witnessIndex].data)) != input.owner:
return False
return TrueSignatures and signature verification are specified here .
The transaction hash is computed as defined here .
For each input of type InputType.Coin or InputType.Message, and predicateLength > 0, verify its predicate .
Given transaction tx, the following checks must pass:
If tx.scriptLength == 0, there is no script and the transaction defines a simple balance transfer, so no further checks are required.
If tx.scriptLength > 0, the script must be executed. For each asset ID assetId in the input set, the free balance available to be moved around by the script and called contracts is freeBalance[assetId]. The initial message balance available to be moved around by the script and called contracts is messageBalance:
freeBalance[assetId] = available_balance(tx, assetId) - unavailable_balance(tx, assetId)
messageBalance = sum_data_messages(tx, 0)Once the free balances are computed, the script is executed . After execution, the following is extracted:
unspentBalance for each asset ID. unspentGas from the $ggas register. size(tx) encompasses the entire transaction serialized according to the transaction format, including witness data.
This ensures every byte of block space either on Fuel or corresponding DA layer can be accounted for.
If the transaction as included in a block does not match this final transaction, the block is invalid.
The cost of a transaction can be described by:
cost(tx) = gas_to_fee(min_gas(tx) + tx.gasLimit - unspentGas, tx.gasPrice)where:
min_gas(tx) is the minimum cost of the transaction in gas, including intrinsic gas fees incurred from:
unspentGas is the amount gas left over after intrinsic fees and execution of the transaction, extracted from the $ggas register. Converting unspent gas to a fee describes how much "change" is left over from the user's payment; the block producer collects this unspent gas as reward. gas_to_fee is a function that converts gas to a concrete fee based on a given gas price. Fees incurred by transaction processing outside the context of execution are collectively referred to as intrinsic fees. Intrinsic fees include the cost of storing the transaction, calculated on a per-byte basis, the cost of processing inputs and outputs, including predicates and signature verification, and initialization of the VM prior to any predicate or script execution. Because intrinsic fees are independent of execution, they can be determined a priori and represent the bare minimum cost of the transaction.
A naturally occurring result of a variable gas limit is the concept of minimum and maximum fees. The minimum fee is, thus, the exact fee required to pay the fee balance, while the maximum fee is the minimum fee plus the gas limit:
min_gas = min_gas(tx)
max_gas = min_gas + (tx.witnessBytesLimit - tx.witnessBytes) * GAS_PER_BYTE + tx.gasLimit
min_fee = gas_to_fee(min_gas, tx.gasPrice)
max_fee = gas_to_fee(max_gas, tx.gasPrice) The cost of the transaction cost(tx) must lie within the range defined by [min_fee, max_fee]. min_gas is defined as the sum of all intrinsic costs of the transaction known prior to execution. The definition of max_gas illustrates that the delta between minimum gas and maximum gas is the sum of:
tx.gasLimit Note that gasLimit applies to transactions of type Script. gasLimit is not applicable for transactions of type Create and is defined to equal 0 in the above formula.
A transaction cost cost(tx), in gas, greater than max_gas is invalid and must be rejected; this signifies that the user must provide a higher gas limit for the given transaction. min_fee is the minimum reward the producer is guaranteed to collect, and max_fee is the maximum reward the producer is potentially eligible to collect. In practice, the user is always charged intrinsic fees; thus, unspentGas is the remainder of max_gas after intrinsic fees and the variable cost of execution. Calculating a conversion from unspentGas to an unspent fee describes the reward the producer will collect in addition to min_fee.
This section defines VM postcondition validity rules for transactions: the requirements for a transaction to be valid after it has been executed.
Given transaction tx, state state, and contract set contracts, the following checks must pass.
If change outputs are present, they must have:
0; an amount of unspentBalance + floor((unspentGas * tx.gasPrice) / GAS_PRICE_FACTOR) amount of the unspent free balance for that asset ID after VM execution is complete 0; an amount of the initial free balance plus (unspentGas * tx.gasPrice) - messageBalance amount of the initial free balance for that asset ID. Transaction processing is completed by removing spent UTXOs from the state and adding created UTXOs to the state.
The coinbase transaction is a mechanism for block creators to collect transaction fees.
In order for a coinbase transaction to be valid:
mintAmount doesn't exceed the total amount of fees processed from all other transactions within the same block. mintAssetId matches the assetId that fees are paid in (assetId == 0). The minted amount of the coinbase transaction intrinsically increases the balance corresponding to the inputContract.
This means the balance of mintAssetId is directly increased by mintAmount on the input contract,
without requiring any VM execution. Compared to coin outputs, intrinsically increasing a contract balance to collect
coinbase amounts prevents the accumulation of dust during low-usage periods.