Transaction Costing

What are transaction costs?

At the very high-level, transaction costs can be broken down into the following categories:

Item	Unit of Measurement	Description
Execution	Execution Cost Unit	This is to account for the CPU usage during the execution of a transaction. In this context, execution refers to the process of determining the state changes and other side effects by interpreting the instructions one-by-one upon some existing world state.
Finalisation	Finalisation Cost Unit	This is to account for the CPU usage during the finalisation of a transaction. In this context, finalisation refers to the process of committing state changes derived from a transaction and other auxiliary indices.
Storage	Bytes	This is to account for the additional data being added to a Node database. There are currently two type of storage costs: State Storage - The substates Archive Storage - Transaction payload, events and logs
Royalties	XRD and USD	The amount of XRD paid to blueprint developers and component owners for the use of the code or component instances.
Tip	Percentage	The extra amount of XRD paid to validators for the processing of a transaction. This is designed to help prioritise transaction when there is a traffic jam.

How does costing work?

Transaction costing is done through the costing module within the System.

This module is responsible for tracking the fee balance, counting execution and finalisation cost units and applying the costs listed above, with the help of a fee reserve.

At the beginning of a transaction, the fee reserve is provided with a loan of XRD to bootstrap transaction execution. 

• The amount is defined as:

execution_cost_unit_price  * ( 1 + tip_percentage / 100) * execution_cost_unit_loan

• This loan must be repaid before execution_cost_unit_loan number of execution cost units are consumed ("repay condition"), otherwise the transaction is rejected.

After that, transaction execution starts, during which two processes happen

• Kernel and system sends **execution cost events **(such as CPU usage, royalties) to the costing module:
  • To deduct the fee balance
  • To increase the cost unit counter
  • To repay system loan and apply deferred costs, if repay condition is met
• System sends vault lock fee events to the costing module:
  • To credit fee balance

Once execution is done, costing module is instructed to apply finalisation cost and storage cost.

Costing Parameters

The following parameters are used by the costing module.

Protocol defined parameters

Name	Value	Description
execution_cost_unit_price	0.00000005	The price of execution cost unit in XRD.
execution_cost_unit_limit	100,000,000	The maximum number of execution cost units that a transaction can consume.
execution_cost_unit_loan	4,000,000	The number of execution cost units loaned from the system, to bootstrap transaction execution.
finalization_cost_unit_price	0.00000005	The price of finalisation cost unit in XRD.
finalization_cost_unit_limit	50,000,000	The maximum number of finalisation cost units that a transaction can consume.
usd_price	16.666666666666666666	The price of USD in XRD.  1 XRD = 0.06 USD
state_storage_price	0.00009536743	The price of state storage. 1 MiB = 6 USD
archive_storage_price	0.00009536743	The price of archive storage.  1MiB = 6 USD

Transaction defined parameters

Name	Description
tip_percentage	The tip percentage specified by the transaction.
free_credit_in_xrd	The free credit amount specified by a preview request.

Fee Table

The table below further defines the cost of each costing entry.

Category	Entry	Description	Cost
Execution (execution cost unit)	VerifyTxSignatures	Verify transaction signature.	Variable: num_of_signature * 7000
	ValidateTxPayload	Verify transaction payload.	Variable: size(payload) * 40
	RunNativeCode	Run native code.	Native code execution is billed in native execution unit per function based on table here. 34 native execution units = 1 execution cost unit
	RunWasmCode	Run WASM code.	WASM code execution is billed in WASM execution units per instruction based on weights here. 3000 WASM execution units = 1 execution cost unit
	PrepareWasmCode	Prepare WASM code.	Variable: size(wasm) * 2
	BeforeInvoke	Before function invocation.	Variable: size(input) * 2
	AfterInvoke	After function invocation.	Variable: size(output) * 2
	AllocateNodeId	Allocate a new node id. Node is the lower level representation of an object or key value store.	Fixed: 97
	CreateNode	Create a new node.	Variable: size(substate) + 456
	DropNode	Drop a node.	Variable: size(substate) +1143
	PinNode	Pin a node to a device (heap or track).	Variable: 12 + IO access
	MoveModule	Move module from one node to another.	Variable: 140 + IO access
	OpenSubstate	Open a substate of a node.	Variable: 303 + IO access
	ReadSubstate	Read the value of a substate.	Variable: If from heap: 65 + size(substate) * 2 + IO access If from track: 113 + size(substate) * 2 + IO access
	WriteSubstate	Update the value of a substate.	Variable: 218 + size(substate) * 2 + IO access
	CloseSubstate	Cloes a substate.	Fixed: 129
	MarkSubstateAsTransient	Marks a substate a transient. Transient substates are not committed after transaction.	Fixed: 55
	SetSubstate	Set the value of a substate	Variable: 133 + size(substate) * 2 + IO access
	RemoveSubstate	Remove a substate	Variable: 717 + IO access
	ScanKeys	Scan substate keys in a collection	Variable: 498 + IO access
	ScanSortedSubstates	Scan substates in a collection	Variable: 187 + IO access
	DrainSubstates	Drain substates in a collection	Variable: 273 * num_of_substates + 272 + IO access
	LockFee	Lock fee	Fixed: 500
	QueryFeeReserve	Query state of fee reserve	Fixed: 500
	QueryActor	Query actor of this call frame	Fixed: 500
	QueryTransactionHash	Query the transaction hash	Fixed: 500
	GenerateRuid	Generate a RUID	Fixed: 500
	EmitEvent	Emit an event	Variable: 500 + size(event) * 2
	EmitLog	Emit a log	Variable: 500 + size(log) * 2
	Panic	Panic and abort execution	Variable: 500 + size(message) * 2
Finalisation (finalisation cost unit)	CommitStateUpdates	Commit state updates	Variable, per substate: Insert or update: 100,000 + size(substate) / 4 Delete: 100,000
	CommitEvents	Commit events	Variable, per event: 5,000 + size(event) / 4
	CommitLogs	Commit logs	Variable, per log: 1,000 + size (log) / 4
Storage (XRD)	IncreaseStateStorageSize	Increase the size of the state storage	Variable, per byte: 0.00009536743
	IncreaseArchiveStorageSize	Increase the size of the archive storage	Variable, per byte: 0.00009536743

IO access cost:

• Read from database and found: 40,000 + size / 10
• Read from database and not found: 160,000

Costing Runtime APIs

The system exposes a list of API to query the current state of the fee reserve.

See WASM API and Scrypto rustdoc.

Fee Distribution

Cost	To Proposer	To Validator Set	To Burn	To Royalty Owners
Execution	25%	25%	50%	0
Finalisation	25%	25%	50%	0
Storage	25%	25%	50%	0
Royalty	0	0	0	100%
Tip	100%	0	0	0