Transaction Fees¶

Introduction¶

Tanssi appchains are built with a modular framework called Substrate. With this framework, you can build unique ways to handle transaction fees. For example, most transactions use a specific module called Transaction Payment. However, transaction fees on Tanssi EVM-compatible appchains can be charged at an EVM execution level, bypassing other fee-related modules.

Under the hood, for execution time, instead of working with a gas-based mechanism, all Tanssi appchains work with a weight-based mechanism. Weight refers to the time (in picoseconds) it takes to validate a block. Generally speaking, for both EVM and non-EVM Tanssi appchains, all function calls have a weight associated with them, which sets limits on storage input/output and computation. For Tanssi EVM appchains, there is a gas-to-weight mapping that fully complies with the expected gas requirements for Ethereum API-based tools.

A transaction fee scheme is applied on top of the weight-based mechanism to ensure economic incentives are in line to limit the execution time, computation, and number of calls (database read/writes) to perform operations. Transaction fees are fundamental to preventing network spam, as they represent the cost of using the Tanssi appchain service. Consequently, a user interacting with the network through a specific function call will pay a transaction fee determined by a baseline fee algorithm.

This page covers the fundamentals of transaction fees for Tanssi appchains. It first covers the underlying transaction fee architecture and how it is adapted to a fully EIP-1559-compliant model for Tanssi EVM appchains.

Baseline Fees Calculation¶

Every action that alters the state of a Tanssi appchain incurs a transaction fee. This fee is essential for the network's operation, covering the computational resources required to process transactions, similar to the gas and gas price parameters in EVM-compatible chains like Ethereum

Tanssi appchains modular framework use a weight-based fee calculation mechanism to determine transaction fees. This approach considers various factors, including computational resources and storage operations (inputs/outputs), to reflect the true cost of transactions accurately. By accounting for these elements, the network ensures fair and efficient resource allocation.

Furthermore, Tanssi appchains modularity ensures that EVM-compatible appchains support legacy and EIP-1559 compatible transaction pricing mechanisms, ensuring full compatibility with development environments commonly used in Ethereum.

This section outlines all the different concepts associated with transaction fees for Tanssi appchains.

Weight¶

Broadly speaking, weight refers to the execution time it takes to validate a block, measured in picoseconds. Weight is divided into two separate variables:

• refTime - corresponds to the weight associated with computation time and database reads/writes
• proofSize - corresponds to the weight associated with the size of the Proof-Of-Validity (or PoV for short). The PoV is associated with the relevant state of a transaction, and it is what the Tanssi appchain block producer shares with the relay chain validator to get its block finalized as part of the appchain transaction flow

To find the weights for all function calls, they are benchmarked in a system with reference hardware, and the approximate values of refTime and proofSize are set. This process is repeated for all function calls that consume blockspace and affect the PoV.

For transactions in which the fees are handled by the transaction payment module, all weight-based parameters are then passed through a weight to fee algorithm that converts all to a final value, deducted from the sender's account when executing the function call. The algorithm can be customized, but Tanssi appchains have a constant value set.

For EVM transactions, gas is converted to weight through a gas-to-weight algorithm so that all EVM calls can be mapped to block execution time. Nevertheless, fees are handled at an EVM execution level.

Baseline Transaction Fees¶

With all function calls benchmarked, the transaction fee for each specific call can be obtained. Transaction fees are typically comprised of the following elements:

• BaseFee - baseline cost for a transaction to be included. It accounts for the transaction inclusion overhead, like signature verification. The fee is defined by two separate parameters:
  • ExtrinsicBaseWeight - a constant value that represents the weight of the transaction inclusion overhead
  • WeightToFee - a polynomial function that converts weight to fee
• WeightFee - fee defined by two separate parameters:
  • BenchmarkedWeight - weight that accounts for the complexity (execution time) of a specific call
  • CongestionMultiplier - a function that converts weight to fee and can be adjusted to account for the congestion of the network (weight consumed in the previous block). For Tanssi appchains, this is a constant value
• LengthFee - a fee correlated to the length in bytes of the function call. The fee is defined by two separate parameters:
  • ByteLengthFunctionCall - length in bytes of the call being executed
  • LengthToFee - a function that defines the per-byte fee algorithm. For Tanssi appchains, this is a constant value
• Tip - an optional value that increases the overall fee, increasing the priority of the transaction by incentivizing block producers to include it in the next block

Therefore, in general terms, the transaction fee can be calculated with the following equation:

BaseFee = ExtrinsicBaseWeight * WeightToFee
WeightFee = BenchmarkedWeight  * CongestionMultiplier
LengthFee = ByteLengthFunctionCall * LengthToFee

InclusionFee = BaseFee + WeightFee + LengthFee
FinalFee = InclusionFee + Tip

All non-EVM function calls available to developers use these baseline calculations for transaction fees. Tanssi EVM appchains have an extra layer to translate this fee scheme into an Ethereum-like scheme from an Ethereum JSON-RPC and EVM perspective.

EVM Transaction Fees¶

Tanssi offers templates for full Tanssi EVM-compatible appchains. Such appchains provide an Ethereum-like environment for developers, where they can use Eth-specific libraries like Ethers.js, Hardhat, and Foundry.

In addition, all Tanssi EVM-compatible appchains have an EIP-1559 compatible transaction pricing mechanism for EVM transactions. But they support both commonly used EVM transaction types:

• Type 0 (Legacy) - the transaction fee is calculated through a single gas price value that is included in the signed transaction blob. Because Tanssi EVM-compatible appchains have a dynamic pricing mechanism, gas price must be greater than the current block's baseFee for a transaction to be considered valid
• Type 2 (EIP-1559) - the transaction fee is calculated with a combination of the maxFeePerGas and maxPriorityFeePerGas from the signed transaction blob, and the network's baseFee dynamically changes based on block congestion

Independently of the transaction type, the outcome of all EVM transactions is that there is an associated cost in native tokens that the network must charge.

By default, Tanssi EVM-compatible appchains are configured with the following parameters:

• Minimum BaseFee - the minimum gas price of the network in case there are no transactions for long periods. The default value is set to 1 GWei
• Block Fulness Target (Elasticity) - the target gas used in a block so that the baseFee remains the same. EIP-1559 defines this value as a constant set to 2, meaning that the target usage is 50% of the block gas limit. All Tanssi EVM-compatible appchains are set with the same target
• Maximum BaseFee Increase - the maximum amount the baseFee can increase or decrease, in percent points, based on the previous block target usage. EIP-1559 defines this value as a constant set to 12.5%. Consequently, if the block is full/empty, the baseFee will increase/decrease by 12.5%, and any intermediate values are linearly adjusted. Developers can configure this value for Tanssi EVM-compatible appchains, but the default value is 12.5%

The EVM transaction fee cost associated with all Tanssi EVM-compatible appchains is captured at an EVM execution level. Nevertheless, EVM transactions do take block execution time. Therefore a gas-to-weight algorithm is required to account for the weight consumed by a specific call relative to the gas it is consuming.

Ultimately, the transaction fee and weight associated to an EVM call in a Tanssi EVM-compatible appchain can be calculated with the following formula:

Gas Price = baseFee + maxPriorityFeePerGas < maxFeePerGas ? 
           baseFee + maxPriorityFeePerGas : 
           maxFeePerGas;
Transaction Fee = Gas Price * Gas Used
Transaction Weight = Gas Used * GasToWeight

Transaction Fee = GasPrice * GasUsed
Transaction Weight = GasUsed * GasToWeight

GasToWeight is a constant value set to 25000.