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62 lines
3.5 KiB
Plaintext
:source-highlighter: highlight.js
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:highlightjs-languages: rust
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:github-icon: pass:[<svg class="icon"><use href="#github-icon"/></svg>]
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= pallet_transaction_payment
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Branch/Release: `release-polkadot-v1.3.0`
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== Purpose
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`pallet-transaction-payment` implements transaction fee logic.
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In substrate, every transaction has an associated `call`, and each `call` has its own weight function. The weight function estimates the time it takes to execute the `call`.
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`Config::WeightToFee` is a mapping between the smallest unit of compute (*Weight*) and smallest unit of fee.
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This pallet also exposes
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- how to update fees for the next block based on past fees (`Config::FeeMultiplierUpdate`)
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- how fees are paid (`Config::OnChargeTransaction`)
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The base fee and adjusted weight and length fees constitute the _inclusion fee_, which is the minimum fee for a transaction to be included in a block. The formula of final fee:
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```rust, ignore
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inclusion_fee = base_fee + length_fee + [fee_multiplier_update * weight_fee];
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final_fee = inclusion_fee + tip;
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```
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The inputs are defined below in the glossary and config sections.
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== Glossary
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** `Weight` -- The time it takes to execute runtime logic. By controlling the execution time that a block can consume, weight bounds the storage changes and computation per block.
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** `WeightFee` -- A fee proportional to amount of weight a transaction consumes.
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** `LengthFee` -- A fee proportional to the encoded length of the transaction.
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** `Tip` -- An optional tip. Tip increases the priority of the transaction, giving it a higher chance to be included by the transaction queue.
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== Config link:https://github.com/paritytech/polkadot-sdk/blob/release-polkadot-v1.3.0/substrate/frame/pallet-transaction-payment/src/lib.rs#L445[{github-icon},role=heading-link]
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* Pallet-specific handlers:
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** `OnChargeTransaction` -- Handler for withdrawing, refunding and depositing the transaction fee. Type must implement the trait `OnChargeTransaction<Self>`.
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** `FeeMultiplierUpdate` -- Handler to define how base fees change over time (over blocks). Type must implement the trait `MultiplierUpdate`. Possible assignments include `ConstantFee`, `SlowAdjustingFee`, and `FastAdjustingFee`.
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* Pallet-specific converters:
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** `WeightToFee` -- Mapping between the smallest unit of weight and smallest unit of fee. Type must implement the trait `WeightToFee<Balance = BalanceOf<Self>>`.
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** `LengthToFee` -- Convert a length value into a deductible fee based on the currency type. Type must implement the trait `WeightToFee<Balance = BalanceOf<Self>>`.
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* Pallet-specific constants:
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** `OperationalFeeMultiplier` -- A fee mulitiplier for `Operational` extrinsics to compute "virtual tip" to boost their `priority`. Type must implement the trait `Get<u32>`.
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* Common configs:
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** `RuntimeEvent`
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== Dispatchables
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There are no dispatchables (and no errors) in this pallet. This pallet is only intended to configure the transaction fee logic for a chain.
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**Events:**
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* `TransactionFeePaid(who, actual_fee, tip)` -- a transaction fee was paid by account `who` with total amount of `actual_fee + tip`.
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== More Reading
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** https://www.shawntabrizi.com/blog/substrate/substrate-weight-and-fees/[Substrate Weight & Fees] by Shawn Tabrizi
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* https://docs.substrate.io/build/tx-weights-fees/[Substrate Docs: Transaction Weight & Fees]
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** https://docs.substrate.io/reference/how-to-guides/weights/calculate-fees/#:~:text=Weight%20fee%20%2D%20A%20fee%20calculated,change%20as%20the%20chain%20progresses[Substrate Docs: How to Calculate Fees]
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