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= pallet_transaction_payment

Branch/Release: `release-polkadot-v1.10.0`

== Source Code link:https://github.com/paritytech/polkadot-sdk/blob/release-polkadot-v1.10.0/substrate/frame/transaction-payment/src/lib.rs[{github-icon},role=heading-link]

== Purpose

`pallet-transaction-payment` implements transaction fee logic.

In substrate, every transaction has an associated `call`, and each `call` has its own xref:glossary.adoc#weight[weight] function. The weight function estimates the time it takes to execute the `call`.

`Config::WeightToFee` is a mapping between the smallest unit of compute (*Weight*) and smallest unit of fee.

This pallet also exposes
- how to update fees for the next block based on past fees (`Config::FeeMultiplierUpdate`)
- how fees are paid (`Config::OnChargeTransaction`)

The base fee and adjusted xref:glossary.adoc#weight_fee[weight] and xref:glossary.adoc#length_fee[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:
```rust, ignore
inclusion_fee = base_fee + length_fee + [fee_multiplier_update * weight_fee];
final_fee = inclusion_fee + tip;
```
The inputs are defined below in the glossary and config sections.

== Config

* Pallet-specific handlers:
** `OnChargeTransaction` -- Handler for withdrawing, refunding and depositing the transaction fee. Type must implement the trait  `OnChargeTransaction<Self>`.
** `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`.
* Pallet-specific converters:
** `WeightToFee` -- Mapping between the smallest unit of weight and smallest unit of fee. Type must implement the trait `WeightToFee<Balance = BalanceOf<Self>>`.
** `LengthToFee` -- Convert a length value into a deductible fee based on the currency type. Type must implement the trait `WeightToFee<Balance = BalanceOf<Self>>`.
* Pallet-specific constants:
** `OperationalFeeMultiplier` -- A fee mulitiplier for `Operational` extrinsics to compute "virtual tip" to boost their `priority`. Type must implement the trait `Get<u32>`.
* Common configs:
** `RuntimeEvent`

== Dispatchables

There are no dispatchables (and no errors) in this pallet. This pallet is only intended to configure the transaction fee logic for a chain.

**Events:**

* `TransactionFeePaid(who, actual_fee, tip)` -- a transaction fee was paid by account `who` with total amount of `actual_fee + tip`.

== More Reading

** https://www.shawntabrizi.com/blog/substrate/substrate-weight-and-fees/[Substrate Weight & Fees] by Shawn Tabrizi
* https://docs.substrate.io/build/tx-weights-fees/[Substrate Docs: Transaction Weight & Fees]
** 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]

