pay_fees.md

title	How to Pay Fees
tags	fees transactions developers

There a several ways to pay for transaction fees, thanks to fee abstraction implemented in the Aztec protocol.

This guide shows the various ways this can be done. For more of a background see Fees on Aztec.

Methods to pay a tx fee

Natively via an existing balance of fee juice

Either your own account or someone else's address has a balance of fee juice, and the transaction is paid for from this balance.

Natively via a balance of fee juice to be claimed

Processing this transaction first claims bridged fee juice then is paid for from the balance, all in the same public/private tx.

Privately or publicly via a Fee Paying Contract

A fee paying contract (FPC) is created and nominates a token that it accepts to then pay for txs in fee juice. So a user doesn't need to hold fee juice, they only need the token that the FPC accepts.

Fee Options

Functions pertaining to sending a transaction, such as deploy and send, each include a fee variable defined with the following (optional) parameters:

#include_code user_fee_options yarn-project/aztec.js/src/entrypoint/payload.ts javascript

Fee Payment Method

The paymentMethod is an object for the type of payment. Each of the implementations can be found here. For example:

#include_code fee_juice_method yarn-project/aztec.js/src/fee/fee_juice_payment_method.ts javascript

Gas Settings

#include_code gas_settings_vars yarn-project/stdlib/src/gas/gas_settings.ts javascript

import { Gas_Components, Gas_Settings } from '/components/snippets';

<Gas_Settings />

<Gas_Components />

Other params

Fee and gas padding params can be left to their default values, and the estimateGas boolean can be used when simulating a tx to calculate gas.

With the fee options explained, lets do a paid transaction.

Examples

Pay with FeeJuice

An account can be deployed directly via fee-juice payment if the address has been pre-funded. This is done using the AccountManager as follows:

#include_code pay_fee_juice_deploy yarn-project/end-to-end/src/e2e_fees/account_init.test.ts javascript

Or to send a transaction from an account holding fee juice: (Note: this example is a public token transfer call, but can equally be a private function call)

#include_code pay_fee_juice_send yarn-project/end-to-end/src/e2e_fees/fee_juice_payments.test.ts javascript

The equivalent to specify fees via CLI...

import { CLI_Fees } from '/components/snippets';

<CLI_Fees />

See here to Bridge Fee Juice via the CLI wallet.

Claim and pay

After a user has sent fee juice from L1 to be bridged to L2, a transaction can be made that claims this to pay for itself, and make the transaction, in one.

The claim object:

#include_code claim_type_amount yarn-project/aztec.js/src/api/ethereum/portal_manager.ts javascript

Calling a function on an object (in this case checking the balance of the fee juice contract)

#include_code claim_and_pay yarn-project/end-to-end/src/e2e_fees/fee_juice_payments.test.ts javascript

Fee Paying Contract

Similarly with a fee paying contract, the fee payment method is created and used as follows:

#include_code fpc yarn-project/end-to-end/src/e2e_fees/public_payments.test.ts javascript

In this example, thanks to the FPC's accepted_asset being banana tokens, Alice only needs to hold this token and not fee juice. The function being called happens to also be a transfer of banana tokens to Bob.

More on FPCs here