# x/feegrant
# Abstract
This document specifies the fee grant module. For the full ADR, please see Fee Grant ADR-029 (opens new window).
This module allows accounts to grant fee allowances and to use fees from their accounts. Grantees can execute any transaction without the need to maintain sufficient fees.
# Contents
# Concepts
# Grant
Grant is stored in the KVStore to record a grant with full context. Every grant will contain granter, grantee and what kind of allowance is granted. granter is an account address who is giving permission to grantee (the beneficiary account address) to pay for some or all of grantee's transaction fees. allowance defines what kind of fee allowance (BasicAllowance or PeriodicAllowance, see below) is granted to grantee. allowance accepts an interface which implements FeeAllowanceI, encoded as Any type. There can be only one existing fee grant allowed for a grantee and granter, self grants are not allowed.
FeeAllowanceI looks like:
# Fee Allowance types
There are two types of fee allowances present at the moment:
BasicAllowancePeriodicAllowanceAllowedMsgAllowance
# BasicAllowance
BasicAllowance is permission for grantee to use fee from a granter's account. If any of the spend_limit or expiration reaches its limit, the grant will be removed from the state.
spend_limitis the limit of coins that are allowed to be used from thegranteraccount. If it is empty, it assumes there's no spend limit,granteecan use any number of available coins fromgranteraccount address before the expiration.expirationspecifies an optional time when this allowance expires. If the value is left empty, there is no expiry for the grant.When a grant is created with empty values for
spend_limitandexpiration, it is still a valid grant. It won't restrict thegranteeto use any number of coins fromgranterand it won't have any expiration. The only way to restrict thegranteeis by revoking the grant.
# PeriodicAllowance
PeriodicAllowance is a repeating fee allowance for the mentioned period, we can mention when the grant can expire as well as when a period can reset. We can also define the maximum number of coins that can be used in a mentioned period of time.
basicis the instance ofBasicAllowancewhich is optional for periodic fee allowance. If empty, the grant will have noexpirationand nospend_limit.periodis the specific period of time, after each period passes,period_can_spendwill be reset.period_spend_limitspecifies the maximum number of coins that can be spent in the period.period_can_spendis the number of coins left to be spent before the period_reset time.period_resetkeeps track of when a next period reset should happen.
# AllowedMsgAllowance
AllowedMsgAllowance is a fee allowance, it can be any of BasicFeeAllowance, PeriodicAllowance but restricted only to the allowed messages mentioned by the granter.
allowanceis eitherBasicAllowanceorPeriodicAllowance.allowed_messagesis array of messages allowed to execute the given allowance.
# FeeGranter flag
feegrant module introduces a FeeGranter flag for CLI for the sake of executing transactions with fee granter. When this flag is set, clientCtx will append the granter account address for transactions generated through CLI.
Example cmd:
# Granted Fee Deductions
Fees are deducted from grants in the x/auth ante handler. To learn more about how ante handlers work, read the Auth Module AnteHandlers Guide.
# Gas
In order to prevent DoS attacks, using a filtered x/feegrant incurs gas. The SDK must assure that the grantee's transactions all conform to the filter set by the granter. The SDK does this by iterating over the allowed messages in the filter and charging 10 gas per filtered message. The SDK will then iterate over the messages being sent by the grantee to ensure the messages adhere to the filter, also charging 10 gas per message. The SDK will stop iterating and fail the transaction if it finds a message that does not conform to the filter.
WARNING: The gas is charged against the granted allowance. Ensure your messages conform to the filter, if any, before sending transactions using your allowance.
# Pruning
A queue in the state maintained with the prefix of expiration of the grants and checks them on EndBlock with the current block time for every block to prune.
# State
# FeeAllowance
Fee Allowances are identified by combining Grantee (the account address of fee allowance grantee) with the Granter (the account address of fee allowance granter).
Fee allowance grants are stored in the state as follows:
- Grant:
0x00 | grantee_addr_len (1 byte) | grantee_addr_bytes | granter_addr_len (1 byte) | granter_addr_bytes -> ProtocolBuffer(Grant)
# FeeAllowanceQueue
Fee Allowances queue items are identified by combining the FeeAllowancePrefixQueue (i.e., 0x01), expiration, grantee (the account address of fee allowance grantee), granter (the account address of fee allowance granter). Endblocker checks FeeAllowanceQueue state for the expired grants and prunes them from FeeAllowance if there are any found.
Fee allowance queue keys are stored in the state as follows:
- Grant:
0x01 | expiration_bytes | grantee_addr_len (1 byte) | grantee_addr_bytes | granter_addr_len (1 byte) | granter_addr_bytes -> EmptyBytes
# Messages
# Msg/GrantAllowance
A fee allowance grant will be created with the MsgGrantAllowance message.
# Msg/RevokeAllowance
An allowed grant fee allowance can be removed with the MsgRevokeAllowance message.
# Events
The feegrant module emits the following events:
# Msg Server
# MsgGrantAllowance
| Type | Attribute Key | Attribute Value |
|---|---|---|
| message | action | set_feegrant |
| message | granter | |
| message | grantee |
# MsgRevokeAllowance
| Type | Attribute Key | Attribute Value |
|---|---|---|
| message | action | revoke_feegrant |
| message | granter | |
| message | grantee |
# Exec fee allowance
| Type | Attribute Key | Attribute Value |
|---|---|---|
| message | action | use_feegrant |
| message | granter | |
| message | grantee |
# Client
# CLI
A user can query and interact with the feegrant module using the CLI.
# Query
The query commands allow users to query feegrant state.
# grant
The grant command allows users to query a grant for a given granter-grantee pair.
Example:
Example Output:
# grants
The grants command allows users to query all grants for a given grantee.
Example:
Example Output:
# Transactions
The tx commands allow users to interact with the feegrant module.
# grant
The grant command allows users to grant fee allowances to another account. The fee allowance can have an expiration date, a total spend limit, and/or a periodic spend limit.
Example (one-time spend limit):
Example (periodic spend limit):
# revoke
The revoke command allows users to revoke a granted fee allowance.
Example:
# gRPC
A user can query the feegrant module using gRPC endpoints.
# Allowance
The Allowance endpoint allows users to query a granted fee allowance.
Example:
Example Output:
# Allowances
The Allowances endpoint allows users to query all granted fee allowances for a given grantee.
Example:
Example Output: