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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.

+++ https://github.com/cosmos/cosmos-sdk/blob/691032b8be0f7539ec99f8882caecefc51f33d1f/proto/cosmos/feegrant/v1beta1/feegrant.proto#L75-L81

FeeAllowanceI looks like:

+++ https://github.com/cosmos/cosmos-sdk/blob/691032b8be0f7539ec99f8882caecefc51f33d1f/x/feegrant/fees.go#L9-L32

Fee Allowance types​

There are two types of fee allowances present at the moment:

  • BasicAllowance
  • PeriodicAllowance

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.

+++ https://github.com/cosmos/cosmos-sdk/blob/691032b8be0f7539ec99f8882caecefc51f33d1f/proto/cosmos/feegrant/v1beta1/feegrant.proto#L13-L26

  • spend_limit is the limit of coins that are allowed to be used from the granter account. If it is empty, it assumes there's no spend limit, grantee can use any number of available tokens from granter account address before the expiration.

  • expiration specifies 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_limit and expiration, it is still a valid grant. It won't restrict the grantee to use any number of tokens from granter and it won't have any expiration. The only way to restrict the grantee is 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.

+++ https://github.com/cosmos/cosmos-sdk/blob/691032b8be0f7539ec99f8882caecefc51f33d1f/proto/cosmos/feegrant/v1beta1/feegrant.proto#L28-L73

  • basic is the instance of BasicAllowance which is optional for periodic fee allowance. If empty, the grant will have no expiration and no spend_limit.

  • period is the specific period of time, after each period passes, period_spend_limit will be reset.

  • period_spend_limit specifies the maximum number of coins that can be spent in the period.

  • period_can_spend is the number of coins left to be spent before the period_reset time.

  • period_reset keeps track of when a next period reset should happen.

FeeAccount flag​

feegrant module introduces a FeeAccount 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.

+++ https://github.com/cosmos/cosmos-sdk/blob/d97e7907f176777ed8a464006d360bb3e1a223e4/client/cmd.go#L224-L235

+++ https://github.com/cosmos/cosmos-sdk/blob/d97e7907f176777ed8a464006d360bb3e1a223e4/client/tx/tx.go#L120

+++ https://github.com/cosmos/cosmos-sdk/blob/d97e7907f176777ed8a464006d360bb3e1a223e4/x/auth/tx/builder.go#L268-L277

+++ https://github.com/cosmos/cosmos-sdk/blob/d97e7907f176777ed8a464006d360bb3e1a223e4/proto/cosmos/tx/v1beta1/tx.proto#L160-L181

Example cmd:

./injectived tx gov submit-proposal --title="Test Proposal" --description="My awesome proposal" --type="Text" --from validator-key --fee-account=cosmos1xh44hxt7spr67hqaa7nyx5gnutrz5fraw6grxn --chain-id=testnet --fees="10stake"

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.