x/bank

Abstract

This document specifies the bank module.

The bank module is responsible for handling multi-asset coin transfers between accounts and tracking special-case pseudo-transfers which must work differently with particular kinds of accounts (notably delegating/undelegating for vesting accounts). It exposes several interfaces with varying capabilities for secure interaction with other modules which must alter user balances.

In addition, the bank module tracks and provides query support for the total supply of all assets used in the application.

Supply

The supply functionality:

• passively tracks the total supply of coins within a chain,
• provides a pattern for modules to hold/interact with Coins, and
• introduces the invariant check to verify a chain's total supply.

Total Supply

The total Supply of the network is equal to the sum of all coins from the account. The total supply is updated every time a Coin is minted (eg: as part of the inflation mechanism) or burned (eg: due to slashing or if a governance proposal is vetoed).

Module Accounts

The supply functionality introduces a new type of auth.Account which can be used by modules to allocate tokens and in special cases mint or burn tokens. At a base level these module accounts are capable of sending/receiving tokens to and from auth.Accounts and other module accounts. This design replaces previous alternative designs where, to hold tokens, modules would burn the incoming tokens from the sender account, and then track those tokens internally. Later, in order to send tokens, the module would need to effectively mint tokens within a destination account. The new design removes duplicate logic between modules to perform this accounting.

The ModuleAccount interface is defined as follows:

type ModuleAccount interface {
  auth.Account               // same methods as the Account interface

  GetName() string           // name of the module; used to obtain the address
  GetPermissions() []string  // permissions of module account
  HasPermission(string) bool
}

WARNING! Any module or message handler that allows either direct or indirect sending of funds must explicitly guarantee those funds cannot be sent to module accounts (unless allowed).

The supply Keeper also introduces new wrapper functions for the auth Keeper and the bank Keeper that are related to ModuleAccounts in order to be able to:

• Get and set ModuleAccounts by providing the Name.
• Send coins from and to other ModuleAccounts or standard Accounts (BaseAccount or VestingAccount) by passing only the Name.
• Mint or Burn coins for a ModuleAccount (restricted to its permissions).

Permissions

Each ModuleAccount has a different set of permissions that provide different object capabilities to perform certain actions. Permissions need to be registered upon the creation of the supply Keeper so that every time a ModuleAccount calls the allowed functions, the Keeper can lookup the permissions to that specific account and perform or not the action.

The available permissions are:

• Minter: allows for a module to mint a specific amount of coins.
• Burner: allows for a module to burn a specific amount of coins.
• Staking: allows for a module to delegate and undelegate a specific amount of coins.

Contents

State

The x/bank module keeps state of three primary objects, account balances, denom metadata and the total supply of all balances.

• Supply: 0x0 | byte(denom) -> byte(amount)
• Denom Metadata: 0x1 | byte(denom) -> ProtocolBuffer(Metadata)
• Balances: 0x2 | byte(address length) | []byte(address) | []byte(balance.Denom) -> ProtocolBuffer(balance)

Keepers

The bank module provides these exported keeper interfaces that can be passed to other modules that read or update account balances. Modules should use the least-permissive interface that provides the functionality they require.

Best practices dictate careful review of bank module code to ensure that permissions are limited in the way that you expect.

Blocklisting Addresses

The x/bank module accepts a map of addresses that are considered blocklisted from directly and explicitly receiving funds through means such as MsgSend and MsgMultiSend and direct API calls like SendCoinsFromModuleToAccount.

Typically, these addresses are module accounts. If these addresses receive funds outside the expected rules of the state machine, invariants are likely to be broken and could result in a halted network.

By providing the x/bank module with a blocklisted set of addresses, an error occurs for the operation if a user or client attempts to directly or indirectly send funds to a blocklisted account, for example, by using IBC.

Common Types

Input

An input of a multiparty transfer

// Input models transaction input.
message Input {
  string   address                        = 1;
  repeated cosmos.base.v1beta1.Coin coins = 2;
}

Output

An output of a multiparty transfer.

// Output models transaction outputs.
message Output {
  string   address                        = 1;
  repeated cosmos.base.v1beta1.Coin coins = 2;
}

BaseKeeper

The base keeper provides full-permission access: the ability to arbitrary modify any account's balance and mint or burn coins.

Restricted permission to mint per module could be achieved by using baseKeeper with WithMintCoinsRestriction to give specific restrictions to mint (e.g. only minting certain denom).

// Keeper defines a module interface that facilitates the transfer of coins
// between accounts.
type Keeper interface {
    SendKeeper

    InitGenesis(sdk.Context, *types.GenesisState)
    ExportGenesis(sdk.Context) *types.GenesisState

    GetSupply(ctx sdk.Context, denom string) sdk.Coin
    GetPaginatedTotalSupply(ctx sdk.Context, pagination *query.PageRequest) (sdk.Coins, *query.PageResponse, error)
    IterateTotalSupply(ctx sdk.Context, cb func(sdk.Coin) bool)
    GetDenomMetaData(ctx sdk.Context, denom string) (types.Metadata, bool)
    SetDenomMetaData(ctx sdk.Context, denomMetaData types.Metadata)
    IterateAllDenomMetaData(ctx sdk.Context, cb func(types.Metadata) bool)

    SendCoinsFromModuleToAccount(ctx sdk.Context, senderModule string, recipientAddr sdk.AccAddress, amt sdk.Coins) error
    SendCoinsFromModuleToModule(ctx sdk.Context, senderModule, recipientModule string, amt sdk.Coins) error
    SendCoinsFromAccountToModule(ctx sdk.Context, senderAddr sdk.AccAddress, recipientModule string, amt sdk.Coins) error
    DelegateCoinsFromAccountToModule(ctx sdk.Context, senderAddr sdk.AccAddress, recipientModule string, amt sdk.Coins) error
    UndelegateCoinsFromModuleToAccount(ctx sdk.Context, senderModule string, recipientAddr sdk.AccAddress, amt sdk.Coins) error
    MintCoins(ctx sdk.Context, moduleName string, amt sdk.Coins) error
    BurnCoins(ctx sdk.Context, moduleName string, amt sdk.Coins) error

    DelegateCoins(ctx sdk.Context, delegatorAddr, moduleAccAddr sdk.AccAddress, amt sdk.Coins) error
    UndelegateCoins(ctx sdk.Context, moduleAccAddr, delegatorAddr sdk.AccAddress, amt sdk.Coins) error

    types.QueryServer
}

SendKeeper

The send keeper provides access to account balances and the ability to transfer coins between accounts. The send keeper does not alter the total supply (mint or burn coins).

// SendKeeper defines a module interface that facilitates the transfer of coins
// between accounts without the possibility of creating coins.
type SendKeeper interface {
    ViewKeeper

    InputOutputCoins(ctx sdk.Context, inputs []types.Input, outputs []types.Output) error
    SendCoins(ctx sdk.Context, fromAddr sdk.AccAddress, toAddr sdk.AccAddress, amt sdk.Coins) error

    GetParams(ctx sdk.Context) types.Params
    SetParams(ctx sdk.Context, params types.Params)

    IsSendEnabledCoin(ctx sdk.Context, coin sdk.Coin) bool
    IsSendEnabledCoins(ctx sdk.Context, coins ...sdk.Coin) error

    BlockedAddr(addr sdk.AccAddress) bool
}

ViewKeeper

The view keeper provides read-only access to account balances. The view keeper does not have balance alteration functionality. All balance lookups are O(1).

// ViewKeeper defines a module interface that facilitates read only access to
// account balances.
type ViewKeeper interface {
    ValidateBalance(ctx sdk.Context, addr sdk.AccAddress) error
    HasBalance(ctx sdk.Context, addr sdk.AccAddress, amt sdk.Coin) bool

    GetAllBalances(ctx sdk.Context, addr sdk.AccAddress) sdk.Coins
    GetAccountsBalances(ctx sdk.Context) []types.Balance
    GetBalance(ctx sdk.Context, addr sdk.AccAddress, denom string) sdk.Coin
    LockedCoins(ctx sdk.Context, addr sdk.AccAddress) sdk.Coins
    SpendableCoins(ctx sdk.Context, addr sdk.AccAddress) sdk.Coins

    IterateAccountBalances(ctx sdk.Context, addr sdk.AccAddress, cb func(coin sdk.Coin) (stop bool))
    IterateAllBalances(ctx sdk.Context, cb func(address sdk.AccAddress, coin sdk.Coin) (stop bool))
}

Messages

MsgSend

Send coins from one address to another. +++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0/proto/cosmos/bank/v1beta1/tx.proto#L19-L28

The message will fail under the following conditions:

• The coins do not have sending enabled
• The to address is restricted

Events

The bank module emits the following events:

Handlers

MsgSend

Type	Attribute Key	Attribute Value
transfer	recipient	{recipientAddress}
transfer	amount	{amount}
message	module	bank
message	action	send
message	sender	{senderAddress}

Keeper events

In addition to handlers events, the bank keeper will produce events when the following methods are called (or any method which ends up calling them)

MintCoins

{
  "type": "coinbase",
  "attributes": [
    {
      "key": "minter",
      "value": "{{sdk.AccAddress of the module minting coins}}",
      "index": true
    },
    {
      "key": "amount",
      "value": "{{sdk.Coins being minted}}",
      "index": true
    }
  ]
}

{
  "type": "coin_received",
  "attributes": [
    {
      "key": "receiver",
      "value": "{{sdk.AccAddress of the module minting coins}}",
      "index": true
    },
    {
      "key": "amount",
      "value": "{{sdk.Coins being received}}",
      "index": true
    }
  ]
}

BurnCoins

{
  "type": "burn",
  "attributes": [
    {
      "key": "burner",
      "value": "{{sdk.AccAddress of the module burning coins}}",
      "index": true
    },
    {
      "key": "amount",
      "value": "{{sdk.Coins being burned}}",
      "index": true
    }
  ]
}

{
  "type": "coin_spent",
  "attributes": [
    {
      "key": "spender",
      "value": "{{sdk.AccAddress of the module burning coins}}",
      "index": true
    },
    {
      "key": "amount",
      "value": "{{sdk.Coins being burned}}",
      "index": true
    }
  ]
}

addCoins

{
  "type": "coin_received",
  "attributes": [
    {
      "key": "receiver",
      "value": "{{sdk.AccAddress of the address beneficiary of the coins}}",
      "index": true
    },
    {
      "key": "amount",
      "value": "{{sdk.Coins being received}}",
      "index": true
    }
  ]
}

subUnlockedCoins/DelegateCoins

{
  "type": "coin_spent",
  "attributes": [
    {
      "key": "spender",
      "value": "{{sdk.AccAddress of the address which is spending coins}}",
      "index": true
    },
    {
      "key": "amount",
      "value": "{{sdk.Coins being spent}}",
      "index": true
    }
  ]
}

Parameters

The bank module contains the following parameters:

Key	Type	Example
SendEnabled	[]SendEnabled	[{denom: "stake", enabled: true }]
DefaultSendEnabled	bool	true

SendEnabled

The send enabled parameter is an array of SendEnabled entries mapping coin denominations to their send_enabled status. Entries in this list take precedence over the DefaultSendEnabled setting.

DefaultSendEnabled

The default send enabled value controls send transfer capability for all coin denominations unless specifically included in the array of SendEnabled parameters.