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# 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.

## Blacklisting 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 of 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](http://docs.cosmos.network/master/ibc/).

## Common Types

### Input

An input of a multiparty transfer

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

### Output

An output of a multiparty transfer.

```protobuf
// 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.

```go
// 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) exported.SupplyI
	SetSupply(ctx sdk.Context, supply exported.SupplyI)

	GetDenomMetaData(ctx sdk.Context, denom string) types.Metadata
	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
	MarshalSupply(supplyI exported.SupplyI) ([]byte, error)
	UnmarshalSupply(bz []byte) (exported.SupplyI, 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).

```go
// 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)

	SendEnabledCoin(ctx sdk.Context, coin sdk.Coin) bool
	SendEnabledCoins(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)`.

```go
// 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))
}
```
