Wire up the ICS-29 Fee Middleware to a Cosmos SDK blockchain

Learning Goals

In this section, you will:

• Add the ICS-29 Fee Middleware to a Cosmos SDK blockchain as a module.
• Wire up the ICS-29 Fee Middleware to the IBC transfer stack.

1. Wire up the ICS-29 Fee Middleware as a Cosmos SDK module

The Fee Middleware is not just an IBC middleware, it is also a Cosmos SDK module since it manages its own state and defines its own messages. We will first wire up the Fee Middleware as a Cosmos SDK module, then we will wire it up to the IBC transfer stack.

Cosmos SDK modules are registered in the app/app.go file. The app.go file is the entry point for the Cosmos SDK application. It is where the application is initialized and where the application's modules are registered.

We first need to import the fee module into the app.go file. Add the following import statements to the app.go file:

app/app.go

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1.1. Add the Fee Middleware to the module managers and define its account permissions

Next, we need to add fee module to the module basic manager and define its account permissions. Add the following code to the app.go file:

app/app.go

	// ModuleBasics defines the module BasicManager is in charge of setting up basic,
	// non-dependant module elements, such as codec registration
	// and genesis verification.
	ModuleBasics = module.NewBasicManager(
		// ... other modules
		evidence.AppModuleBasic{},
		transfer.AppModuleBasic{},
		ica.AppModuleBasic{},
		vesting.AppModuleBasic{},
+		ibcfee.AppModuleBasic{},
		consensus.AppModuleBasic{},
		// this line is used by starport scaffolding # stargate/app/moduleBasic
	)

	// module account permissions
	maccPerms = map[string][]string{
		authtypes.FeeCollectorName:     nil,
		distrtypes.ModuleName:          nil,
		icatypes.ModuleName:            nil,
		minttypes.ModuleName:           {authtypes.Minter},
		stakingtypes.BondedPoolName:    {authtypes.Burner, authtypes.Staking},
		stakingtypes.NotBondedPoolName: {authtypes.Burner, authtypes.Staking},
		govtypes.ModuleName:            {authtypes.Burner},
		pooltypes.ModuleName:           nil,
		ibctransfertypes.ModuleName:    {authtypes.Minter, authtypes.Burner},
+		ibcfeetypes.ModuleName:         nil,
		// this line is used by starport scaffolding # stargate/app/maccPerms
	}

Next, we need to add the fee middleware to the module manager. Add the following code to the app.go file:

app/app.go

	app.mm = module.NewManager(
		// ... other modules
		consensus.NewAppModule(appCodec, app.ConsensusParamsKeeper),
		ibc.NewAppModule(app.IBCKeeper),
		params.NewAppModule(app.ParamsKeeper),
		transferModule,
+		ibcfee.NewAppModule(app.IBCFeeKeeper),
		icaModule,
		// this line is used by starport scaffolding # stargate/app/appModule

		crisis.NewAppModule(app.CrisisKeeper, skipGenesisInvariants, app.GetSubspace(crisistypes.ModuleName)), // always be last to make sure that it checks for all invariants and not only part of them
	)

Note that we have added ibcfee.NewAppModule(app.IBCFeeKeeper) to the module manager but we have not yet created nor initialized the app.IBCFeeKeeper. We will do that next.

1.2. Initialize the Fee Middleware keeper

Next, we need to add the fee middleware keeper to the Cosmos App, register its store key, and initialize it.

app/app.go

// App extends an ABCI application, but with most of its parameters exported.
// They are exported for convenience in creating helper functions, as object
// capabilities aren't needed for testing.
type App struct {
	// ... other fields
	UpgradeKeeper         *upgradekeeper.Keeper
	ParamsKeeper          paramskeeper.Keeper
	IBCKeeper             *ibckeeper.Keeper // IBC Keeper must be a pointer in the app, so we can SetRouter on it correctly
+	IBCFeeKeeper          ibcfeekeeper.Keeper
	EvidenceKeeper        evidencekeeper.Keeper
	TransferKeeper        ibctransferkeeper.Keeper
	ICAHostKeeper         icahostkeeper.Keeper
	// ... other fields
}

app/app.go

keys := sdk.NewKVStoreKeys(
		authtypes.StoreKey, authz.ModuleName, banktypes.StoreKey, stakingtypes.StoreKey,
		crisistypes.StoreKey, minttypes.StoreKey, distrtypes.StoreKey, slashingtypes.StoreKey,
		govtypes.StoreKey, paramstypes.StoreKey, ibcexported.StoreKey, upgradetypes.StoreKey,
		feegrant.StoreKey, evidencetypes.StoreKey, ibctransfertypes.StoreKey, icahosttypes.StoreKey,
		capabilitytypes.StoreKey, group.StoreKey, icacontrollertypes.StoreKey, consensusparamtypes.StoreKey,
+		ibcfeetypes.StoreKey,
		// this line is used by starport scaffolding # stargate/app/storeKey
	)

Then initialize the keeper:

warning

Make sure to do the following initialization after the IBCKeeper is initialized and before TransferKeeper is initialized.

app/app.go

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1.3. Add the Fee Middleware to SetOrderBeginBlockers, SetOrderEndBlockers, and genesisModuleOrder

Next, we need to add the fee middleware to the SetOrderBeginBlockers, SetOrderEndBlockers, and genesisModuleOrder functions. Add the following code to the app.go file:

app/app.go

	// During begin block slashing happens after distr.BeginBlocker so that
	// there is nothing left over in the validator fee pool, so as to keep the
	// CanWithdrawInvariant invariant.
	// NOTE: staking module is required if HistoricalEntries param > 0
	app.mm.SetOrderBeginBlockers(
		// ... other modules
		icatypes.ModuleName,
+		ibcfeetypes.ModuleName,
		genutiltypes.ModuleName,
		// ... other modules
		consensusparamtypes.ModuleName,
		// this line is used by starport scaffolding # stargate/app/beginBlockers
	)

	app.mm.SetOrderEndBlockers(
		// ... other modules
		icatypes.ModuleName,
+		ibcfeetypes.ModuleName,
		capabilitytypes.ModuleName,
		// ... other modules
		consensusparamtypes.ModuleName,
		// this line is used by starport scaffolding # stargate/app/endBlockers
	)

	// NOTE: The genutils module must occur after staking so that pools are
	// properly initialized with tokens from genesis accounts.
	// NOTE: Capability module must occur first so that it can initialize any capabilities
	// so that other modules that want to create or claim capabilities afterwards in InitChain
	// can do so safely.
	genesisModuleOrder := []string{
		// ... other modules
		icatypes.ModuleName,
+		ibcfeetypes.ModuleName,
		evidencetypes.ModuleName,
		// ... other modules
		consensusparamtypes.ModuleName,
		// this line is used by starport scaffolding # stargate/app/initGenesis
	}
	app.mm.SetOrderInitGenesis(genesisModuleOrder...)
	app.mm.SetOrderExportGenesis(genesisModuleOrder...)

2. Wire up the ICS-29 Fee Middleware to the IBC Transfer stack

2.1. Wire up the ICS-29 Fee Middleware to the TransferKeeper

The ICS-29 Fee Middleware Keeper implements ICS4Wrapper interface. This means that the IBCFeeKeeper wraps the IBCKeeper.ChannelKeeper and that it can replace the use of the ChannelKeeper for sending packets, writing acknowledgements, and retrieving the IBC channel version.

We need to replace the ChannelKeeper with the IBCFeeKeeper in the TransferKeeper. To do this, we need to modify the TransferKeeper initialization in the app.go file.

app/app.go

	// Create Transfer Keepers
	app.TransferKeeper = ibctransferkeeper.NewKeeper(
		appCodec,
		keys[ibctransfertypes.StoreKey],
		app.GetSubspace(ibctransfertypes.ModuleName),
-		app.IBCKeeper.ChannelKeeper,
+		app.IBCFeeKeeper,
		app.IBCKeeper.ChannelKeeper,
		&app.IBCKeeper.PortKeeper,
		app.AccountKeeper,
		app.BankKeeper,
		scopedTransferKeeper,
	)

2.2. Wire up the ICS-29 Fee Middleware to the TransferModule

Currently, our app/app.go only contains the transfer module, which is a regular SDK AppModule (that manages state and has its own messages) that also fulfills the IBCModule interface and therefore has the ability to handle both channel handshake and packet lifecycle callbacks.

note

The transfer module is instantiated two times, once as a regular SDK module and once as an IBC module.

app/app.go

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We therefore need to "convert" the transferIBCModule to an IBC application stack that includes both the transferIBCModule and the ICS-29 Fee Middleware. Modify the app.go file as follows:

app/app.go

	transferModule := transfer.NewAppModule(app.TransferKeeper)
-	transferIBCModule := transfer.NewIBCModule(app.TransferKeeper)
+
+	/**** IBC Transfer Stack ****/
+	var transferStack ibcporttypes.IBCModule
+	transferStack = transfer.NewIBCModule(app.TransferKeeper)
+	transferStack = ibcfee.NewIBCMiddleware(transferStack, app.IBCFeeKeeper)

And finally, we need to add the transferStack to the ibcRouter. Modify the app.go file as follows:

app/app.go

	ibcRouter.AddRoute(icahosttypes.SubModuleName, icaHostIBCModule).
-		AddRoute(ibctransfertypes.ModuleName, transferIBCModule)
+		AddRoute(ibctransfertypes.ModuleName, transferStack)

This completes the wiring of the ICS-29 Fee Middleware to the IBC transfer stack! See a full example of the app.go file with the fee middleware wired up here and the diff here. Test that the application is still running with ignite chain serve --reset-once, and quit with q.