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Synopsis

Learn how to integrate IBC to your application and send data packets to other chains. This document outlines the required steps to integrate and configure the IBC module to your Cosmos SDK application and send fungible token transfers to other chains.

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Integrating the IBC module

Integrating the IBC module to your SDK-based application is straightforward. The general changes can be summarized in the following steps:
  • Add required modules to the module.BasicManager
  • Define additional Keeper fields for the new modules on the App type
  • Add the module’s StoreKeys and initialize their Keepers
  • Set up corresponding routers and routes for the ibc module
  • Add the modules to the module Manager
  • Add modules to Begin/EndBlockers and InitGenesis
  • Update the module SimulationManager to enable simulations

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Module BasicManager and ModuleAccount permissions

The first step is to add the following modules to the BasicManager: x/capability, x/ibc, and x/ibc-transfer. After that, we need to grant Minter and Burner permissions to the ibc-transfer ModuleAccount to mint and burn relayed tokens. 
/ app.go
var (

  ModuleBasics = module.NewBasicManager(
    / ...
    capability.AppModuleBasic{
},
    ibc.AppModuleBasic{
},
    transfer.AppModuleBasic{
}, / i.e ibc-transfer module
  )

  / module account permissions
  maccPerms = map[string][]string{
    / other module accounts permissions
    / ...
    ibctransfertypes.ModuleName:    {
    authtypes.Minter, authtypes.Burner
},
)

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Application fields

Then, we need to register the Keepers as follows: 
/ app.go
type App struct {
  / baseapp, keys and subspaces definitions

  / other keepers
  / ...
  IBCKeeper        *ibckeeper.Keeper / IBC Keeper must be a pointer in the app, so we can SetRouter on it correctly
  TransferKeeper   ibctransferkeeper.Keeper / for cross-chain fungible token transfers

  / make scoped keepers public for test purposes
  ScopedIBCKeeper      capabilitykeeper.ScopedKeeper
  ScopedTransferKeeper capabilitykeeper.ScopedKeeper

  / ...
  / module and simulation manager definitions
}

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Configure the Keepers

During initialization, besides initializing the IBC Keepers (for the x/ibc, and x/ibc-transfer modules), we need to grant specific capabilities through the capability module ScopedKeepers so that we can authenticate the object-capability permissions for each of the IBC channels. 
func NewApp(...args) *App {
  / define codecs and baseapp

  / add capability keeper and ScopeToModule for ibc module
  app.CapabilityKeeper = capabilitykeeper.NewKeeper(appCodec, keys[capabilitytypes.StoreKey], memKeys[capabilitytypes.MemStoreKey])

  / grant capabilities for the ibc and ibc-transfer modules
    scopedIBCKeeper := app.CapabilityKeeper.ScopeToModule(ibchost.ModuleName)
    scopedTransferKeeper := app.CapabilityKeeper.ScopeToModule(ibctransfertypes.ModuleName)

  / ... other modules keepers

  / Create IBC Keeper
  app.IBCKeeper = ibckeeper.NewKeeper(
    appCodec, keys[ibchost.StoreKey], app.GetSubspace(ibchost.ModuleName), app.StakingKeeper, app.UpgradeKeeper, scopedIBCKeeper,
  )

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

  / .. continues
}

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Register Routers

IBC needs to know which module is bound to which port so that it can route packets to the appropriate module and call the appropriate callbacks. The port to module name mapping is handled by IBC’s port Keeper. However, the mapping from module name to the relevant callbacks is accomplished by the port Router on the IBC module. Adding the module routes allows the IBC handler to call the appropriate callback when processing a channel handshake or a packet. Currently, a Router is static so it must be initialized and set correctly on app initialization. Once the Router has been set, no new routes can be added. 
/ app.go
func NewApp(...args) *App {
  / .. continuation from above

  / Create static IBC router, add ibc-transfer module route, then set and seal it
    ibcRouter := port.NewRouter()

ibcRouter.AddRoute(ibctransfertypes.ModuleName, transferModule)
  / Setting Router will finalize all routes by sealing router
  / No more routes can be added
  app.IBCKeeper.SetRouter(ibcRouter)

  / .. continues

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Module Managers

In order to use IBC, we need to add the new modules to the module Manager and to the SimulationManager in case your application supports simulations. 
/ app.go
func NewApp(...args) *App {
  / .. continuation from above

  app.mm = module.NewManager(
    / other modules
    / ...
    capability.NewAppModule(appCodec, *app.CapabilityKeeper),
    ibc.NewAppModule(app.IBCKeeper),
    transferModule,
  )

  / ...

  app.sm = module.NewSimulationManager(
    / other modules
    / ...
    capability.NewAppModule(appCodec, *app.CapabilityKeeper),
    ibc.NewAppModule(app.IBCKeeper),
    transferModule,
  )

  / .. continues

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Application ABCI Ordering

One addition from IBC is the concept of HistoricalEntries which are stored on the staking module. Each entry contains the historical information for the Header and ValidatorSet of this chain which is stored at each height during the BeginBlock call. The historical info is required to introspect the past historical info at any given height in order to verify the light client ConsensusState during the connection handshake. The IBC module also has BeginBlock logic as well. This is optional as it is only required if your application uses the localhost client to connect two different modules from the same chain.
Only register the ibc module to the SetOrderBeginBlockers if your application will use the localhost (aka loopback) client.
/ app.go
func NewApp(...args) *App {
  / .. continuation from above

  / add staking and ibc modules to BeginBlockers
  app.mm.SetOrderBeginBlockers(
    / other modules ...
    stakingtypes.ModuleName, ibchost.ModuleName,
  )

  / ...

  / 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.
  app.mm.SetOrderInitGenesis(
    capabilitytypes.ModuleName,
    / other modules ...
    ibchost.ModuleName, ibctransfertypes.ModuleName,
  )

  / .. continues
IMPORTANT: The capability module must be declared first in SetOrderInitGenesis
That’s it! You have now wired up the IBC module and are now able to send fungible tokens across different chains. If you want to have a broader view of the changes take a look into the SDK’s SimApp.