---
sidebar_position: 7
description: Build an understanding of how to create and send packets across blockchains and navigate between blockchains.
title: "Inter-Blockchain Communication: Basics"
---

# Inter-Blockchain Communication: Basics

The Inter-Blockchain Communication protocol (IBC) is an important part of the Cosmos SDK ecosystem. The Hello World tutorial is a time-honored tradition in computer programming. This tutorial builds an understanding of how to create and send packets across blockchain. This foundational knowledge helps you navigate between blockchains with the Cosmos SDK.

**You will learn how to**

- Use IBC to create and send packets between blockchains.
- Navigate between blockchains using the Cosmos SDK and the Ignite CLI Relayer.
- Create a basic blog post and save the post on another blockchain.

## What is IBC?

The Inter-Blockchain Communication protocol (IBC) allows blockchains to talk to each other. IBC handles transport across different sovereign blockchains. This end-to-end, connection-oriented, stateful protocol provides reliable, ordered, and authenticated communication between heterogeneous blockchains. 

The [IBC protocol in the Cosmos SDK](https://ibc.cosmos.network/main/ibc/overview) is the standard for the interaction between two blockchains. The IBCmodule interface defines how packets and messages are constructed to be interpreted by the sending and the receiving blockchain.

The IBC relayer lets you connect between sets of IBC-enabled chains. This tutorial teaches you how to create two blockchains and then start and use the relayer with Ignite CLI to connect two blockchains.

This tutorial covers essentials like modules, IBC packets, relayer, and the lifecycle of packets routed through IBC.

## Create a blockchain

Create a blockchain app with a blog module to write posts on other blockchains that contain the Hello World message. For this tutorial, you can write posts for the Cosmos SDK universe that contain Hello Mars, Hello Cosmos, and Hello Earth messages.

For this simple example, create an app that contains a blog module that has a post transaction with title and text.

After you define the logic, run two blockchains that have this module installed.

- The chains can send posts between each other using IBC.

- On the sending chain, save the `acknowledged` and `timed out` posts.

After the transaction is acknowledged by the receiving chain, you know that the post is saved on both blockchains.

- The sending chain has the additional data `postID`.

- Sent posts that are acknowledged and timed out contain the title and the target chain of the post. These identifiers are visible on the parameter `chain`. The following chart shows the lifecycle of a packet that travels through IBC.

![The Lifecycle of an IBC packet in the Blog Module](./images/packet_sendpost.png)

## Build your blockchain app

Use Ignite CLI to scaffold the blockchain app and the blog module.

### Build a new blockchain

To scaffold a new blockchain named `planet`:

```bash
ignite scaffold chain planet --no-module
cd planet
```

A new directory named `planet` is created in your home directory. The `planet` directory contains a working blockchain app.

### Scaffold the blog module inside your blockchain

Next, use Ignite CLI to scaffold a blog module with IBC capabilities. The blog module contains the logic for creating blog posts and routing them through IBC to the second blockchain.

To scaffold a module named `blog`:

```bash
ignite scaffold module blog --ibc
```

A new directory with the code for an IBC module is created in `planet/x/blog`. Modules scaffolded with the `--ibc` flag include all the logic for the scaffolded IBC module.

### Generate CRUD actions for types

Next, create the CRUD actions for the blog module types.

Use the `ignite scaffold list` command to scaffold the boilerplate code for the create, read, update, and delete (CRUD) actions.

These `ignite scaffold list` commands create CRUD code for the following transactions:

- Creating blog posts

  ```bash
  ignite scaffold list post title content creator --no-message --module blog
  ```

- Processing acknowledgments for sent posts

  ```bash
  ignite scaffold list sentPost postID title chain creator --no-message --module blog
  ```

- Managing post timeouts

  ```bash
  ignite scaffold list timedoutPost title chain creator --no-message --module blog
  ```

The scaffolded code includes proto files for defining data structures, messages, messages handlers, keepers for modifying the state, and CLI commands.

### Ignite CLI Scaffold List Command Overview

```
ignite scaffold list [typeName] [field1] [field2] ... [flags]
```

The first argument of the `ignite scaffold list [typeName]` command specifies the name of the type being created. For the blog app, you created `post`, `sentPost`, and `timedoutPost` types.

The next arguments define the fields that are associated with the type. For the blog app, you created `title`, `content`, `postID`, and `chain` fields.

The `--module` flag defines which module the new transaction type is added to. This optional flag lets you manage multiple modules within your Ignite CLI app. When the flag is not present, the type is scaffolded in the module that matches the name of the repo.

When a new type is scaffolded, the default behavior is to scaffold messages that can be sent by users for CRUD operations. The `--no-message` flag disables this feature. Disable the messages option for the app since you want the posts to be created upon reception of IBC packets and not directly created from a user's messages.

### Scaffold a sendable and interpretable IBC packet

You must generate code for a packet that contains the title and the content of the blog post.

The `ignite packet` command creates the logic for an IBC packet that can be sent to another blockchain.

- The `title` and `content` are stored on the target chain.

- The `postID` is acknowledged on the sending chain.

To scaffold a sendable and interpretable IBC packet:

```bash
ignite scaffold packet ibcPost title content --ack postID --module blog
```

Notice the fields in the `ibcPost` packet match the fields in the `post` type that you created earlier.

- The `--ack` flag defines which identifier is returned to the sending blockchain.

- The `--module` flag specifies to create the packet in a particular IBC module.

The `ignite packet` command also scaffolds the CLI command that is capable of sending an IBC packet:

```bash
planetd tx blog send-ibcPost [portID] [channelID] [title] [content]
```

## Modify the source code

After you create the types and transactions, you must manually insert the logic to manage updates in the database. Modify the source code to save the data as specified earlier in this tutorial.

### Add creator to the blog post packet

Start with the proto file that defines the structure of the IBC packet.

To identify the creator of the post in the receiving blockchain, add the `creator` field inside the packet. This field was not specified directly in the command because it would automatically become a parameter in the `SendIbcPost` CLI command.

```protobuf
// proto/blog/packet.proto
message IbcPostPacketData {
    string title = 1;
    string content = 2;
    string creator = 3; // < ---
}
```

To make sure the receiving chain has content on the creator of a blog post, add the `msg.Creator` value to the IBC `packet`. 

- The content of the `sender` of the message is automatically included in `SendIbcPost` message. 
- The sender is verified as the signer of the message, so you can add the `msg.Sender` as the creator to the new packet before it is sent over IBC.

```go
	// x/blog/keeper/msg_server_ibc_post.go

	// Construct the packet
	var packet types.IbcPostPacketData
	packet.Title = msg.Title
	packet.Content = msg.Content
	packet.Creator = msg.Creator // < ---

	// Transmit the packet
	err := k.TransmitIbcPostPacket(
		ctx,
		packet,
		msg.Port,
		msg.ChannelID,
		clienttypes.ZeroHeight(),
		msg.TimeoutTimestamp,
	)
```

### Receive the post

The methods for primary transaction logic are in the `x/blog/keeper/ibc_post.go` file. Use these methods to manage IBC packets:

- `TransmitIbcPostPacket` is called manually to send the packet over IBC. This method also defines the logic before the packet is sent over IBC to another blockchain app.
- `OnRecvIbcPostPacket` hook is automatically called when a packet is received on the chain. This method defines the packet reception logic.
- `OnAcknowledgementIbcPostPacket` hook is called when a sent packet is acknowledged on the source chain. This method defines the logic when the packet has been received.
- `OnTimeoutIbcPostPacket` hook is called when a sent packet times out. This method defines the logic when the packet is not received on the target chain

You must modify the source code to add the logic inside those functions so that the data tables are modified accordingly.

On reception of the post message, create a new post with the title and the content on the receiving chain.

To identify the blockchain app that a message is originating from and who created the message, use an identifier in the following format:

`<portID>-<channelID>-<creatorAddress>`

Finally, the Ignite CLI-generated AppendPost function returns the ID of the new appended post. You can return this value to the source chain through acknowledgment.

Append the type instance as `PostID` on receiving the packet:

- The context `ctx` is an [immutable data structure](https://docs.cosmos.network/main/core/context#go-context-package) that has header data from the transaction. See [how the context is initiated](https://github.com/cosmos/cosmos-sdk/blob/master/types/context.go#L71)
- The identifier format that you defined earlier
- The `title` is the Title of the blog post
- The `content` is the Content of the blog post

In the `x/blog/keeper/ibc_post.go` file, make sure to import `"strconv"` below `"errors"`:

```go
// x/blog/keeper/ibc_post.go
import (
	//...

	"strconv"

	//...
)
```

Then modify the `OnRecvIbcPostPacket` keeper function with the following code:

```go
func (k Keeper) OnRecvIbcPostPacket(ctx sdk.Context, packet channeltypes.Packet, data types.IbcPostPacketData) (packetAck types.IbcPostPacketAck, err error) {
	// validate packet data upon receiving
	if err := data.ValidateBasic(); err != nil {
		return packetAck, err
	}

	id := k.AppendPost(
		ctx,
		types.Post{
			Creator: packet.SourcePort + "-" + packet.SourceChannel + "-" + data.Creator,
			Title:   data.Title,
			Content: data.Content,
		},
	)

	packetAck.PostID = strconv.FormatUint(id, 10)

	return packetAck, nil
}
```

### Receive the post acknowledgement

On the sending blockchain, store a `sentPost` so you know that the post has been received on the target chain.

Store the title and the target to identify the post.

When a packet is scaffolded, the default type for the received acknowledgment data is a type that identifies if the packet treatment has failed. The `Acknowledgement_Error` type is set if `OnRecvIbcPostPacket` returns an error from the packet.

```go
// x/blog/keeper/ibc_post.go
func (k Keeper) OnAcknowledgementIbcPostPacket(ctx sdk.Context, packet channeltypes.Packet, data types.IbcPostPacketData, ack channeltypes.Acknowledgement) error {
	switch dispatchedAck := ack.Response.(type) {
	case *channeltypes.Acknowledgement_Error:
		// We will not treat acknowledgment error in this tutorial
		return nil
	case *channeltypes.Acknowledgement_Result:
		// Decode the packet acknowledgment
		var packetAck types.IbcPostPacketAck

		if err := types.ModuleCdc.UnmarshalJSON(dispatchedAck.Result, &packetAck); err != nil {
			// The counter-party module doesn't implement the correct acknowledgment format
			return errors.New("cannot unmarshal acknowledgment")
		}

		k.AppendSentPost(
			ctx,
			types.SentPost{
				Creator: data.Creator,
				PostID:  packetAck.PostID,
				Title:   data.Title,
				Chain:   packet.DestinationPort + "-" + packet.DestinationChannel,
			},
		)

		return nil
	default:
		return errors.New("the counter-party module does not implement the correct acknowledgment format")
	}
}
```

### Store information about the timed-out packet

Store posts that have not been received by target chains in `timedoutPost` posts. This logic follows the same format as `sentPost`.

```go
// x/blog/keeper/ibc_post.go
func (k Keeper) OnTimeoutIbcPostPacket(ctx sdk.Context, packet channeltypes.Packet, data types.IbcPostPacketData) error {
	k.AppendTimedoutPost(
		ctx,
		types.TimedoutPost{
			Creator: data.Creator,
			Title:   data.Title,
			Chain:   packet.DestinationPort + "-" + packet.DestinationChannel,
		},
	)

	return nil
}

```

This last step completes the basic `blog` module setup. The blockchain is now ready!

## Use the IBC modules

You can now spin up the blockchain and send a blog post from one blockchain app to the other. Multiple terminal windows are required to complete these next steps.

### Test the IBC modules

To test the IBC module, start two blockchain networks on the same machine. Both blockchains use the same source code. Each blockchain has a unique chain ID.

One blockchain is named `earth` and the other blockchain is named `mars`.

The `earth.yml` and `mars.yml` files are required in the project directory:

```yaml
# earth.yml
accounts:
  - name: alice
    coins: ["1000token", "100000000stake"]
  - name: bob
    coins: ["500token", "100000000stake"]
validator:
  name: alice
  staked: "100000000stake"
faucet:
  name: bob
  coins: ["5token", "100000stake"]
genesis:
  chain_id: "earth"
init:
  home: "$HOME/.earth"
```

```yaml
# mars.yml
accounts:
  - name: alice
    coins: ["1000token", "1000000000stake"]
  - name: bob
    coins: ["500token", "100000000stake"]
validator:
  name: alice
  staked: "100000000stake"
faucet:
  host: ":4501"
  name: bob
  coins: ["5token", "100000stake"]
host:
  rpc: ":26659"
  p2p: ":26658"
  prof: ":6061"
  grpc: ":9092"
  grpc-web: ":9093"
  api: ":1318"
genesis:
  chain_id: "mars"
init:
  home: "$HOME/.mars"
```

Open a terminal window and run the following command to start the `earth` blockchain:

```bash
ignite chain serve -c earth.yml
```

Open a different terminal window and run the following command to start the `mars` blockchain:

```bash
ignite chain serve -c mars.yml
```

### Remove Existing Relayer and Ignite CLI Configurations

If you previously used the relayer, follow these steps to remove exiting relayer and Ignite CLI configurations:

- Stop your blockchains and delete previous configuration files:

    ```bash
    rm -rf ~/.ignite/relayer
    ```

If existing relayer configurations do not exist, the command returns `no matches found` and no action is taken.

### Configure and start the relayer

First, configure the relayer. Use the Ignite CLI `configure` command with the `--advanced` option:

```bash
ignite relayer configure -a \
  --source-rpc "http://0.0.0.0:26657" \
  --source-faucet "http://0.0.0.0:4500" \
  --source-port "blog" \
  --source-version "blog-1" \
  --source-gasprice "0.0000025stake" \
  --source-prefix "cosmos" \
  --source-gaslimit 300000 \
  --target-rpc "http://0.0.0.0:26659" \
  --target-faucet "http://0.0.0.0:4501" \
  --target-port "blog" \
  --target-version "blog-1" \
  --target-gasprice "0.0000025stake" \
  --target-prefix "cosmos" \
  --target-gaslimit 300000
```

When prompted, press Enter to accept the default values for `Source Account` and `Target Account`. 

The output looks like:

```
---------------------------------------------
Setting up chains
---------------------------------------------

🔐  Account on "source" is "cosmos1xcxgzq75yrxzd0tu2kwmwajv7j550dkj7m00za"

 |· received coins from a faucet
 |· (balance: 100000stake,5token)

🔐  Account on "target" is "cosmos1nxg8e4mfp5v7sea6ez23a65rvy0j59kayqr8cx"

 |· received coins from a faucet
 |· (balance: 100000stake,5token)

⛓  Configured chains: earth-mars
```

In a new terminal window, start the relayer process:

```bash
ignite relayer connect
```

Results:

```
------
Paths
------

earth-mars:
    earth > (port: blog) (channel: channel-0)
    mars  > (port: blog) (channel: channel-0)

------
Listening and relaying packets between chains...
------
```

### Send packets

You can now send packets and verify the received posts:

```bash
planetd tx blog send-ibc-post blog channel-0 "Hello" "Hello Mars, I'm Alice from Earth" --from alice --chain-id earth --home ~/.earth
```

To verify that the post has been received on Mars:

```bash
planetd q blog list-post --node tcp://localhost:26659
```

The packet has been received:

```yaml
Post:
- content: Hello Mars, I'm Alice from Earth
  creator: blog-channel-0-cosmos1aew8dk9cs3uzzgeldatgzvm5ca2k4m98xhy20x
  id: "0"
  title: Hello
pagination:
  next_key: null
  total: "1"
```

To check if the packet has been acknowledged on Earth:

```bash
planetd q blog list-sent-post
```

Output:

```yaml
SentPost:
- chain: blog-channel-0
  creator: cosmos1aew8dk9cs3uzzgeldatgzvm5ca2k4m98xhy20x
  id: "0"
  postID: "0"
  title: Hello
pagination:
  next_key: null
  total: "1"
```

To test timeout, set the timeout time of a packet to 1 nanosecond, verify that the packet is timed out, and check the timed-out posts:

```bash
planetd tx blog send-ibc-post blog channel-0 "Sorry" "Sorry Mars, you will never see this post" --from alice --chain-id earth --home ~/.earth --packet-timeout-timestamp 1
```

Check the timed-out posts:

```bash
planetd q blog list-timedout-post
```

Results:

```yaml
TimedoutPost:
- chain: blog-channel-0
  creator: cosmos1fhpcsxn0g8uask73xpcgwxlfxtuunn3ey5ptjv
  id: "0"
  title: Sorry
pagination:
  next_key: null
  total: "2"
```

You can also send a post from Mars:

```bash
planetd tx blog send-ibc-post blog channel-0 "Hello" "Hello Earth, I'm Alice from Mars" --from alice --chain-id mars --home ~/.mars --node tcp://localhost:26659
```

List post on Earth:

```bash
planetd q blog list-post
```

Results:

```yaml
Post:
- content: Hello Earth, I'm Alice from Mars
  creator: blog-channel-0-cosmos1xtpx43l826348s59au24p22pxg6q248638q2tf
  id: "0"
  title: Hello
pagination:
  next_key: null
  total: "1"
```

## Congratulations 🎉

By completing this tutorial, you've learned to use the Inter-Blockchain Communication protocol (IBC).

Here's what you accomplished in this tutorial:

- Built two Hello blockchain apps as IBC modules
- Modified the generated code to add CRUD action logic
- Configured and used the Ignite CLI relayer to connect two blockchains with each other
- Transferred IBC packets from one blockchain to another
