Simulate parachains in a test network

⚠️ Update Notice:

Please read Substrate to Polkadot SDK page first.

You can use the zombienet command-line tool to set up a local test network to simulate a relay chain with validators and parachain collator nodes. You can configure the test network to include multiple validators and parachains with multiple collators.

This tutorial illustrates how to set up a basic test network with the following configuration:

Four validators
Two parachains
One collator per parachain
One message passing channel that enables the parachains to exchange messages

Prepare a working folder with the binaries

The zombienet command-line interface relies on a configuration file to specify the characteristics of the test network, including the name and location of the binaries, Docker image, or Kubernetes deployment to use.

This tutorial illustrates how to configure a test network that uses the native relay chain and collator binaries, so the first step in setting up your test network is to prepare a working folder with the binaries you'll need.

To prepare a working folder with the binaries for the test network:

Open a new terminal shell on your computer, if needed.
Change to your home directory and create a new folder to hold the binaries required to generate a test network.

For example:
```
mkdir binaries
```
If you’re setting up the test network on Linux, you can download the Polkadot binary from Releases into your working folder. If you’re setting up the test network on macOS or want to compile the binary yourself, continue to the next step.
Clone the Polkadot repository by running a command similar to the following:
```
git clone https://github.com/paritytech/polkadot-sdk
```
Change to the root of the polkadot directory by running the following command:
```
cd polkadot
```
Checkout the latest release of Polkadot.

Release branches use the naming convention release-v<n.n.n>. For example, the release branch used in this tutorial is release-v1.0.0 which is in master branch. You can check out a more recent release branch instead of using git checkout release-v1.1.0. You can find information about recent releases and what's included in each release on the Releases tab.
Compile the relay chain node by running the following command:
```
cargo build --release
```
Copy the Polkadot binary into your working binaries folder by running a command similar to the following:
```
cp ./target/release/polkadot ../binaries/polkadot-v1.0.0
```
As this example illustrates, it's generally a good practice to append the version of polkadot to the binary name to keep the files in the binaries folder organized.
Change to your home directory.

Add the parachain binary

Your working folder now has the binary for the relay chain, but you also need the binary for the parachain collator nodes. You can add the parachain collator binary to your working folder by cloning the substrate-parachain-template repository. By default, compiling the substrate-parachain-template creates a parachain collator binary that is configured with the paraId 1000. You can use this paraId for the first parachain in the test network.

To add the parachain collator binary to the working folder:

Clone the substrate-parachain-template repository by running the following command:

git clone https://github.com/substrate-developer-hub/substrate-parachain-template

Change to the root of the parachain template directory by running the following command:
```
cd substrate-parachain-template
```
Check out the release branch that matches the release branch you used to configure the relay chain.

For example:
```
git checkout polkadot-v1.0.0
```
Compile the parachain template collator by running the following command:
```
cargo build --release
```
You now have a parachain collator binary for paraId 1000.
Copy the parachain binary into your working binaries folder by running a command similar to the following:
```
cp ./target/release/parachain-template-node ../binaries/parachain-template-node-v1.0.0
```
In this example, your working directory is$HOME/binaries or ~/binaries so you must navigate up a directory level from your current substrate-parachain-template root directory. As this example illustrates, it's generally a good practice to append the version to the binary name to keep the files in the binaries folder organized.

Configure the test network settings

Now that you have the binaries you need in a working folder, you are ready to configure the settings for the test network that Zombienet will use.

To download and configure Zombienet:

Download the appropriate Zombienet executable for the Linux or macOS operating system.

Depending on your security settings, you might need to explicitly allow access to the executable.

If you want the executable to be available system-wide, run commands similar to the following after downloading the executable:
```
chmod +x zombienet-macos
cp zombienet-macos /usr/local/bin
```
Verify that Zombienet is installed correctly by running the following command:
```
./zombienet-macos --help
```
If command-line help is displayed, the Zombienet is ready to configure.

Create a configuration file for Zombienet by running the following command:

touch config.toml

You are going to use the configuration file to specify the following information:

Location of the binaries for the test network.
The relay chain specification—rococo-local—to use.
Information about the four relay chain validators.
Identifiers for parachains included in the test network.
Information about the collators for each parachains.
WebSocket endpoint ports to use to connect to each node.

For example:

[relaychain]

default_command = "../binaries/polkadot-v1.0.0"
default_args = [ "-lparachain=debug" ]

chain = "rococo-local"

  [[relaychain.nodes]]
  name = "alice"

  [[relaychain.nodes]]
  name = "bob"

  [[relaychain.nodes]]
  name = "charlie"

  [[relaychain.nodes]]
  name = "dave"

[[parachains]]
id = 1000
cumulus_based = true

  [parachains.collator]
  name = "parachain-A-1000-collator01"
  command = "../binaries/parachain-template-node-v1.0.0"

[[parachains]]
id = 1001
cumulus_based = true

  [parachains.collator]
  name = "parachain-B-1001-collator01"
  command = "../binaries/parachain-template-node-v1.0.0"

Save your changes and close the file.
Start the test network using this configuration file by running a command similar to the following:
```
./zombienet-macos spawn config.toml -p native
```
The command displays information about the test network nodes being started.

Take note of the relay chain and parachain node endpoints. For example, the direct link to the relay chain endpoints should look similar to the following:
The direct link to the parachain collator endpoints should look similar to the following:
- parachain-1000-collator: https://polkadot.js.org/apps/?rpc=ws://127.0.0.1:52206#/explorer
- parachain-1001-collator: https://polkadot.js.org/apps/?rpc=ws://127.0.0.1:52210#/explorer
After all of the nodes are running, you can interact with your nodes by opening the Polkadot/Substrate Portal and connecting to any of the node endpoints.

Open a message passing channel

Now that you have your test network up, you can open horizontal relay message passing channels to enable communication between parachain A (1000) and parachain B (1001). Because channels are unidirectional, you need to

Send a request to open channel from parachain A (1000) to parachain B (1001).
Accept the request on parachain B (1001).
Send a request to open channel from parachain B (1001) to parachain A (1000).
Accept the request on parachain A (1000).

Zombienet simplifies opening these channels by enabling you to include basic channel settings in the configuration file for testing purposes.

To set up communication between the parachains in the test network:

Open the config.toml file in a text editor.
Add channel information similar to the following to the configuration file:
```
[[hrmp_channels]]
sender = 1000
recipient = 1001
max_capacity = 8
max_message_size = 8000

[[hrmp_channels]]
sender = 1001
recipient = 1000
max_capacity = 8
max_message_size = 8000
```
Note that the values you set for max_capacity and maxmessagesize shouldn't exceed the values defined for the hrmpChannelMaxCapacity and hrmpChannelMaxMessageSize parameters for the relay chain.

To check the configuration settings for the current relay chain using the Polkadot/Substrate Portal:
- Click Developer and select Chain State.
- Select configuration, then select activeConfig().
- Check the following parameter values:
```
hrmpChannelMaxCapacity: 8
hrmpChannelMaxTotalSize: 8,192
hrmlChannelMaxMessageSize: 1,048,576
```
Save your changes and close the file.
Restart Zombienet by running the following command:
```
./zombienet-macos spawn config.toml -p native
```
You now have a test network with a bidirectional HRMP channel open between the parachains A (1000) and parachain B (1001).

You can use the Polkadot/Substrate Portal to connect to the parachains and send messages.
Click Developer and select Extrinsics.
Select polkadotXcm or xcmPallet, then select sent(dest, message) to craft the XCM messages you want to send.

You should note that XCM messages are like other transactions and require the sender to pay for the execution of the operation. All of the information required must be included in the message itself. For information about how to craft messages using XCM after you've opened HRMP channels, see Cross-consensus communication and Transfer assets with XCM.

Where to go next

For a more complex preconfigured environment that uses Zombienet, download and explore the Trappist playground. For more information about the properties you can set in the configuration file, see the Network definition specification.