Prepare your first contract

⚠️ Update Notice:

Please read Substrate to Polkadot SDK page first.

As you learned in Blockchain basics decentralized applications are most often written as smart contracts.

Although Substrate is primarily a framework and toolkit for building custom blockchains, it can also provide a platform for smart contracts.

This tutorial demonstrates how to build a basic smart contract to run on a Substrate-based chain.

In this tutorial, you'll explore using ink! as a programming language for writing Rust-based smart contracts.

Before you begin

Before you begin, verify the following:

You have good internet connection and access to a shell terminal on your local computer.
You are generally familiar with software development and using command-line interfaces.
You are generally familiar with blockchains and smart contract platforms.
You have installed Rust and set up your development environment as described in Install.

Tutorial objectives

By completing this tutorial, you will accomplish the following objectives:

Learn how to create a smart contract project.
Build and test a smart contract using the ink! smart contract language.
Deploy a smart contract on a local Substrate node.
Interact with a smart contract using the cargo-contract CLI

Update your Rust environment

For this tutorial, you need to add some Rust source code to your Substrate development environment.

To update your development environment:

Open a terminal shell on your computer.
Update your Rust environment by running the following command:
```
rustup component add rust-src
```
Verify that you have the WebAssembly target installed by running the following command:
```
rustup target add wasm32-unknown-unknown --toolchain nightly
```
If the target is installed and up-to-date, the command displays output similar to the following:
```
info: component 'rust-std' for target 'wasm32-unknown-unknown' is up to date
```

Install `cargo-contract` CLI Tool

cargo-contract is a command-line tool which you will use to build, deploy, and interact with your ink! contracts.

Note that in addition to Rust, installing cargo-contract requires a C++ compiler that supports C++17.

Modern releases of gcc, clang, as well as Visual Studio 2019+ should work.

Add the rust-src compiler component:

rustup component add rust-src

Install the latest version of cargo-contract:

cargo install --force --locked cargo-contract --version 2.0.0-rc

Verify the installation and explore the commands available by running the following command:

cargo contract --help

Install the Substrate Contracts Node

To simplify this tutorial, you can download a precompiled Substrate node for Linux or macOS.

The precompiled binary includes the FRAME pallet for smart contracts by default.

To install the contracts node on macOS or Linux:

Open the Releases page.
Download the appropriate compressed archive for your local computer.
Open the downloaded file and extract the contents to a working directory.

If you can't download the precompiled node, you can compile it locally with a command similar to the following. You can find the latest tag on the Releases page:

cargo install contracts-node --git https://github.com/paritytech/substrate-contracts-node.git --tag <latest-tag> --force --locked

You can find the latest tag to use on the Tags page.

You can verify the installation by running substrate-contracts-node --version.

Create a new smart contract project

You are now ready to start developing a new ink! smart contract project.

To generate the files for an ink! project:

Open a terminal shell on your computer.
Create a new project folder named flipper by running the following command:
```
cargo contract new flipper
```
Change to the new project folder by running the following command:
```
cd flipper/
```

List all of the contents of the directory by running the following command:

ls -al

You should see that the directory contains the following files:

-rwxr-xr-x   1 dev-doc  staff   285 Mar  4 14:49 .gitignore
-rwxr-xr-x   1 dev-doc  staff  1023 Mar  4 14:49 Cargo.toml
-rwxr-xr-x   1 dev-doc  staff  2262 Mar  4 14:49 lib.rs

Like other Rust projects, the Cargo.toml file is used to provide package dependencies and configuration information.

The lib.rs file is used for the smart contract business logic.

Explore the default project files

By default, creating a new ink! project generates some template source code for a very simple contract.

This contract has one function — flip() — that changes a Boolean variable from true to false and a second function — get() — that gets the current value of the Boolean.

The lib.rs file also contains two functions for testing that the contract works as expected.

As you progress through the tutorial, you'll modify different parts of the starter code. By the end of the tutorial, you'll have a more advanced smart contract - See more examples here.

To explore the default project files:

Open a terminal shell on your computer, if needed.
Change to project folder for the flipper smart contract, if needed:
Open the Cargo.toml file in a text editor and review the dependencies for the contract.
Open the lib.rs file in a text editor and review the macros, constructors, and functions defined for the contract.
- The #[ink::contract] macro defines the entry point for your smart contract logic.
- The #[ink(storage) macro defines a structure to stores a single boolean value for the contract.
- The new and default functions initialize the boolean value to false.
- There's a #[ink(message) macro with a flip function to change the state of the data stored for the contract.
- There's a #[ink(message) macro with a get function to get the current state of the data stored for the contract.

Test the default contract

At the bottom of the lib.rs source code file, there are simple test cases to verify the functionality of the contract. These are annotated using the #[ink(test)] macro. You can test whether this code is functioning as expected using the offchain test environment.

To test the contract:

Open a terminal shell on your computer, if needed.
Verify that you are in the flipper project folder, if needed.
Use the test subcommand to execute the default tests for the flipper contract by running the following command:
```
cargo test
```
The command should compile the program and display output similar to the following to indicate successful test completion:
```
running 2 tests
test flipper::tests::it_works ... ok
test flipper::tests::default_works ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```

Build the contract

After testing the default contract, you are ready to compile this project to WebAssembly.

To build the WebAssembly for this smart contract:

Open a terminal shell on your computer, if needed.
Verify that you are in the flipper project folder.
Compile the flipper smart contract by running the following command:
```
cargo contract build
```
This command builds a WebAssembly binary for the flipper project, a metadata file that contains the contract Application Binary Interface (ABI), and a .contract file that you use to deploy the contract.

For example, you should see output similar to the following:
```
Original wasm size: 35.5K, Optimized: 11.9K

The contract was built in DEBUG mode.

Your contract artifacts are ready. You can find them in:
/Users/dev-doc/flipper/target/ink

- flipper.contract (code + metadata)
- flipper.wasm (the contract's code)
- flipper.json (the contract's metadata)
```
The .contract file includes both the business logic and metadata. This is the file that tooling (e.g UIs) expect when you want to deploy your contract on-chain.

The .json file describes all the interfaces that you can use to interact with this contract. This file contains several important sections:
- The spec section includes information about the functions—like constructors and messages—that can be called, the events that are emitted, and any documentation that can be displayed. This section also includes a selector field that contains a 4-byte hash of the function name and is used to route contract calls to the correct functions.
- The storage section defines all the storage items managed by the contract and how to access them.
- The types section provides the custom data types used by the contract.

Start the Substrate Contracts Node

If you have successfully installed the substrate-contracts-node, it's time to start a local node.

Start the contracts node in local development mode by running the following command:

substrate-contracts-node --log info,runtime::contracts=debug 2>&1

The extra logging is useful for development.

You should see output in the terminal similar to the following:

2023-01-30 23:08:49.835  INFO main sc_cli::runner: Substrate Contracts Node
2023-01-30 23:08:49.836  INFO main sc_cli::runner: ✌️  version 0.23.0-87a3d76c880
2023-01-30 23:08:49.836  INFO main sc_cli::runner: ❤️  by Parity Technologies <admin@parity.io>, 2021-2023
2023-01-30 23:08:49.836  INFO main sc_cli::runner: 📋 Chain specification: Development
2023-01-30 23:08:49.836  INFO main sc_cli::runner: 🏷  Node name: profuse-grandmother-6287
2023-01-30 23:08:49.836  INFO main sc_cli::runner: 👤 Role: AUTHORITY
2023-01-30 23:08:49.836  INFO main sc_cli::runner: 💾 Database: ParityDb at /tmp/substrateCu3FVo/chains/dev/paritydb/full
2023-01-30 23:08:49.836  INFO main sc_cli::runner: ⛓  Native runtime: substrate-contracts-node-100 (substrate-contracts-node-1.tx1.au1)
2023-01-30 23:08:54.570  INFO main sc_service::client::client: 🔨 Initializing Genesis block/state (state: 0x27d2…a1d8, header-hash: 0x6a05…1669)
2023-01-30 23:08:54.573  INFO main sub-libp2p: 🏷  Local node identity is: 12D3KooWG4h1FpwAhybzyMxoEGQgY8SbrLb4F5FB6mCBZCY6u7W1
2023-01-30 23:08:58.643  INFO main sc_service::builder: 📦 Highest known block at #0
2023-01-30 23:08:58.643  INFO tokio-runtime-worker substrate_prometheus_endpoint: 〽️ Prometheus exporter started at 127.0.0.1:9615
2023-01-30 23:08:58.644  INFO                 main sc_rpc_server: Running JSON-RPC HTTP server: addr=127.0.0.1:9933, allowed origins=None
2023-01-30 23:08:58.644  INFO                 main sc_rpc_server: Running JSON-RPC WS server: addr=127.0.0.1:9944, allowed origins=None
2023-01-30 23:09:03.645  INFO tokio-runtime-worker substrate: 💤 Idle (0 peers), best: #0 (0x6a05…1669), finalized #0 (0x6a05…1669), ⬇ 0 ⬆ 0
2023-01-30 23:09:08.646  INFO tokio-runtime-worker substrate: 💤 Idle (0 peers), best: #0 (0x6a05…1669), finalized #0 (0x6a05…1669), ⬇ 0 ⬆ 0

Note that no blocks will be produced unless we send an extrinsic to the node. This is because the substrate-contracts-node uses Manual Seal as its consensus engine.

Deploy the contract

At this point, you have completed the following steps:

Installed the packages for local development.
Generated the WebAssembly binary for the flipper smart contract.
Started the local node in development mode.

The next step is to deploy the flipper contract on your Substrate chain.

However, deploying a smart contract on Substrate is a little different than deploying on traditional smart contract platforms.

For most smart contract platforms, you must deploy a completely new blob of the smart contract source code each time you make a change.

For example, the standard ERC20 token has been deployed to Ethereum thousands of times.

Even if a change is minimal or only affects some initial configuration setting, each change requires a full redeployment of the code.

Each smart contract instance consumes blockchain resources equivalent to the full contract source code, even if no code was actually changed.

In Substrate, the contract deployment process is split into two steps:

Upload the contract code to the blockchain.
Create an instance of the contract.

With this pattern, you can store the code for a smart contract like the ERC20 standard on the blockchain once, then instantiate it any number of times.

You don't need to reload the same source code repeatedly, so your smart contract doesn't consume unnecessary resources on the blockchain.

Uploading the ink! Contract Code

For this tutorial, you use the cargo-contract CLI tool to upload and instantiate the flipper contract on a Substrate chain.

Start your node using substrate-contracts-node --log info,runtime::contracts=debug 2>&1
Go to the flipper project folder.
Build the contract using cargo contract build.

Upload and instantiate your contract using:

cargo contract instantiate --constructor new --args "false" --suri //Alice --salt $(date +%s)

Some notes about the command:

The instantiate command will do both the upload and instantiate steps for you.
We need to specify the contract constructor to use, which in this case is new()
We need to specify the argument to the constructor, which in this case is false
We need to specify the account uploading and instantiating the contract, which in this case is the default development account of //Alice
During development we may want to upload the instantiate the same contract multiple times, so we specify a salt using the current time. Note that this is optional.

After running the command confirming that we're happy with the gas estimatation we should see something like this:

Dry-running new (skip with --skip-dry-run)
  Success! Gas required estimated at Weight(ref_time: 328660939, proof_size: 0)
Confirm transaction details: (skip with --skip-confirm)
Constructor new
      Args false
 Gas limit Weight(ref_time: 328660939, proof_size: 0)
Submit? (Y/n):
    Events
     Event Balances ➜ Withdraw
       who: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
       amount: 98.986123μUNIT
     Event System ➜ NewAccount
       account: 5GRAVvuSXx8pCpRUDHzK6S1r2FjadahRQ6NEgAVooQ2bB8r5
     ... snip ...
     Event TransactionPayment ➜ TransactionFeePaid
       who: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
       actual_fee: 98.986123μUNIT
       tip: 0UNIT
     Event System ➜ ExtrinsicSuccess
       dispatch_info: DispatchInfo { weight: Weight { ref_time: 2827629132, proof_size: 0 }, class: Normal, pays_fee: Yes }

  Contract 5GRAVvuSXx8pCpRUDHzK6S1r2FjadahRQ6NEgAVooQ2bB8r5

We will need the Contract address to call the contract, so make sure you don't lose it.

Calling the Deployed ink! Contract

We can not only upload and instantiate contracts using cargo-contract, we can also call them!

`get()` Message

When we initialized the contract we set the initial value of the flipper to false. We can confirm this by calling the get() message.

Since we are only reading from the blockchain state (we're not writing any new data) we can use the --dry-run flag to avoid submitting an extrinsic.

cargo contract call --contract $INSTANTIATED_CONTRACT_ADDRESS --message get --suri //Alice --dry-run

Some notes about the command:

The address of the contract we want to call had to be specified using the --contract flag
This can be found in the output logs of the cargo contract instantiate command
We need to specify the contract message to use, which in this case is get()
We need to specify the account callling the contract, which in this case is the default development account of //Alice
We specify --dry-run to avoid submitting an extrinsic on-chain

After running the command should see something like this:

Result Success!
Reverted false
    Data Tuple(Tuple { ident: Some("Ok"), values: [Bool(false)] })

We're interested in the value here, which is false as expected.

`flip()` Message

The flip() message changes the storage value from false to true and vice versa.

To call the flip() message we will need to submit an extrinsic on-chain because we are altering the state of the blockchain.

To do this we can use the following command:

cargo contract call --contract $INSTANTIATED_CONTRACT_ADDRESS --message flip --suri //Alice

Notice that we changed the message to flip and removed the --dry-run flag.

After running we expect to see something like:

Dry-running flip (skip with --skip-dry-run)
    Success! Gas required estimated at Weight(ref_time: 8013742080, proof_size: 262144)
Confirm transaction details: (skip with --skip-confirm)
     Message flip
        Args
   Gas limit Weight(ref_time: 8013742080, proof_size: 262144)
Submit? (Y/n):
      Events
       Event Balances ➜ Withdraw
         who: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
         amount: 98.974156μUNIT
       Event Contracts ➜ Called
         caller: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
         contract: 5GQwxP5VTVHwJaRpoQsK5Fzs5cERYBzYhgik8SX7VAnvvbZS
       Event TransactionPayment ➜ TransactionFeePaid
         who: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
         actual_fee: 98.974156μUNIT
         tip: 0UNIT
       Event System ➜ ExtrinsicSuccess
         dispatch_info: DispatchInfo { weight: Weight { ref_time: 1410915697, proof_size: 13868 }, class: Normal, pays_fee: Yes }

If we call the get() message again we can see that the storage value was indeed flipped!

Result Success!
Reverted false
    Data Tuple(Tuple { ident: Some("Ok"), values: [Bool(true)] })

Next steps

Congratulations!

In this tutorial, you learned:

How to create a new smart contract project using the ink! smart contract language.
How to test and build a WebAssembly binary for a simple default smart contract.
How to start a working Substrate-based blockchain node using the contracts node.
How to deploy a smart contract by connecting to a local node and uploading and instantiating the contract.
How to interact with a smart contract using the cargo-contract CLI tool.

Additional smart contract tutorials build on what you learned in this tutorial and lead you deeper into different stages of contract development.

You can learn more about smart contract development in the following topics: