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substrate-api-client

substrate-api-client a library written in Rust for connecting to the substrate's RPC interface via WebSockets allowing to

  • Compose extrinsics, send them and subscribe to updates (synchronously).
  • supports no_std builds. Only the rpc-client is std only. For no_std builds, a custom rpc client needs to be implemented.
  • Watch events and execute code upon events.
  • Parse and print the node metadata.

Prerequisites

In order to build the substrate-api-client and the examples, Rust and the wasm target are needed. For Linux:

curl https://sh.rustup.rs -sSf | sh
# Install the rust toolchain specified in rust-toolchain.toml
rustup show

Substrate node

To execute the examples, a running substrate node is needed. You can download a node artifact from substrate directly: https://github.com/paritytech/substrate or run the kitchensink-node with docker:

docker run -p 9944:9944 -p 9933:9933 -p 30333:30333 parity/substrate:latest --dev --ws-external --rpc-external

For more information, please refer to the substrate repository.

Examples

To run an example, clone the substrate-api-client repository and run the desired example directly with the cargo command:

git clone https://github.com/scs/substrate-api-client.git
cd substrate-api-client
cargo run -p ac-examples --example get_storage

or download the already built binaries from GitHub Actions and run them without any previous building:

# Add execution rights to the chosen example.
chmod +x <example>
# And run it.
./<example>

Set the output verbosity by prepending RUST_LOG=info or RUST_LOG=debug.

The following examples can be found in the examples folder:

no_std build

Almost everything in the api-client, except for the rpc-clients and a few additional features, is no_std compatible. Many helpful features, such as extrinsic and call creation (see the macros), metadata and event types (see the node-api and primitives) are available in no_std right away. However, to directly connect to a Substrate node a RPC client is necessary. Because websocket connection features are often hardware dependent, a generic no_std RPC client implementation is hardly possible. So for most use cases a self-implemented RPC client is required. To make this as simple as possible, the interface between the Api, which provides all the features, and the RPC client, providing the node connection, is kept very basic. Check out the following explanations for more info.

Import

To import the api-client in no_std make sure the default features are turned off and disable_target_static_assertions is enabled:

# In the Cargo.toml import the api-client as following:
substrate-api-client = { git = "https://github.com/scs/substrate-api-client.git", default-features = false, features = ["disable_target_static_assertions"] }

RPC Client

Depending on the usage, there are two traits that the RPC Client needs to implement.

Request

For simple requests (send one request and receive one answer) the trait Request is required:

/// Trait to be implemented by the ws-client for sending rpc requests and extrinsic.
pub trait Request {
	/// Sends a RPC request to the substrate node and returns the answer as string.
	fn request<R: DeserializeOwned>(&self, method: &str, params: RpcParams) -> Result<R>;
}

By implementing this trait with a custom RPC client, most basic functionalities of the Api can already be used. Currently, there is no no_std example available. But the tungstenite_client provides a relatively simple std example. If a websocket library is available in your no_std environment, then your implementation may look similar.

Subscription

A little more complex is the second trait Subscribe, which does not only send a subscription request to the node, it also keeps listening and updating accordingly. Two traits need to be implemented for this feature. The Subscribe trait itself:

/// Trait to be implemented by the ws-client for subscribing to the substrate node.
pub trait Subscribe {
	type Subscription<Notification>: HandleSubscription<Notification>
	where
		Notification: DeserializeOwned;

	fn subscribe<Notification: DeserializeOwned>(
		&self,
		sub: &str,
		params: RpcParams,
		unsub: &str,
	) -> Result<Self::Subscription<Notification>>;
}

and the HandleSubscription trait, which is returned by the subscribe function:

/// Trait to use the full functionality of jsonrpseee Subscription type
/// without actually enforcing it.
pub trait HandleSubscription<Notification: DeserializeOwned> {
	/// Returns the next notification from the stream.
	/// This may return `None` if the subscription has been terminated,
	/// which may happen if the channel becomes full or is dropped.
	fn next(&mut self) -> Option<Result<Notification>>;

	/// Unsubscribe and consume the subscription.
	fn unsubscribe(self) -> Result<()>;
}

Refering to the std example of the tungstenite, the HandleSubscription impl can be looked up here. It implements a simple channel receiver, waiting for the sender of the websocket client to send something. The Subscribe implementation can be found here.

A more complex RPC client, but also with more functionalities, is the jsonrpsee client.

Alternatives

Parity offers a Rust client with similar functionality: https://github.com/paritytech/subxt

Acknowledgements

The development of substrate-api-client is financed by web3 foundation's grant programme.

We also thank the teams at

Projects using substrate-api-client

If you intend to or are using substrate-api-client, please add your project here

In alphabetical order