@polkadot-api/substrate-client

This TypeScript package provides low-level bindings to the Substrate JSON-RPC Interface, enabling interaction with Substrate-based blockchains.

Usage

Start by creating a SubstrateClient object with the exported function createClient. To create one, you need a ConnectProvider provider defined in @polkadot-api/json-rpc-provider for establishing a connection to a specific blockchain client.

For instance, you can use @polkadot-api/sc-provider to get a substrate-connect provider for connecting to the Polkadot relay chain through a light client:

import { getScProvider, WellKnownChain } from "@polkadot-api/sc-provider"
import { createClient } from "@polkadot-api/substrate-client"

const scProvider = getScProvider()
const { relayChain } = scProvider(WellKnownChain.polkadot)

const client = createClient(relayChain)

Request

Invoke any method defined in the JSON-RPC Spec using client.request(method, params, abortSignal?). This returns a promise resolving with the response from the JSON-RPC server.

const genesisHash = await client.request("chainSpec_v1_genesisHash", [])

All promise-returning functions exported by this package accept an AbortSignal for operation cancellation.

ChainHead

Operations within the chainHead group of functions involve subscriptions and interdependencies between methods. The client has a function that simplifies the interaction with these group.

Calling client.chainHead(withRuntime, onFollowEvent, onFollowError) will start a chainHead_unstable_follow subscription, and will return a handle to perform operations with the chainHead.

const chainHead = client.chainHead(
  true,
  (event) => {
    // ...
  },
  (error) => {
    // ...
  },
)

The handle provides one method per each of the functions defined inside chainHead: chainHead_unstable_body, chainHead_unstable_call, chainHead_unstable_header, chainHead_unstable_storage, and chainHead_unstable_unpin.

The JSON-RPC Spec for chainHead specifies that these functions return an operationId, and that the resolved response for the call will come through the chainHead_unstable_follow subscription, linking it through this operationId.

substrate-client's chainHead is an abstraction over this: The events emitted through the client.chainHead() callback are only the ones initiated from the JSON-RPC Server. The promise returned by any of the chainHead's handle functions will resolve with the respective event.

const chainHead = client.chainHead(
  true,
  async (event) => {
    if (event.type === "newBlock") {
      const body = await chainHead.body(event.blockHash)
      // body is a string[] containing the SCALE-encoded values within the body
      processBody(body)

      chainHead.unpin([event.blockHash])
    }
  },
  (error) => {
    // ...
  },
)

header

Calls chainHead_unstable_call and returns a promise that resolves with the SCALE-encoded header of the block

const header = await chainHead.header(blockHash)

body

Calls chainHead_unstable_body and returns a promise that will resolve with an array of strings containing the SCALE-encoded extrinsics found in the block

const body = await chainHead.body(blockHash)

call

Calls chainHead_unstable_header and returns a promise that resolves with the encoded output of the runtime function call

const result = await chainHead.call(blockHash, fnName, callParameters)

storage

Calls chainHead_unstable_storage and returns a promise that resolves with the value returned by the JSON-RPC server, which depends on the type parameter. See the JSON-RPC spec for chainHead_unstable_storage for the details on the usage.

// string with the SCALE-encoded value
const value = await chainHead.storage(blockHash, "value", key, childTrie)

// string with the hash value
const hash = await chainHead.storage(blockHash, "hash", key, childTrie)

// string with the merkle value
const items = await chainHead.storage(
  blockHash,
  "closestDescendantMerkleValue",
  key,
  childTrie,
)

// array of key-value pairs
const items = await chainHead.storage(
  blockHash,
  "descendantsValues",
  key,
  childTrie,
)

// array of key-hash pairs
const hashes = await chainHead.storage(
  blockHash,
  "descendantsHashes",
  key,
  childTrie,
)

storageSubscription

While storage only can resolve for one specific item, the JSON-RPC specification allows to resolve multiple items within the same call. For this case, substrate-client also offers a lower-level version called chainHead.storageSubscription(hash, inputs, childTrie, onItems, onError, onDone, onDiscardedItems) that emits the storage items as they get resolved by the JSON-RPC server:

const abort = chainHead.storageSubscription(
  hash,
  [
    { key, type },
    /* ... each item */
  ],
  null,
  (items) => {
    // items is an array of { key, value?, hash?, closestDescendantMerkleValue? }
  },
  onError,
  onDone,
  (nDiscardedItems) => {
    // amount of discarded items, as defined by the JSON-RPC spec.
  },
)

storageSubscription returns a function to cancel the operation.

unpin

Calls chainHead_unstable_unpin and returns a promise that will resolve after the operation is done.

chainHead.unpin(blockHashes)

unfollow

To close the chainHead subscription, call chainHead.unfollow().

Transaction

transaction group of functions also deals with subscriptions through submitAndWatch. SubstrateClient also abstracts over this:

const cancelRequest = client.transaction(
  transaction, // SCALE-encoded transaction
  (event) => {
    // ...
  },
  (error) => {
    // ...
  },
)

// call `cancelRequest()` to abort the transaction (`transaction_unstable_stop`)

The event emitted through the callback are fully typed, and can be discriminated through event.type

switch (event.type) {
  case "validated":
    break
  case "broadcasted":
    const { numPeers } = event
    break
  case "bestChainBlockIncluded":
  case "finalized":
    const { block } = event
    break
  case "dropped":
  case "error":
  case "invalid":
    const { error } = event
    break
}

Destroy

Call client.destroy() to disconnect from the provider.