Chain Interaction¶

Introduction¶

The Chain Interaction method group lets a Product read on-chain state, including account balances, storage items, and runtime constants, and subscribe to changes on those values. It is the foundation most Products build on. Reading state is usually the first thing a Product does after lifecycle setup.

The Product SDK wraps this surface as @parity/product-sdk-chain-client, which gives the Product a typed PAPI client per connected chain. The same code works whether the Product runs inside a Host, where calls route through the Host's chain client, or outside one in development, where the client falls back to a direct WebSocket connection.

Conceptual Contract¶

The group exposes the chain-client surface a Product expects, with the Host handling the routing:

• Connecting to one or more chains: A preset path (getChainAPI(env)) returns a ready-to-use client for a known environment. A Bring Your Own Descriptors path (createChainClient) lets the Product compose any set of chains. Both paths produce the same client shape.
• Reading storage: Every connected chain exposes a typed .query surface for storage items and a .constants surface for runtime constants via PAPI's typed descriptors.
• Subscribing to changes: The underlying PAPI client exposes watchValue(address) and similar primitives for live subscriptions. The Host mediates the connection but the subscription surface is the standard PAPI one.
• Reading multiple chains in parallel: Independent connections per chain let reads compose with Promise.all.
• Dropping to the raw PAPI client: .raw exposes the underlying PolkadotClient for advanced flows the typed surface does not cover, such as custom remote procedure call methods, low-level submissions, and raw storage subscriptions.

For the conceptual model and the Product-side how-to, see Read Chain State.

Where to Go Next¶