---
title: Chain Interaction Method Group
description: TrUAPI method group for reading chain state and subscribing to changes, behind @parity/product-sdk-chain-client and the typed PAPI API exposed per chain.
categories:
- Apps
- Reference
url: https://docs.polkadot.com/reference/apps/protocol/truapi/chain-interaction/
word_count: 419
token_estimate: 648
version_hash: sha256:e6f10e0e5ef719cccadde1b640a77ce429816cf132bac6762ddc896e30606703
last_updated: '2026-06-16T14:17:44+00:00'
---

# 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`](https://www.npmjs.com/package/@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](/apps/build/read-chain-state/).

!!! warning "Provisional"
    Subscription error semantics (reconnection, missed-events recovery), per-Host differences in how the underlying connection is routed, and the precise set of presets shipped with the SDK are still being finalized.

## Where to Go Next

<div class="grid cards" markdown>

- <span class="badge guide">Guide</span> **Read Chain State**

    ---

    The Product-side how-to: preset and BYOD connection paths, reading storage / constants / account state, multi-chain reads.

    [:octicons-arrow-right-24: Get Started](/apps/build/read-chain-state/)

- <span class="badge learn">Learn</span> **Signing**

    ---

    The signing method group that complements chain reads and lets a Product react to state changes on-chain.

    [:octicons-arrow-right-24: Reference](/reference/apps/protocol/truapi/signing/)

</div>