Channels¶

Introduction¶

pallet-statement-store supports an optional channel field on every statement. When a new submission carries the same channel as an existing live statement from the same account, the pallet replaces the older one in the node's pool — and the replacement propagates to subscribed peers, who drop the old statement in favor of the new one.

The result is a last-write-wins primitive on top of the standard pub/sub: each channel name, scoped per-account, maps to a single live value at any moment.

The Product SDK abstracts this as ChannelStore<T> in @parity/product-sdk-statement-store. The underlying storage primitive is the same — every channel value is still a statement going through the same lifecycle — but the read surface looks like a key-value map keyed by channel name.

When to Reach for Channels¶

Three patterns are the canonical fit:

• Presence indicators: "Alice is online." Only the latest value matters; older states should be replaced as the user goes idle and active again.
• Multiplayer cursors and other live UI state: The user has moved their cursor to position N; the previous position is no longer relevant.
• "Now playing" / live status: Only the current track matters, not the history of what was playing earlier in the session.

For append-only events — chat messages, action logs, social-feed posts — channels are the wrong primitive. Each event needs to live independently; using a channel would erase the previous message every time the user sent a new one. For those use cases, keep using client.publish directly.

Scope and Replacement Rules¶

Channels are scoped per-account. The pallet's replacement rule only matches statements from the same signer — one user cannot overwrite another user's channel value. Two users posting on the same channel name produce two independent live values, one per account.

The replacement is atomic from a subscriber's perspective: the older statement is evicted from the pool and the newer one inserted in the same step. A subscriber sees the new value; the old value, if it was in flight to the subscriber's node, is dropped before it reaches the application callback.

The Channel-Store API¶

The SDK's ChannelStore<T> exposes the standard surface:

• channels.write(name, value): Publish a new value on the named channel. The SDK hashes the channel name with Blake2b-256, attaches the per-account scoping, and submits as a statement.
• channels.read(name): Return the latest value seen on that channel locally.
• channels.readAll(): Return the full map of (channel name → latest value) the local node has seen.
• channels.onChange(callback): Fire on every transition (a new write replacing an older value).

ChannelStore automatically timestamps the value if your T omits a timestamp, so consumers can reason about ordering when needed.

Where to Go Next¶