--- title: Statement Store Subscriptions description: How a subscriber registers a topic filter against the People Chain node — primary topic, optional secondary topic, dedup behavior, and unsubscription. categories: - Apps - Reference url: https://docs.polkadot.com/reference/apps/infrastructure/statement-store/subscriptions/ word_count: 586 token_estimate: 820 version_hash: sha256:62055a454cad743ab43770ab9c5fda25da4e0cd4995f2e588bed32756b1d6b76 last_updated: '2026-06-16T14:17:44+00:00' --- # Subscriptions ## Introduction A Product subscribes to the Statement Store by registering a topic filter with a People Chain node. The node forwards only the statements matching that filter to the Product's callback; everything else on the gossip stream is dropped before it reaches the Product. The filter is the Product's lever — pick it correctly and the Product sees exactly the traffic it wants. The Product SDK wraps this as `client.subscribe(callback, options?)` in `@parity/product-sdk-statement-store`. Underneath, it calls the node's `statement_subscribeStatement` JSON-RPC with the appropriate filter, decodes payloads into the typed `T`, and dedupes by content hash before invoking the callback. ## How Topics Filter Two topics shape every subscription: - **The primary topic**: The Product's identity on the gossip layer. The SDK derives it as `blake2b(appName)` and tags every submission your Product publishes with that primary topic. Subscribers from _other_ Products on the same network see only their own traffic, and your subscribers see only yours — the node filters traffic from other Products at the boundary before it reaches your callback. - **The optional `topic2`**: Scopes a subscription further within your Product's primary topic. Pass it as a room id, a document id, or any other secondary key. The SDK hashes it with Blake2b-256 and adds it to the filter; only statements that match both topics are forwarded. A Product subscribing without `topic2` sees every statement published anywhere in your Product. With `topic2`, it sees only the slice that matches the secondary key. Both shapes are routinely useful — global Product chat needs the full stream; a specific room needs the scoped one. ## What the Callback Receives The subscription delivers each matching statement to your callback decoded into your typed `T` and deduped by channel and content hash. The dedup means: - A statement that propagates to the Product's node via multiple gossip paths fires the callback only once. - A statement replaced on the same channel by a newer one (see [Channels](/reference/apps/infrastructure/statement-store/channels/)) fires the callback for the newer value only. ## Subscription Lifecycle `client.subscribe` returns an `Unsubscribable` whose `unsubscribe()` method tears down the JSON-RPC subscription on the node and stops the callback from receiving further events. Tear subscriptions down when the Product no longer needs them — typically on view unmount or process shutdown — so the node doesn't hold open a filter your Product has stopped reading. ## Initial-State Behavior Some `subscribe` implementations return the current statement pool matching the filter (a backfill of statements that haven't expired yet) plus an ongoing stream of new matches; some return only the ongoing stream. The Product SDK's default returns the current pool plus the ongoing stream, so a subscriber that joins late still sees recently published statements that haven't aged out. !!! warning "Provisional" The exact backfill behavior, the per-node pool size limits visible to subscribers, and the reconnection / missed-events semantics when a Product's node loses gossip connectivity are still being finalized. ## Where to Go Next
- Learn **Channels** --- The last-write-wins primitive built on the same gossip layer — `topic2` filters by scope; channels filter by replacement semantics. [:octicons-arrow-right-24: Reference](/reference/apps/infrastructure/statement-store/channels/) - Learn **Lifecycle** --- Where in the lifecycle a subscription attaches (step 4: gossip propagation) and what the dedup behavior protects against. [:octicons-arrow-right-24: Reference](/reference/apps/infrastructure/statement-store/lifecycle/)