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# SyncKit SSE Push Notifications |
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SyncKit provides a Server-Sent Events (SSE) endpoint that notifies connected clients in real time when new data is available. Instead of polling, your app can subscribe to a persistent connection and pull only when something changes. |
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## Endpoint |
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``` |
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GET /api/sync/subscribe?app_id={app_id} |
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Authorization: Bearer <token> |
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``` |
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Requires a valid SyncKit JWT token. The `app_id` query parameter specifies which sync app to subscribe to. |
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## Event Format |
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When another device pushes changes, all other connected devices for the same app and user receive a `changed` event: |
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``` |
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event: changed |
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data: |
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``` |
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The event carries no payload (data is always E2E encrypted and never sent over SSE). On receiving `changed`, initiate a pull to fetch actual changes. |
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## Keepalive |
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The server sends a comment line every 30 seconds to keep the connection alive and prevent proxy timeouts: |
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``` |
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: keepalive |
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``` |
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This is a standard SSE comment and is ignored by conforming clients. |
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## Client Integration |
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### Basic Flow |
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1. Authenticate and obtain a SyncKit JWT token |
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2. Open an SSE connection to `/api/sync/subscribe?app_id={app_id}` |
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3. On receiving a `changed` event, call the pull endpoint to fetch new changes |
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4. Reconnect on connection drop (most SSE libraries handle this automatically) |
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### With the Rust SDK |
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The `synckit-client` crate provides `SyncNotifyStream` for SSE integration. `SyncKitClient::subscribe` opens the connection; `next_change()` yields `Some(())` for each `changed` event and `None` when the stream ends: |
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```rust |
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let mut stream = client.subscribe().await?; |
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// In your sync loop: |
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while stream.next_change().await.is_some() { |
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// The server signaled new changes, pull them. |
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let (changes, new_cursor, _has_more) = client.pull(device_id, cursor).await?; |
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cursor = new_cursor; |
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} |
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// next_change() returned None: the stream ended. Reconnect (typically after a |
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// short delay), refreshing the token first if it has expired. |
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``` |
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Keepalive comments are parsed and ignored for you. The stream does **not** auto-reconnect: when `next_change()` returns `None`, the caller reconnects. |
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### Selective Sync |
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Combine SSE notifications with `PullFilter` to pull only specific tables or time ranges: |
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```rust |
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let filter = PullFilter { |
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tables: Some(vec!["tasks".into()]), |
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since: Some(last_sync_time), |
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..Default::default() |
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}; |
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let (changes, new_cursor, has_more) = |
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client.pull_filtered(device_id, cursor, filter).await?; |
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``` |
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This reduces bandwidth and processing when your app only needs a subset of changes. |
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## Design Notes |
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- One SSE connection per (app, user) pair |
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- The broadcast channel buffers up to 16 messages. If a client falls behind, missed events are skipped. The next pull will catch up regardless. |
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- SSE works through standard HTTP proxies and load balancers. The 30-second keepalive stays within common proxy timeout defaults. |
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## See Also |
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- [SyncKit Cloud Sync](./synckit.md): Full sync API reference |
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- [OAuth2 PKCE](./oauth.md): Authentication for SyncKit apps |
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