Keep client state current with server changes using subscriptions, deltas, and resync, without drift or memory growth. Use when a UI must reflect data that other users or systems are changing.
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---
name: realtime-data-sync
description: Keep client state current with server changes using subscriptions, deltas, and resync, without drift or memory growth. Use when a UI must reflect data that other users or systems are changing.
---
# Realtime data sync
Most realtime features are not collaborative editing but live data: a
dashboard, a list, a status that must not go stale. The failure mode is
quiet drift, where the client believes something the server stopped
believing an hour ago.
## Method
1. **Subscribe narrowly.** Push only what this client is displaying,
because broad subscriptions waste bandwidth and leak data the user
should not receive (see realtime-permissions).
2. **Send deltas with a sequence number.** Changes rather than whole
payloads, numbered so the client can detect a gap rather than
silently missing one.
3. **Resync on gap or reconnect.** Detecting a missed sequence triggers
a full refresh of that resource, which is the only reliable recovery
from divergence.
4. **Reconcile push with fetch.** Data arriving by subscription and by
request must merge under one rule, or the two paths overwrite each
other in ways that depend on timing.
5. **Bound what the client holds.** Live subscriptions accumulate state
in long-lived sessions, so eviction is part of the design rather than
an afterthought.
6. **Coalesce high-frequency updates.** Rendering every change to a fast
moving value wastes the frame budget; batch to the refresh rate the
user can perceive (see batching-and-debouncing).
7. **Show staleness when the connection drops.** An interface that looks
live while disconnected is actively misleading, particularly for
operational data.
## Boundaries
- Push keeps data fresh; it does not guarantee delivery, so periodic
reconciliation remains necessary.
- Realtime for data that changes rarely is usually unjustified
complexity over simple polling.
- Ordering guarantees come from your protocol rather than the transport
(see delivery-guarantees).
