Realtime Transport: SSE vs WebSocket vs Long Poll
Select the right transport based on interaction model, reliability needs, and infra constraints.
Pattern level: macro
TL;DR
Choose WebSocket for frequent bidirectional interaction, SSE for simpler server-to-client streams, and long poll as compatibility fallback.
Problem this solves
- Defines latency, ordering, and reconnect behavior for live updates.
- Shapes infra complexity and cost for persistent connections.
Decision criteria
- WebSocket: best for high-frequency duplex traffic.
- SSE: best for one-way updates with simpler infra.
- Long poll: best for environments with strict proxy/network constraints.
Layered depth
Intro
Transport choice is a product decision: latency profile, scale model, and developer ergonomics change together.
Decision criteria
Start with required interaction directionality and update frequency, then evaluate reconnect/replay needs.
Implementation detail
- Define heartbeat and timeout policy per transport.
- Standardize event envelope with sequence IDs across all transports.
- Implement replay from snapshot + cursor to recover after disconnect.
Failure modes
- Silent disconnects create stale UI with no reconnect attempt.
- Sequence gaps after reconnect cause duplicate or missing state.
- Proxy buffering breaks SSE freshness assumptions.
Pitfalls
- Coupling domain events directly to transport protocol frames.
- Ignoring fallback behavior for captive portals and constrained networks.
Testing and accessibility
Testing + Accessibility Rubric
| Category | Level | Requirement | Done When |
|---|---|---|---|
| functional | unit | Incoming events respect sequence and dedupe semantics. | Out-of-order, duplicate, and missing event tests are all covered. |
| functional | integration | Reconnect path correctly hydrates snapshot and applies deltas. | Reconnect test proves no duplicate rows or missing updates. |
| a11y | integration | Live updates are announced without overwhelming screen readers. | Announcement batching and wording pass manual SR checks. |
| a11y | e2e | Focus remains stable when real-time updates occur. | Keyboard-only user can continue primary task uninterrupted. |
Production trade-offs
- WebSocket improves interaction quality but requires stronger connection lifecycle handling.
- SSE simplifies operations but is limited for duplex workloads.