05Design patternsPushing real-time updates
05 · Reusable pattern
Pushing real-time updates
Choose WebSockets, server-sent events, long polling, or push notifications by delivery need.Lesson spine
What you need to understand.
Real-time delivery combines a durable source of change with connection ownership, fan-out, and a repair path.
Polling, long polling, SSE, WebSockets
Choose by directionality, update frequency, browser support, proxy behavior, and reconnect needs.
Connection tier
Scale on concurrent sockets and egress, keep gateways mostly stateless, and map users or topics to the gateways that own them.
Broker fan-out
Publish committed events to topic partitions; gateways subscribe only to topics needed by their local connections.
Sequence and replay
Give events ordered IDs within a stream so reconnecting clients can request everything after their last cursor.
Presence
Treat heartbeats and cursors as ephemeral state with TTLs; do not mix them with durable document or message truth.
Adaptive delivery
Push while active, fall back to notification or polling when offline, and collapse low-value updates under pressure.
Before the boxes
Frame the decision.
What must work
Choose WebSockets, server-sent events, long polling, or push notifications by delivery need.
What changes the design
Connections · updates/s · fan-out · message size · reconnect · delay
What owns the truth
Identify the component that commits authoritative state, then separate synchronous confirmation from derived work.
What stays simple
Do not add global coordination, multi-region writes, or a specialized store until a requirement earns the complexity.
Architecture map
Trace ownership, not just traffic.
Follow the decision from left to right. Every arrow should have a reason.
Commits change
Fans out by topic
Owns sessions
Maps users
Receives sequence
Repairs gaps
Wakes offline devices
Walk one representative request across every arrow. Say whether the handoff is synchronous or asynchronous, what identity makes a retry safe, and which step changes authoritative state.
- 01
Publisher — Commits change Define the output contract before moving to the next owner.
- 02
Event bus — Fans out by topic Define the output contract before moving to the next owner.
- 03
Connection gateway — Owns sessions Define the output contract before moving to the next owner.
- 04
Presence registry — Maps users Define the output contract before moving to the next owner.
- 05
Client — Receives sequence Define the output contract before moving to the next owner.
- 06
Sync API — Repairs gaps Define the output contract before moving to the next owner.
- 07
Push provider — Wakes offline devices Confirm the result and emit the evidence needed to reconcile it.
Decision table
Make the trade-offs explicit.
| Decision | Defensible position | Cost to acknowledge |
|---|---|---|
| Primary mechanism | WebSockets earn bidirectionality; SSE simplifies server-to-client streams. | The stronger guarantee usually adds coordination, latency, state, or operational work. |
| Sync vs. async | Keep only correctness-critical confirmation synchronous. Move derived views, notifications, analytics, and cleanup behind a durable boundary. | Async work needs idempotency, lag monitoring, replay, and a product definition for partial completion. |
| Simple vs. scaled | Begin with one logical owner and a clear API. Partition or replicate only the resource proven to be the first bottleneck. | Migration requires stable identities, versioned contracts, backfill, and a rollback path. |
Failure review
Design the recovery path.
Topic-specific risk
Reconnect can create missed, duplicate, or reordered updates.
ResponsePersist enough identity and state to distinguish retry, resume, compensation, and operator repair.
Dependency timeout
A timeout is ambiguous: the remote side may have failed, succeeded, or still be running.
ResponseUse deadlines, bounded backoff with jitter, idempotency keys, and a status or reconciliation path.
Overload or skew
Average capacity can look healthy while a tenant, key, partition, region, or expensive request saturates one owner.
ResponseExpose queue depth and hot-key share, apply backpressure, isolate tenants, and degrade optional work before correctness.
Evidence + level bar
Prove the design can be operated.
Health of the promise
Measure user-visible latency or freshness, correctness drift, saturation, retry volume, and time to recover. Alert on the failed promise—not only CPU.
Complete and clear
Finish the happy path, identify the state owner, choose reasonable building blocks, and explain one scale mechanism.
Trade-offs and failure
Separate read and write paths, define consistency, explain partitioning, and make duplicate or partial failure safe.
Evolution and operations
Discuss multi-region boundaries, migration, tenant isolation, capacity, observability, and how the architecture changes over time.
Interview language
Open the deep dive with a claim.
“For Pushing real-time updates, the decision I want to make explicit is this: WebSockets earn bidirectionality; SSE simplifies server-to-client streams. I’ll trace the state-changing path first, show where the result becomes durable, then test the design against the highest-risk failure and our target scale.”
08 · Retrieval check
Can you defend it without the page?
- For Pushing real-time updates, where is the correctness boundary and which failure would you test first?
- Which component owns committed truth, and what event or response proves the commit?
- Where is the first scaling or coordination bottleneck under the stated envelope?
- What happens after an ambiguous timeout or duplicate operation?
- Which complexity would you remove at one hundredth of the scale?