Choose ordering and retry settings by first defining what “in order” means for your chat: a sender’s own sequence, one shared sequence within a room, or a canonical order across regions. Then decide whether your system should favor avoiding message loss or avoiding duplicates. A publish timeout can leave the outcome unknown, so reliable designs pair bounded retries with stable event IDs, duplicate-safe consumers, and a separate replay path for subscribers that disconnect.
Start by defining the ordering guarantee your product needs
“Messages arrive in order” can mean several different things. Write down the invariant the user should observe before choosing a provider setting or retry schedule.
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- Per sender: a user’s own messages appear in the order they sent them.
- Per conversation: everyone in one room sees the same sequence, including a deterministic relationship between messages, edits, deletes, and reactions.
- Across regions: clients connected in different regions eventually agree on one canonical history.
- Per subscriber: each connected client processes events in its own arrival order, without requiring a shared order across clients.
Prefer the narrowest scope that preserves the product’s meaning. A global sequencer or cross-region coordination can add complexity and latency. A timestamp alone is not a dependable causal order: clocks can differ, simultaneous events need a tie-breaker, and the service’s acceptance order can differ from client send time.
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Understand what a vendor’s ordering guarantee covers
Ably distinguishes regional realtime order from canonical global ordering (CGO). Near-simultaneous publications from different regions can be observed in different orders by subscribers in those regions, while relevant history requests use canonical ordering. Ably also documents a rare recovery edge case: order may not be maintained if the server holding connection state is recycled during recovery. See Ably’s message-ordering documentation.
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For Ably Chat, messages and update events are delivered to clients connected to a particular region in the order that region receives them. Each message also has a lexicographically sortable serial that can support deterministic sorting; that does not mean observed local order always matches global time-based order. See Ably Chat messages.
PubNub assigns a server-side timetoken when it accepts a message. History is ordered by timetoken within a channel, but live arrival can differ from timetoken order and between subscribers. Do not infer a global order across separate channels from one channel’s sequence. See PubNub’s publishing documentation and pub/sub overview.
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Choose what matters more: avoiding loss or avoiding duplicates
At-most-once delivery avoids redelivery but can lose an event. At-least-once delivery favors eventual delivery but allows duplicates. “Exactly once” is an end-to-end property: broker deduplication alone cannot guarantee exactly-once side effects if a handler or external system can apply the same operation twice. Ably explains the trade-offs in its idempotency documentation.
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A timeout does not prove that a publish failed. The service may have accepted the event while the acknowledgment was lost. Retrying can therefore publish the same logical event again. Treat that result as outcome unknown, not automatically as “not accepted.”
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- Create a stable event ID or idempotency key before the first publish attempt, and reuse it for every retry of that logical event.
- Retry transient failures with bounded backoff and a maximum attempt count or event age that fits the product’s latency budget. There is no universal timing schedule.
- Make consumers idempotent: record processed event IDs alongside their effects when possible, so receiving the same logical event again does not apply the effect twice.
- If silent loss would be worse than delay, retain an outbox entry or surface a failure for recovery instead of discarding the event.
PubNub documents that its SDKs retry subscribe operations by default, not publish operations; applications decide whether to retry failed publishes. A retry after an ambiguous failure can produce a distinct duplicate message because separate publishes are not deduplicated by payload. The REST-only qos=1 option has its own documented semantics and is not exposed by PubNub SDKs. Consult the current PubNub publish contract for the API and SDK you use.
Recover subscribers separately from publishing
Publish retry tries to get an outgoing event accepted. Subscriber recovery catches a client up after a disconnect or gap. Treat them as separate mechanisms.
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- Persist the last event the application successfully processed—not just the last event the transport delivered—when correctness depends on replay.
- After reconnecting or detecting a sequence gap, resume from the saved cursor or retrieve persisted history.
- Page through missed events and deduplicate replay against event IDs already applied.
- Monitor gaps and duplicates so the application can detect when recovery is incomplete.
PubNub describes automatic replay for short gaps and history retrieval for longer gaps when persistence is enabled. Retention and buffer limits depend on service configuration, so check the selected account and SDK. Its publishing documentation describes the recovery behavior.
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Ably documents connection-state recovery and ordered replay under ordinary recovery, with the rare server-state-recycle edge case described above. If maintaining strict order after every disconnection is a hard invariant, validate the recovery contract for the actual SDK and consider application-level reordering using the appropriate sequence metadata. See Ably’s ordering documentation.
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Compare options against the same decision criteria
| Decision axis | Questions to answer |
|---|---|
| Ordering scope | Is the guarantee per publisher, channel, connection, region, or canonical global history? What happens with concurrent publishers? |
| Identifier and tie-break | Is there a sequence, serial, or timetoken? Is it sortable and unique within the scope your application needs? |
| Publish acknowledgment | Does success mean accepted by the broker, placed in connected subscribers’ buffers, or processed by consumers? |
| Retry policy | Which API or SDK retries automatically? Which errors are retryable? What happens after an ambiguous timeout? |
| Idempotency | Can the broker deduplicate a repeated logical publish? Do consumers need a stable key and durable processed-ID store? |
| Recovery | Is missed data replayed automatically, fetched from history, or unavailable? Which cursor must the client persist? |
| Product trade-off | Is a visible duplicate worse than a lost or delayed event? Can the UI reconcile optimistic sends? |
Keep vendor examples specific to their APIs
| Service or API | Documented behavior | What not to assume |
|---|---|---|
| Ably Realtime | Documentation distinguishes regional realtime order from canonical global ordering. Separate REST requests at high rates can arrive out of order; the documentation suggests rate limiting, batching, or Realtime clients when ordering matters. Recovery has a documented rare edge case. Details. | Do not assume regional receipt order is a single global order or that recovery preserves strict ordering under every edge case. |
| Ably Chat | Messages and update events are ordered for clients connected to a particular region by that region’s receipt order; sortable serials support deterministic sorting. Details. | Do not equate the local sequence with global time-based order. |
| PubNub | Messages receive server-side channel timetokens. Live delivery is at-most-once by default on a stable connection; reconnect replay can repeat an item, while disconnection or buffer limits can result in missed messages. Publish retry decisions belong to the application, and persistence supports catch-up. Details. | Do not assume payload-based deduplication, identical live arrival order for subscribers, or one channel’s ordering applies across channels. |
| Slack Events API | For inbound webhook events, Slack documents up to three retries at nearly immediate, one-minute, and five-minute intervals, with retry attempt and reason headers. Details. | This is an API-specific webhook delivery contract, not a recommended retry setting for chat message publication generally. |
Use this implementation checklist
- State the ordering invariant and its scope in product terms.
- Use a stable event ID across retries and replays.
- Set a bounded retry policy based on the event’s latency budget and the consequences of loss.
- Make consumers and external side effects idempotent where possible.
- Persist the application’s processed-event cursor and define a replay or history path.
- Verify the exact service, API, SDK version, persistence configuration, and recovery contract before relying on a guarantee.
- Instrument publish timeouts, retry attempts, duplicates, sequence gaps, and replay outcomes.
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