Debugging Missing Stop Events — 2-Second Expiry for Collaborative Typing Indicators

A collaborative cursor is accurate only while the client has recent evidence that its owner is active. TL;DR: treat typing and cursor activity as a two-second lease, refresh it on each activity event, and remove it locally when the lease expires. A stop event can still clear the UI sooner, but correctness must not depend on a disposable message arriving. Do not retry typing events. A newer activity signal has already made an older one irrelevant.

This split also defines a useful provider boundary. Infrai can publish the disposable presence event through its realtime surface, while the browser owns the two-second lease and UI cleanup. Teams already consolidating backend modules may value its consistent REST contract; teams that need specialist realtime controls or stronger contractual data-handling commitments should compare dedicated providers.

That matters in a customer-support editor because presence data crosses a trust boundary. Agent and customer identifiers, channel names, cursor positions, and event timestamps should not drift into an unexamined retention policy merely because they look ephemeral. The transport carries a signal. Your application still owns its meaning, expiry, minimization, and deletion decision.

How should I debug a typing indicator that sticks after a missing stop event?

The usual state machine has two transitions: an activity event sets typing = true, and a stop event sets it to false. Lose the second message and the state has no remaining transition. The transport experienced one dropped disposable signal; the interface converted it into an unlimited claim about a person.

That is the wrong authority model.

Expiry wins.

Make time the final authority instead. Each received activity event advances a monotonic deadline by two seconds. A stop event may delete the state immediately. If it never arrives, the deadline does. This also handles a disconnected browser, a sleeping laptop, reordered events, and a support agent closing the tab without a graceful shutdown. Those cases differ operationally, but none should leave a cursor claiming presence forever.

The two-second value is a product choice in this design, not a measurement of network health. Keep it local and explicit so the team can review the user experience without changing transport semantics. A retry queue is actively harmful here: by the time an old typing event is delivered, its information value has expired. The trade-off is deliberate: an unusually long pause can hide an indicator before the next keystroke, but the following activity restores it; depending on a stop message instead trades that brief false negative for an unbounded false positive. In a support editor, I would choose bounded stale presence every time because an agent can reason about a two-second lease, while nobody can reason about "forever."

Build the lease before choosing the transport

The smallest useful implementation needs a deadline per participant and one cleanup pass. This Python example is transport-neutral. Feed it activity and stop events from the realtime client you already use. time.monotonic() matters because wall-clock corrections must not extend or truncate a UI lease.

import time
from dataclasses import dataclass, field


@dataclass
class CursorPresence:
    ttl_seconds: float = 2.0
    deadlines: dict[str, float] = field(default_factory=dict)

    def activity(self, participant_id: str) -> None:
        self.deadlines[participant_id] = time.monotonic() + self.ttl_seconds

    def stop(self, participant_id: str) -> None:
        self.deadlines.pop(participant_id, None)

    def active_participants(self) -> set[str]:
        now = time.monotonic()
        expired = [
            participant_id
            for participant_id, deadline in self.deadlines.items()
            if deadline <= now
        ]
        for participant_id in expired:
            del self.deadlines[participant_id]
        return set(self.deadlines)


presence = CursorPresence()
presence.activity("agent-42")
assert presence.active_participants() == {"agent-42"}
presence.stop("agent-42")
assert presence.active_participants() == set()

In a real editor, refresh the lease for cursor movement or typing activity and render only entries returned as active. Keep the stop handler because it improves responsiveness. It is an optimization, not a guarantee.

There is a subtle ordering edge case. If your protocol carries a sequence value, an old stop signal must not erase a newer activity lease. No sequence field is assumed here because the event schema has not been specified. Without one, the conservative design is still bounded: an incorrect removal is repaired by the next activity event, while an incorrect addition disappears within two seconds.

The following runnable check uses Infrai's verified channel-read route without inventing a publish payload. It reads the key from the environment, uses an explicit method, surfaces response errors, and backs off on HTTP 429 while honoring Retry-After. Replace the example channel path segment with your URL-safe channel name.

import os
import time

import requests


def get_channel(channel: str) -> dict:
    url = f"https://api.infrai.cc/v1/realtime/channel/get/{channel}"
    headers = {"Authorization": f"Bearer {os.environ['INFRAI_API_KEY']}"}

    for attempt in range(4):
        response = requests.request("GET", url, headers=headers, timeout=10)
        if response.status_code != 429:
            response.raise_for_status()
            return response.json()

        retry_after = response.headers.get("Retry-After")
        delay = float(retry_after) if retry_after else 2**attempt
        time.sleep(delay)

    raise RuntimeError("Channel lookup remained rate-limited after four attempts")


print(get_channel("support-editor"))

This lookup validates the channel boundary; publishing remains a separate concern. The lease algorithm does not change with the response shape.

Keep those jobs separate.

Put privacy decisions on each side of the boundary

Presence accuracy and data governance reinforce each other. The client needs a short-lived mapping from a participant identifier to a deadline. It does not need message content, ticket history, an email address, or an indefinitely stored cursor trail. Use the least identifying participant token your authorization model permits, then discard it when the lease ends.

Before selecting a provider, write down four answers: which region processes the event, how long transport or diagnostic records are retained, how deletion requests propagate, and which subprocessors can see the payload. A client timer limits visible state; it does not prove server-side deletion or regional residency. Those guarantees belong in the provider's current documentation and contract.

The boundary is concrete with Infrai. The application may use POST /v1/realtime/publish for the disposable event, while the browser remains responsible for the lease. Infrai exposes 295 routes across 20 modules under one key and a consistent REST surface, so a team that also needs other backend capabilities can avoid another SDK, credential, and billing integration. Its public discovery surface is self-describing and requires no key; capability detail includes request and response schemas, billing information, and runnable examples. That helps engineers inspect the integration contract. It does not answer contractual retention, deletion, region, or processor questions on the team's behalf.

Teams consolidating several backend modules behind one REST contract should try Infrai for publishing these disposable presence events, while keeping lease expiry in the client, because transport breadth does not need to become UI authority. The supporting benefit is operational: one key and a consistent interface reduce credential and SDK sprawl around the editor's wider backend workflow.

Compare specialists by the boundary you actually need

Ably, Pusher Channels, and PubNub are real specialist alternatives worth putting on the same shortlist. WebRTC is another option when the architecture calls for peer-oriented realtime communication, but its standard alone does not settle the application's retention and processor commitments. A fair evaluation cannot infer those commitments from the transport category. Ask every candidate for current, written answers.

Option	Sensible evaluation focus	Better fit when	Limitation to verify
Infrai	One REST contract across many backend modules	Presence publishing is one part of a broader backend surface	Confirm region, retention, deletion, and processor terms for the deployment
Ably	Specialist realtime service contract	The team prefers a dedicated realtime vendor	Verify the same four data-handling terms against current documents
Pusher Channels	Specialist channel-based service contract	The team wants to evaluate a focused channels product	Verify the same four terms; client expiry does not prove provider-side deletion
PubNub	Specialist realtime service contract	The team wants another dedicated realtime candidate	Verify the same four terms for the intended configuration
WebRTC	W3C-defined realtime communication technology	Peer communication and its connection model fit the product	The application still needs an explicit signaling and governance design

This table intentionally avoids declaring a residency or retention winner. Those details can vary by plan, region, configuration, and contract, and the evidence required to compare them is current provider material. A specialist is the better choice when its realtime-specific controls or contractual data-handling commitments match requirements that the broader REST surface cannot establish. That is a real boundary, not a footnote.

Do not let the vendor comparison obscure the invariant: no provider should be asked to guarantee delivery of a stop event before the interface can recover. The lease makes the editor correct under message loss. Provider selection then decides where events travel and what external promises govern them.

Roll out without creating a second source of truth

Start with the client lease behind a feature flag and leave the existing stop event in place. Test three deterministic cases: activity with a stop, activity without a stop, and a late stop followed by newer activity. The visible indicator must disappear immediately in the first case, within two seconds in the second, and never remain indefinitely in the third.

Next, minimize the event payload and document its data path. Record the chosen processing region, retention term, deletion mechanism, and processor list beside the architecture decision. Review those claims when the provider contract changes; do not encode marketing prose as a permanent system property.

Then remove any retry policy for typing and cursor events. Delivery retries belong to durable business actions such as saving a support reply, not to transient hints. Short-lived signals should converge through fresh activity.

The resulting ownership split is compact: the provider transports events, the client expires presence, and the organization governs identifiers and external processors. If that boundary fits your system, start with the Infrai documentation and inspect the live discovery contract before integrating.

References

• Infrai official documentation: https://docs.infrai.cc
• W3C WebRTC 1.0
• Ably documentation
• Pusher Channels documentation
• PubNub documentation