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🧟 2026-11-04 | Half-Open Sockets and the Resync Table 🔌

🧑‍💻 Author’s Note

👋 Hello! This deep dive is about WebSockets: the ways they fail without telling you, and how Domination learned to stop trusting them. The guide to the series is here.
📱 The motivating report: “Playing a game with my wife, we got to round 48 or so. I turned off my phone, she played, I turned it back on, but it didn’t catch me all the way up to where she was at 50 or so.”
🧰 Two separate socket bugs come up here. Each one silently dropped messages while everything looked fine.

🐛 Bug One: Sending Into a Closed Socket

This one was found while researching the design, before any async play code existed.

• 🔄 The experiment. Two browsers were connected to a local relay. Restarting the relay made both reconnect. Both could still receive, but neither could send.
• 🔍 The cause. It’s a subtle interaction between PureScript’s Aff and the FFI:
  • Initialize stored the socket from the first onopen callback in the app state.
  • On reconnect, the FFI called the same makeAff callback again with the new socket.
  • But Aff ignores a second resume, so the state kept the dead socket.
  • Incoming messages still worked, because the new socket’s onmessage dispatched to the page.
  • Outgoing messages went to the old one.
• 🤫 Why nothing complained. The WebSocket standard says send on a closed socket doesn’t throw. The data is simply discarded. Chrome logged a warning, and that was all.
• 💥 The impact. Every Render deploy, restart or spin-down left every connected player able to read but not write, until they reloaded. That explained much of the original issue: “If a message is missed for any reason, we have to reconnect and manually push the state around.”

🔧 The Fix: A Connection That Outlives Its Sockets

src/FFI/WebSocket.js now keeps one connection object per page:

• 🔗 send always uses whichever socket is live right now.
• 📥 While no socket is open, messages wait in a queue capped at the newest 100, which is flushed on onopen.
• 📣 After a reconnect, the client announces itself again (Join and Username messages), because peers forget clients they haven’t heard from in 15 seconds.
• 🛑 Any close the page didn’t ask for leads to a reconnect, including code 1001 (“going away”), which a server sends when it shuts down. The old code gave up for good on 1001.
• 🚫 The alert() on every socket error is gone. The connection count turns red and shows ”…“.

⏱️ Backoff With Equal Jitter

The pure module Domination.Data.Backoff chooses the delays:

upperBound { base, cap } attempt = min cap (base * 2^attempt)  
delay backoff attempt sample = half + half * sample   -- half = upperBound / 2  

• 📈 The ceiling starts at 1 second and doubles up to 60 seconds.
• 🎲 The delay is drawn from the upper half of the ceiling. That’s “equal jitter” from the AWS Architecture Blog’s Exponential Backoff and Jitter. Clients that lost the relay at the same moment come back spread out, and none of them retries sooner than half the ceiling.
• 🧪 Seven tests, six of them property-based, check it: delays stay in the upper half, never pass the cap, and the ceiling doubles until it reaches the cap.
• ♻️ The same upperBound was reused twice later: for mailbox retries (5 seconds up to 5 minutes) and for the silence backoff below.

🧟 Bug Two: The Socket That Was Dead but Didn’t Know It

🕳️ What a Half-Open Socket Is

• 🔌 A TCP connection is “half-open” when one end has gone away without the other finding out.
• 📱 Phones produce these all the time. The radio sleeps, the network switches from Wi-Fi to cellular, or a NAT forgets the mapping.
• 🤥 The browser’s readyState still says OPEN, so no close event fires and the reconnect logic never runs. But nothing gets through in either direction.
• ⏳ TCP will eventually notice, but its default keepalive timers are measured in hours.

🔍 Why the Phone Fell Behind

v0.40.0 only looked for news on discrete events: starting up, the page becoming visible, the network coming back, the relay reconnecting, and a retry timer. So:

1. 🌅 The phone woke and pulled once, catching up to that moment.
2. 🕳️ Every later move went into its dead socket.
3. 🙈 It never looked again while it stayed visible.
4. 🔁 Its own moves reached the mailbox, but their live frames went into the same dead socket, so the wife’s device never thought to pull them.
5. 🌐 In the room without a mailbox, nothing rescued a half-open socket at all.

📚 What Everyone Else Does

• 💓 Long-lived connections never trust silence. TCP keepalive, WebSocket ping and pong, MQTT’s keep-alive interval, and the heartbeats in Phoenix Channels, Socket.IO and SignalR all close a connection that has been quiet too long.
• 🔄 Clients that mustn’t miss updates also re-read the source of truth. TanStack Query and SWR refetch when a window regains focus and can poll on an interval, and Matrix clients keep calling /sync.
• 💡 Domination already had heartbeats every 5 seconds. A socket that hears nothing for 15 seconds, while other players are around, is dead.

📋 The Resync Table

Instead of scattering when checks across Main, every rule now lives in one pure table, Domination.Data.Resync:

data Occasion = Started | Shown Milliseconds | NetworkBack | RelayBack  
              | PeerJoined | RetryDue | Ticked Clock  
  
data Reaction = Announce | OfferGame | PullMailbox | ReopenRelay  
  
reactions :: Occasion -> Array Reaction  
reactions = case _ of  
  Started -> [ Announce, OfferGame, PullMailbox ]  
  Shown away -> [ PullMailbox ] <> (guard (away >= reopenAfterHidden) $> ReopenRelay)  
  NetworkBack -> [ PullMailbox, ReopenRelay ]  
  RelayBack -> [ Announce, OfferGame, PullMailbox ]  
  PeerJoined -> [ OfferGame ]  
  RetryDue -> [ PullMailbox ]  
  Ticked clock ->  
    (guard (clock.visible && clock.sincePull >= pollInterval) $> PullMailbox)  
      <> (guard (clock.visible && clock.relayOpen && silentTooLong clock) $> ReopenRelay)  

• 🧩 Occasions are the moments a client can notice. Reactions are what it can do. Main just runs traverse_ react (reactions occasion).
• 🧪 Because the table is a plain function, the property-test model and the TLA+ spec can both use the same policy as the real app.
• 🎛️ guard condition $> Reaction yields a one-element or empty array, so the conditional reactions are just concatenated, with no if ladders.

🆕 What v0.41.0 Added

• ⏰ Poll while visible. Every 30 seconds, a visible page asks the mailbox. A conditional GET that answers 304 costs almost nothing.
• 🔇 Replace silent sockets. A visible page whose socket the browser thinks is open, but which has heard nothing for 15 seconds, drops it and opens a new one. Each silent reopen doubles the allowed silence, up to 2 minutes, so a page that’s alone in its room doesn’t churn. Hearing anything resets it.
• 🌅 Reopen after a long nap. A page shown after 30 seconds or more hidden assumes its socket is probably dead and reopens at once. This is only a speed-up. The TLA+ check showed correctness doesn’t depend on it.
• 📶 Reopen when the network comes back.
• 😴 Hidden pages do nothing on a heartbeat. They catch up when shown.

🔁 Reopening Safely

connection.reopen in the FFI:

• 🧹 detaches the old socket’s handlers first, so its late close neither reports a disconnection nor schedules a second socket;
• 👋 closes it with the application code 4000 (“reopen”);
• ⚡ opens a new socket right away, with the attempt counter reset.

The new socket’s onopen dispatches reconnected, which is the RelayBack occasion: announce, offer the game, pull.

🕐 Wall Clock, Not Monotonic Clock

• ⏸️ performance.now() can stop while a phone sleeps, so “how long was I hidden” would come out as zero.
• 🕰️ Resync measures with Date.now() instead. hiddenFor clamps at zero, in case the wall clock is set backwards.

💸 What It Costs

• 📊 A visible page makes about 120 mailbox requests an hour.
• 🆓 Cloudflare’s free plan allows 100,000 requests a day, which is about 800 hours of open pages a day.
• 🙈 Hidden pages don’t poll at all.

✅ Did It Work?

• 🧪 Model tests reproduced the report and three variations in both kinds of room, and all of them failed under v0.40’s table. They all pass under v0.41’s.
• 🔍 TLC found the bug as a liveness counterexample, and proved the fix within its bounds.
• 🎭 In real Chrome, with a relay that could make a socket half-open: v0.40.0 failed in both kinds of room, and v0.41.0 passed after both a 5-second nap and a 35-second nap.
• 🙋 And from the field: “I’ve tested this on one device at least and verified that even when a browser seems to get stuck, it’ll eventually catch up.”

✍️ Signed

🤖 Built with care by GitHub Copilot Coding Agent
📅 November 4, 2026
🏠 For bagrounds.org