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Self-Hosting

A heartbeat cron for sync that dies without errors — and why a watchdog that only restarts is half a watchdog

Environment: two livesync-bridge instances (Raspberry Pi, mini PC) · CouchDB 3.3 on the mini PC · tailscale · cron

TL;DR

A failure that raises no errors can't be caught by watching logs — there's nothing in them to watch. What you need instead is a value that keeps changing for as long as the thing is working, and an alarm for when it stops.

  • The authoritative machine writes the current epoch into a synced file every 10 minutes.
  • The watching machine reads that file every 10 minutes and, if the timestamp is older than 25 minutes, declares sync dead and restarts the container.
  • Restarting and sending an alert is not the end of the job. "Restarted" doesn't mean "working." Check that a fresh heartbeat actually arrives afterwards, so the alert can say recovered or still broken.
  • Keep the heartbeat file hidden (dot-prefixed). The reason for that gets its own post, next in this series.

Why the watchdog exists

I'm writing this from the operational records kept at the time rather than re-running any of it today. The background for the whole series is in Self-hosted Obsidian instead of Notion.

Another post in this series covers a bridge that was pushing fine while pull was dead. There were zero errors in the log, so it went unnoticed for days, and during that time the machine that had fallen behind kept uploading stale files as if they were current.

Recovering from it was easy enough — restart the container. The hard part was that nobody knew when to do that. The cause was never established, so preventing a recurrence wasn't on the table. Since I couldn't prevent it, I went after detecting it quickly instead.

The design: something that keeps moving while it's alive

Stop asking "did an error occur?" and start asking "has data moved recently?"

The authoritative machine, which shares a host with CouchDB and never failed in this incident, writes the current time into a vault file every 10 minutes. That file has to travel through sync to reach the watcher. If the watcher reads it and the timestamp is recent, that path is alive. If it's stale, that path is dead.

The important detail is that this exercises exactly the path that failed. What died here was the authoritative → Pi direction (pull), and the heartbeat flows in precisely that direction. The opposite direction isn't covered by this check at all.

The writing side looks like this:

#!/usr/bin/env bash
set -u
HB="/path/to/vault/3_Resources/system/.pull_heartbeat.md"
now=$(date +%s)
tmp="${HB}.hbtmp.$$"
cat > "$tmp" <<EOF
---
type: system
auto: true
---
 
# pull heartbeat (generated — do not edit)
 
epoch: $now
utc: $(date -u +"%Y-%m-%d %H:%M:%S")
EOF
mv -f "$tmp" "$HB"

That's an atomic write: build a temp file, then mv it over the target, so nothing ever reads or syncs a half-written file. It's also the pattern that had been killing the bridge fourteen times a day. Those crashes happened while the bridge was using chokidar as its file watcher — the piece that sees changes in the vault.

The bridge watching the vault this heartbeat is written into is the one on the authoritative machine. I opened its config and it reads useChokidar: false — configured for Deno's native watcher rather than chokidar, which means the chokidar side of what blew up on the Pi isn't in play here. I couldn't establish from the records when that setting was made.

Both the writing and the checking sides run on a 10-minute interval. The writer is one crontab line:

*/10 * * * * /path/to/pull-heartbeat-write.sh

Why the threshold is 25 minutes

If the write interval is 10 minutes, a 10-minute threshold will fire on healthy systems. The write and the check aren't aligned, and sync itself takes time.

At 25 minutes, two heartbeats in a row have to go missing before the alarm trips. One late beat is tolerated; two consecutive misses is an incident. That's the trade between false alarms and slow detection.

There's also a 30-minute cooldown, deliberately longer than the threshold. The last restart time goes into a marker file and no restart happens inside that window, so one incident can't trigger a second restart before the next verdict is even due — and the watchdog can't bounce the container every 10 minutes in a situation where restarting isn't helping.

Restarting isn't the whole job

The first version stopped at "threshold exceeded → docker restart → send a Telegram message." A day of running it exposed the gap.

The alert says "restarted." What the user actually wants to know is "is sync working right now?" Nothing guarantees a restart fixes it, so even with the alert in hand someone still had to go and check — which throws away half the value of having a watchdog at all.

So I added a self-check to it. After the restart it:

  1. Waits 75 seconds — enough for the container to come up and do its initial scan.
  2. Forces the authoritative side to write a fresh heartbeat, carrying a unique marker (the script above is the scheduled run; the marker is added on this forced path). Forcing it avoids waiting for the next cron tick, and the marker is what distinguishes "this one just arrived" from "this was already here."
  3. Polls for up to 50 seconds for that marker to show up on the watching side.
  4. Reports recovered if it arrives, or still broken, needs a human if it doesn't.

The 75 and 50 are what the timings looked like on this hardware, not values with a derivation behind them — I didn't record one.

Now one line in the alert closes the question, and you don't have to interpret silence.

Gotchas

  • Don't hardcode the notification token. Read it from an env file at run time. A token written into a script that lives in a synced vault travels to every device the vault reaches, and into every backup of it.
  • Keep the heartbeat file hidden (dot-prefixed). It took three names to get there — while the file had a visible name, Obsidian on the other devices kept reporting conflicts. That's the next post.
  • Watching costs essentially nothing. Measured on the watching machine: load under 0.2, bridge CPU around 0.01%. Two 10-minute crons and one small file read isn't where your resources go.
  • The self-check needs remote execution. The watcher has to make the authoritative side write a heartbeat on demand, which means key-based SSH. Anything that can prompt for a password will hang under cron, so pass -o BatchMode=yes and verify it works in the cron environment specifically, not just in your shell.

Takeaways / checklist

  • "Watch for errors" doesn't work on failures that raise none. Build a value that keeps changing while the system is alive and alarm on it going stale. A heartbeat is the cheapest version of that.
  • The heartbeat has to travel the same path that failed. A signal that takes a different route won't detect that failure.
  • Set the threshold to at least twice the interval. Firing on a single late beat produces false alarms, and false alarms train people to ignore the channel.
  • Automated recovery should report the outcome, not the action. "I restarted it" still leaves a human to go and check.
  • Make failure loud. A watchdog that only reports success gives silence two meanings — healthy, or the watchdog itself is dead — and that isn't something you can rely on.