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A PouchDB `changes` feed that hangs forever with no error — `heartbeat` was never the missing piece

Environment: two livesync-bridge instances (Raspberry Pi, mini PC) · CouchDB 3.3 on the mini PC · pouchdb-core / pouchdb-adapter-http / pouchdb-replication 9.0.0 · tailscale HTTPS

TL;DR

If a feed opened with .changes({live: true}) stops without an error and without completing, setting or raising a heartbeat interval is not the answer.

  • live: true becomes continuous internally, and pouchdb-adapter-http puts heartbeat=10000 on continuous feeds by default. The option was never missing.
  • The adapter's own changes path computes a requestTimeout and then never refers to it again. Nothing puts a deadline on that fetch, so when the socket dies quietly the feed waits forever.
  • That's why a reconnect wired to .on("error") never runs. What you actually need is something outside the feed asking "did anything arrive recently?" rather than "is the connection alive?"
  • Also, retry isn't a changes() option at all — it belongs to replication. One of the two lines I added was being ignored from the start.
  • And pull kept stalling after those two lines went in. The restarts are still in the watchdog log (see the verification section).

This is the post where I check my own diagnosis and find it wrong. Here's the walk-through.

The symptom: self-healing was in place and it kept dying anyway

The incident itself is reconstructed from the operational records kept at the time; I read the source quotes and the watchdog log below myself today, on both machines — the hub and the one that was failing — and the interpretation is today's as well. Below, "the hub" is the mini PC that also runs CouchDB, and "the machine that was failing" is the Raspberry Pi. The background for the whole series is in Self-hosted Obsidian instead of Notion.

An earlier post in this series covers pull dying without a single line of error; to catch it I added a heartbeat watchdog cron that restarts the container when the heartbeat goes stale. The watchdog worked. It treats a heartbeat older than 25 minutes as dead, and since both the check and the heartbeat run on ten-minute cycles, detection lands 15 to 35 minutes after the feed dies.

The restart alerts kept coming. In the watchdog's first two days alone: 2 on 07-14, 4 on 07-15. Both numbers come from counting the watchdog's own alerts, and before it existed there was no way to count this failure at all — so "it got worse" isn't a claim I can make, and two days is far too little to call a trend either. The log further down shows that on most days that saw a restart, it was only one or two. What moved me wasn't a trend so much as the sheer fact of getting several alerts a day.

The watchdog shrank the exposure window from days to under an hour, but that window wasn't safe. As the earlier post establishes, a machine whose pull is dead keeps pushing its stale files up as though they were current. Frequent alerts meant frequent exposure to exactly that. So this time I went after the cause instead of the restart.

What the log showed: nothing happened at the moment it died

I read the bridge log from just before the point where pull stopped.

  • Pull events were normal right up to the last heartbeat it logged.
  • After that, nothing. No error, no reconnect attempt, no stream-finished line.

That was the clue. This code logs WATCH: FINISHED when the stream completes normally, and logs the exception when one is thrown. Neither appeared, which means that as far as the code was concerned the connection was still alive. So it did nothing.

The diagnosis I made — and why it was wrong

The bridge receives changes through this call in DirectFileManipulatorV2.ts. This is from DirectFileManipulatorV2.ts.bak.20260716, the pre-patch backup still sitting on the machine that was failing, read today:

.changes({
  include_docs: true,
  since: this.since,
  selector: { type: { $ne: "leaf" } },
  live: true,
})
.on("change", async (change) => { /* … */ })

No heartbeat, no retry, and the reconnect logic hangs off .on("error").

One thing to get straight first: the heartbeat this section is about is not the heartbeat file the watchdog cron uses. This one is a parameter on the _changes request PouchDB sends to CouchDB — "send me a liveness signal every N milliseconds," defaulting to 10000. Same word, different layer.

Anyway, at the time I wrote it up like this:

With no heartbeat, the client never learns the connection dropped → no error is raised → the reconnect on .on("error") never runs.

And I added two lines. This snippet is from that machine's copy of the file, read this morning; I changed those same lines again that evening, which comes up below:

  live: true,
  retry: true,        // added
  heartbeat: 30000,   // added

That diagnosis was wrong. Going back to shore up the evidence for this post, I opened the cached pouchdb-adapter-http 9.0.0 source in the hub machine's container, and found three things.

First, heartbeat was never missing. pouchdb-core maps live onto continuous:

if ('live' in opts && !('continuous' in opts)) {
  opts.continuous = opts.live;
}

and the HTTP adapter fills in a default heartbeat for continuous feeds:

const DEFAULT_HEARTBEAT = 10000;
// …
if (opts.continuous && !('heartbeat' in opts)) {
  opts.heartbeat = DEFAULT_HEARTBEAT;
}

So the original request already carried heartbeat=10000, which means CouchDB was already sending keep-alives every ten seconds (inferred from the request parameters — I didn't watch the packets). All my heartbeat: 30000 did was stretch that interval from ten seconds to thirty.

Second, retry isn't a changes() option. The string doesn't appear anywhere in pouchdb-core or pouchdb-adapter-http; it belongs to replicate/sync. The retry: true in LiveSyncReplicator.ts that I'd cited as precedent is a replication option, not a changes option. The replication code hand-picks the keys it forwards into the changes call — in pouchdb-replication 9.0.0, read from the same place, heartbeat and timeout are copied across explicitly and retry is nowhere on that list. retry is an option the replication layer uses itself, and it isn't on the copy list, so it never reaches changes(). I saw the same name on a different API and assumed it carried over. That line was a no-op from the moment I wrote it.

Third, the real problem was somewhere else. The adapter computes a deadline for the request:

let requestTimeout = ('timeout' in opts) ? opts.timeout : 30 * 1000;
// …
if ('heartbeat' in opts && opts.heartbeat &&
   (requestTimeout - opts.heartbeat) < CHANGES_TIMEOUT_BUFFER) {
    requestTimeout = opts.heartbeat + CHANGES_TIMEOUT_BUFFER;
}

And this is the one line that matters most in this post.

In this file, requestTimeout is computed and then never referenced again.

The value is derived through several branches and then never passed anywhere. The fetch gets an AbortController signal, and nothing ever aborts that controller on a timer. There is no client-side deadline on this path at all.

What the code says: nobody sets a deadline

Put the three together:

  • Given a request with heartbeat, CouchDB holds the connection open indefinitely and just sends keep-alives at that interval. The server never hangs up first.
  • On the client, nothing applies a timeout to that fetch.
  • So when the TCP connection quietly disappears somewhere in the middle, nothing on either end reports an error. The socket looks open and the data simply never comes.

.on("error") cannot fire in that situation, so the reconnect never runs. That would explain the silence in the log (though whether this incident actually took that path is something I couldn't confirm; see the limits of the log in the verification section, and "What I could not verify" below).

It's easy to misread what a heartbeat does here. Keep-alives only surface a failure if the receiving side is measuring a deadline. With nobody measuring, the signal can stop and nothing happens — and in this stack, nothing was measuring.

The plugin does expose this trade-off as a setting — "Use timeouts instead of heartbeats" — and here is its description in full:

If this option is enabled, PouchDB will hold the connection open for 60 seconds, and if no change arrives in that time, close and reopen the socket, instead of holding it open indefinitely. Useful when a proxy limits request duration but can increase resource usage.

Two caveats about reading that.

  • The stated purpose is proxies that cap request duration. The docs don't claim it exists to detect dead connections, so leaning on this sentence as evidence for my argument would be overreaching.
  • "PouchDB will hold the connection open for 60 seconds" isn't literal either. All the setting does in code is put heartbeat: setting.useTimeouts ? false : 30000 into the replication options. With the option on, the heartbeat is switched off and nothing passes a timeout option either — I searched the plugin's replication path and the adapter on the hub machine for one — while the adapter only forwards a timeout as a request parameter when it's given. So neither heartbeat nor timeout goes out on the request. That leaves CouchDB's own default doing the 60-second counting — the server hangs up, not a client timer.

So what surfaces a dead connection is periodically ending the connection, not the heartbeat. And with that setting on, the timing falls to the server rather than the client — nothing on the client is timing anything here at all. The option I picked is on the wrong side of that trade-off.

(The #953 post in this series blames a streaming connection opened without a heartbeat for the failure it covers. That isn't a contradiction — it's a different code path: there, as that post describes, the plugin's fast fetch opens that connection; here the bridge goes through PouchDB, whose adapter fills in a default heartbeat.)

Verification: it kept stalling after those two lines went in

Everything above comes from reading the source. So I went and checked what actually happened.

First, the patch really was on the machine that was failing. Its DirectFileManipulatorV2.ts carried the two lines with a comment, and DirectFileManipulatorV2.ts.bak.20260716 was sitting next to it. That machine's deno.jsonc pins pouchdb-core and pouchdb-adapter-http at 9.0.0 too — the same versions as the source quoted above.

And the watchdog cron had been keeping a log the whole time. Every time it found the heartbeat older than the 25-minute threshold and actually restarted the bridge, it left a line:

2026-07-14 02:10:01 STALE age=1800s > 1500s -> restarting bridge

The patch went in on 2026-07-16. Counting the events on either side:

  • Before: 2 on 07-14, 4 on 07-15
  • After: 2 on 07-18, 2 on 07-19, 1 on 07-21, 2 on 07-22, 2 on 07-24, 1 on 07-27, 2 on 08-07, 2 on 08-08, 1 on 08-11 (the count window ends on 2026-08-11)

(The patch date comes from the backup filename DirectFileManipulatorV2.ts.bak.20260716 on that machine and from the records, which agree.)

The stalls didn't stop. There's one on 2026-08-11. So the log contradicts my claim that two lines fixed it.

It's worth being precise about what this log can tell you, though. The cron only knows that the heartbeat went stale. Whether all fifteen took the infinite-wait path described above, or whether crashes, network drops, and container trouble are mixed in, is something this log can't distinguish. What's established is fifteen restarts after the patch, each triggered by pull stopping. Nor does that mean fifteen distinct failures — the cron has a 30-minute cooldown, so one long outage can collapse into a single line, or produce a second one once the cooldown expires.

One of the things that might be mixed in there did eventually surface. On 2026-08-12 I rebuilt the bridge on the machine that had been failing, and it died twice in a row on startup with a Corrupted document exception. The throw comes from assembling a document whose chunks are missing, it sits outside the try, and an unhandled rejection takes the whole process with it — a mechanism another post in this series covers in full (its example is from the hub machine).

That still doesn't let me say how many of the fifteen it accounts for; there's nothing in the logs from back then that would tell them apart.

The rate does look lower: 15 over the 26 days after the patch — about 0.6 a day — against roughly three a day across the two days before it. But nothing here lets me credit the patch for it. The "before" sample is the two days right after the watchdog went in, and the "after" is lumpy — an eleven-day gap between 07-27 and 08-07, then clusters. Plenty of other things changed in this stack over the same weeks.

One thing I can say: pull still stops.

What to do about it

Honestly, this post has no "and then it was fixed" ending. There are two things I can say from here.

One — the code points toward turning the heartbeat off. Passing heartbeat: false makes the adapter omit the parameter entirely:

if ('heartbeat' in opts) {
  // If the heartbeat value is false, it disables the default heartbeat
  if (opts.heartbeat) {
    params.heartbeat = opts.heartbeat;
  }
}

which leaves CouchDB's own default timeout (60 seconds) in charge, so the server closes the stream once 60 seconds pass with no change.

What happens next is the part that matters, and the adapter source settles it: a continuous feed re-issues the request from the last sequence as soon as a response comes back.

if ((opts.continuous && !(limit && leftToFetch <= 0)) || !finished) {
  // Queue a call to fetch again with the newest sequence number
  pouchdbUtils.nextTick(function () { fetchData(lastFetchedSeq, fetched); });
}

And a few lines up in the same callback: The changes feed may have timed out with no results / if so reuse last update sequence. Timing out is a case this code plans for, and picking up from the last sequence means nothing is dropped across the gap.

With the heartbeat off, the request looks a lot like the one that kept failing in the #953 post. The difference is the loop above: as that post observed, the #953 case is a one-shot initial fetch, where a closed stream got misread as "everything received," whereas a continuous feed answers a closed stream by asking again from the last sequence.

That isn't a cure-all, though, and it splits into two cases:

  • If the connection dies between requests, or the next one fails to establish, an error is thrown and the existing reconnect runs.
  • If the in-flight request itself becomes a black hole, the server's close notification has to travel back down that same dead path, so it never arrives. And with no deadline on the client side, the result is the same infinite wait.

So the structurally correct answer is putting a deadline on the client, and in this version neither option gives you one. The 60 seconds that heartbeat: false relies on is counted by the server, not by a client timer, and the client-side value is the one the adapter computes and never uses.

When I drafted this I hadn't put heartbeat: false into the bridge. That evening I did, on the machine that had been failing — replacing the heartbeat: 30000 line quoted above (backed up as .bak.20260812). Every snippet and log in this post therefore predates that change. What I don't have yet is a result — settling that means counting the same watchdog log for a few more weeks, and it gets its own post. One caveat worth recording now: a second change went in on that same restart (I cleared out the old documents with broken chunk references — the ones that caused the crash above). So if the rate does drop, there'll be no clean way to tell which change did it.

Two — what actually kept this stack running was measuring from the outside. The heartbeat watchdog cron never asks whether the connection is alive. It asks whether a file actually arrived recently. That verdict holds regardless of what the library is doing internally, and it turned out to be the only thing that kept papering over this defect.

Which leaves the stack in a state I can at least describe precisely: the cause is unconfirmed, this version of the library offers no deadline to set, a cron outside it is acting as the safety net, and the next thing to try is already deployed with a verdict weeks out. Not a good place to be, but at least I know which bucket each piece falls into.

What I could not verify

  • Why the connection dies is still not established. What's settled above is that the code enforces no deadline; what actually severed the connection each time it died is something I never confirmed by reproducing it.
  • I don't know where the apparent drop in frequency comes from. The sample is thin and the confounders are many, as above.
  • Whether heartbeat: false reduces the recurrence is still unknown. That needs a few more weeks of log counting, and even then the second change in the same cycle muddies the attribution.
  • The option changes here went into this stack's own copy rather than upstream, so a single git pull in that repository removes all of them.

Takeaways / checklist

  • Don't infer behavior from an option's name — open the adapter you're actually using. heartbeat being absent from the argument list didn't mean it was absent from the request; a layer below was filling in the default.
  • Don't assume an option on one API exists on another. retry exists for replication and not for changes. An option that's silently ignored won't even tell you you're wrong.
  • A value that's computed and never used is itself a clue. requestTimeout is derived through several careful branches and then dropped, which is precisely the evidence that nothing here enforces a deadline.
  • Keep-alives are only worth anything if something is measuring the gaps. Before making the other side send more signals, decide what cuts the connection when they stop arriving.
  • Before writing "fixed", count the log that's already piling up. The evidence was there from the start — the watchdog cron had been leaving a line every time it restarted the bridge. I never counted those lines. "Root cause fixed" went into the record on my say-so, and that sentence got quoted for the better part of a month.