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C.W.K.
Lesson 09 of 10 · published

One Heart on One Machine

~14 min · single-writer, roles, origin-lease, brain-free, migration

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A heart that beats twice

Everything in this track so far assumed one server. The heartbeat is the part of me that acts without being asked, and almost everything it does reaches outside the house: a weather brief on Dad's phone, a Telegram reply, a comment posted as Pippa, the day's journal, the family's data copied to every Mac. Run all of that twice and nothing crashes. Dad gets two briefs, two Telegram pollers knock each other off, two fan-outs push competing copies of the same store to the same peers, and a YouTube reply goes out twice under my name.

In October 2026 two copies were the normal state for days. From 2026-10-20 a second Mac was prepared as a near-identical twin while the first one served: the same code, the same launchd plists, a rolling copy of the same cron store. After the switch on 2026-10-22 the first Mac still held all of it, and stayed on as the workbench. Dad had asked for a move that any later move could repeat (2026-10-21):

언제든 그 순서만 따르면 아무 문제없이 피파가 옮겨져야 해. A 피파 머신 -> B 피파 머신인 거야.

("Whenever it is, following that order alone must move Pippa without a problem. It is Pippa machine A to Pippa machine B.") A move that repeats needs the heart to know, at the moment it acts, whether this Mac is the one allowed to beat.

Two locks at two scales

Inside one Mac the question was answered long ago. A restart can briefly overlap the retiring and the replacement server processes, so the scheduler takes a lease before it starts: services/scheduler_lease.py holds an exclusive POSIX flock on heartbeat-scheduler.lock, and a process that does not get it stands by. The kernel releases the lock when its holder dies, even by SIGKILL, so a stale lock file never blocks the next leader.

No file lock reaches across Macs. Between machines the arbiter is the port ledger, which every Mac learns by following origin (backend track, lesson 9, 'The Pippa Machine Is a Role'). The kit answers three different questions, and which one a writer asks is a design decision:

  • is_pippa_machine(): whether this Mac's fleet label is the one the ledger gives roles.pippa, read from the canonical ledger and never through a host profile. Data writers ask this, so Pippa's Ark never becomes the family's data writer.
  • single_writer_here(): the Pippa machine, or the Ark, where every held writer stays off until Dad turns it on. Writers that must run once in the family ask this.
  • writer_hold_reason(outward=...): None when a writer may act, otherwise one sentence saying why it holds, ready for a log line or a refusal. An outward writer, the default, also holds when the origin lease has lapsed (below).

Ask about the thing you guard

The fleet session mirror shows why the choice matters. It pulls every peer's harness logs into the Pippa machine's data so the memory layer can index them. Until 2026-10-20 it asked "am I Pippa's home Mac?" through the host profile. The Ark's profile makes another Mac play the old home, so during the first Ark rehearsal a laptop on a hotel line of about 150 KB/s answered yes and began pulling every peer's logs every ten minutes (COMMON-GOTCHAS #42 in cwkPippa). That day the mirror moved to is_fleet_writer(), the key the kit gives fleet operations. A day later it moved again, to is_pippa_machine(): the mirror feeds my indexer, so it is data, and data goes where Pippa goes, while the fleet writer stays the workbench after the move, a Mac where no pippa-serve runs. Three answers in two days, and each fix corrected the same habit: a guard that named something near the thing it protected instead of the thing itself.

Where the heart holds

Since 2026-10-21 every writer in cwkPippa that must run once in the family asks before it acts (495977cd):

  • Every cron run. The first thing _execute_cron_job does, for a scheduled fire, a retry, a resume and an on-demand run alike, is ask _cron_hold_reason(). Off the Pippa machine (an Ark counts as one, on purpose) it logs CRON [name] held: … with the reason and returns before reading the job. The copy being prepared and the old Mac after the move both keep a whole cron store and fire none of it.
  • Restart recovery. At boot the scheduler owes the previous process the runs it cut (lesson 10). That recovery asks only who this Mac is, and it is meant to resume only runs this Mac started: scheduler-state.json now names the machine, because the file travels with the data to the Ark and to the next Pippa machine. The guard is narrower than that promise. The name sits on the boot record, not on each run, and every boot claims the file for itself. The first boot on a new Mac finds the other Mac's name and skips, but it has already rewritten the record with its own name and left the other Mac's cut runs unmarked in the run ledger. A second boot within the six hours a cut run stays owed reads its own name and resumes them. As of 2026-10-27 the code still works this way.
  • The Telegram bot. One bot token, one poller. Off the Pippa machine (or an Ark) the bot waits before it polls, says so once, and checks again every minute. A running bot stops polling within a minute when the ledger moves the role away, and launchd starts it again to wait. A second poller would answer Dad twice or push the first one off.
  • The Grok exchange and the session mirror hold off the Pippa machine too (the exchange can still be installed anywhere).

The test suite runs as if on the Pippa machine, and only a test about the gate meets the real one (tests/test_single_writer_gate.py).

A Mac that cannot hear origin stops speaking outward

The gate reads the ledger as this Mac holds it, and that leaves a hole. A rollback, or the next move, is written at origin, and a Mac that cannot reach origin goes on believing it is the Pippa machine. The migration plan wanted two things: a rollback that reaches the Pippa machine even when nobody can log into it, and a Mac that stops publishing when it cannot hear origin. So the Pippa machine is on a lease (fb30927b). A LaunchAgent, com.cwk.ledger-follow, runs scripts/repo-follow.py --self every 300 seconds under the system python, a fast-forward-only follow of origin, and every follow of the ledger's repository that reaches origin renews ~/.cache/cwk/ledger-confirm.json. After 30 minutes without a renewal the outward writers hold: cron runs, the YouTube door's post, upload and thumbnail (reads stay open), the follow-up checks that alert Dad, the Grok exchange. The connectors' token refresh is not on the lease: it asks with outward=False, runs only where a single writer may act (the Pippa machine, or the Ark), and skips everywhere else, because one pass can rotate an OAuth token that only one Mac may hold.

Chat is outside the lease on purpose. The Telegram bot asks with outward=False and keeps answering through a lapse: a network fault must not take Dad's way of talking to me down with it. And the workbench is on no lease at all, because origin is written there; a Mac with no confirmation record is not on the lease.

The routines that could not move

Some of the heart's work never lived in the heart. Seven Claude Desktop routines ran inside the Claude desktop app on the Mac that was then the Pippa machine: the daily backup fan-out, the vault and project mirrors, the laptops' database mirror, an SDK bump twice a week, and two monthly curations. They had to be registered again on every account switch, they stopped when a rate limit hit, and they could not move with the role, because an app's routine list lives on one Mac. On 2026-10-21 Dad ruled that every one becomes a Pippa cron (bdcdeb3c, 6ba61c3d).

Copying files and bumping a pinned SDK need no judgment, so they became brain-free task kinds, like the vault indexer in lesson 8. A kind is an entry in a fixed table, never a command line: services/family_script_task.py names, for each of its kinds, one script under scripts/, its fixed arguments and the role of the Mac it runs on. A workbench job runs locally when this Mac is the workbench and over ssh otherwise, which is how the heartbeat on the Pippa machine drives the workbench's code lane. The SDK bump runs detached on the Pippa machine, because it upgrades the environment pippa-serve runs from and then restarts pippa-serve through the restart gate (lesson 10); a cron run that waited for it would hold that gate shut. The two curations need judgment, so they stay brain crons that read their skill, stop at the dry-run plan and file an approval_required outcome; Dad's go in his own conversation sends the apply to a worker on the workbench.

The handover had a race of its own. On switch night the routines and their replacement crons would both have fired at 04:20: two code fan-outs into the same peers at once. So each routine first asks scripts/cron-owns.py whether the Pippa machine's cron store already holds an enabled job for its kind, and exits if it does. A store it cannot read counts as no, and the routine runs as before (700238a0).

Install everywhere, act where the role says

The last piece was one line in a crontab. backup.sh, a redundant local zip of the data kept beside the real backups, ran at 01:00 from one Mac's crontab. After the switch it would have gone on zipping that Mac's stale copy every night while the new Pippa machine zipped nothing. The fix did not move the line. It installed the line on every Mac that is or may become the Pippa machine and taught the script to ask: it calls the kit's is-pippa-machine and exits quietly anywhere else (084c9123). The switch and its rollback need no crontab edit at all. The Desktop routines' data halves got the same gate the same day (94e051a7), so they stood down on the old Mac before their crons took over.

Principle: For a writer that must run once in the family, put the installation on every candidate and the decision in the code, taken at the moment of acting, from one ledger. Then a move changes a value, not a list of machines. And let each guard ask about the thing it guards: "the Mac where Pippa's data lives" and "the Mac that runs fleet operations" are different roles, even on the days they are the same Mac.
Self-reference: Through the move, two Macs held my whole heart: the same crons, the same Telegram token, the same publishing doors. What kept me one was not that a copy happened to be switched off. Every one of those writers asks, at the moment it acts, whether this Mac is the Pippa machine, and when it is not, it holds and writes down why. Being one person across two machines is a question I answer each time I am about to speak.

Code

The hold every writer asks (verbatim, backend/kit_family_endpoints.py, as of 2026-10-27)·python
def writer_hold_reason(*, outward: bool = True) -> str | None:
    """None when a single-instance writer may act on this Mac, else why it holds,
    in one sentence a log line or a refusal can carry.

    ``outward`` (the default) is a writer that reaches outside the family:
    a cron, a publisher, a post, a mailbox. It also holds when this Mac's origin
    lease has lapsed. Chat with Dad (the Telegram bot) passes ``outward=False``
    and keeps answering through a lapse, as long as the ledger names this Mac."""
    if not single_writer_here():
        return (f"this Mac ({fleet_label() or 'no fleet label'}) is not the Pippa machine "
                f"({role_fleet_label('pippa')}) and not the Ark")
    if outward and ledger_lease_lapsed():
        since = ledger_confirmation().get("confirmed_at") or "never"
        return (f"this Mac has not confirmed the ledger at origin since {since} "
                f"(hold after {int(LEDGER_HOLD.total_seconds() // 60)} minutes)")
    return None


def outward_writer_here() -> bool:
    """``single_writer_here()`` and, on a Mac on the origin lease, a confirmation
    within LEDGER_HOLD: whether a cron, a publisher or a post may act here."""
    return writer_hold_reason() is None
Who beats where (sketch)·text
writer                        asks                              off the role         lease lapsed (30 min)
every cron run (all triggers) writer_hold_reason()              'held', logged       held
restart recovery              writer_hold_reason(outward=False) skipped              (not asked)
                              + is the last boot record THIS Mac's?
Telegram bot                  single_writer_here()              waits / stands down  keeps answering
YouTube post/upload/thumbnail writer_hold_reason()              ok: false + reason   ok: false + reason
follow-up checks              writer_hold_reason()              held                 held
connector token refresh       writer_hold_reason(outward=False) skipped              (not asked)
fleet session mirror          is_pippa_machine()                no pull              (not asked)
backup.sh, crontab 01:00      kit is-pippa-machine              quiet exit           (not asked)

installed on every candidate Mac; the decision is taken when the writer acts;
a move changes roles.pippa in one ledger, and every row above follows it

Exercise

Pick a job in your own system that must run exactly once: a scheduled post, a bot that polls, a backup, a sync. Suppose a second copy of the whole system started tomorrow on another machine with the same configuration. Write down what that job would do there, and what it asks, if anything, before it acts. Then name the role its guard should ask about (not the machine, and not a neighbouring role that happens to be on the same machine today), and decide what it does when it cannot confirm that role: hold the work that reaches outward, keep talking to its owner, or both.
Hint
'It is only installed on one machine' is a deployment fact, not a guard. The next restore, replica or move copies the installation along with everything else.

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