halkyon/docs/adr/0030-halkyon-train.md

# ADR 0030: Halkyon Train, what it does and where its data comes from

- **Status:** decided 27–28 Oct 2026, by the developer (D1–D40 below). The shell (D12)
  is built; recording (D13–D28) is built on Android, iOS waiting for a Mac; Health
  Connect (D29–D31) is built; the Loop's setup and keeping (7a, D34–D36) and its
  daily data (7b, D37, D38) and its sessions (7c, D39) are built; its firmware (D10,
  D40) is being built.
- **Numbering:** 0025 is held by the federation draft (stashed), 0026–0029 by the .NET
  rewrite on the local branch `rewrite-dotnet`, abandoned on 27 Oct; Train takes the next
  free number so that neither is ever read as this one.
- **Followed by:** [ADR 0031](0031-the-apps-icons.md) (the apps' icons, Train's among them).
- **Relates to:** ADR 0014 (halkyon_base, which Train's server includes like every other),
  ADR 0016 (one account across an operator's services; Train is one of them).

## Context

Train is the third of Halkyon's apps after Learn and Todo, so far only named. On 27 Oct
the developer asked for it to "encompass advanced features for running and cycling", to
connect to a Polar Loop, and for rowing machines' APIs to be looked into (WaterRower,
Kettler, Christopeit, BH Fitness, Tunturi), the Christopeit WP 5000 in detail. For running
they asked for "GPS tracking, automatic stop (*at least* the functionality of e.g.
F-Droid's OpenTracks)", HIIT, music in tandem with the pace, and planning tracks as on
Strava. What was found, 27 Oct:

- **Open Bluetooth profiles carry most of it.** Heart rate (`0x180D`, measurement `0x2A37`
  with RR intervals, hence HRV), running speed and cadence (`0x1814`), cycling speed,
  cadence and power, and the Fitness Machine Service (FTMS, `0x1826`; Rower Data
  `0x2AD1`, Indoor Bike Data, Treadmill Data; resistance set through its control point).
  Any strap, footpod, power meter, smart trainer or FTMS rower works without a vendor's
  account or SDK, on every platform with Bluetooth LE. Pixels have no ANT+ radio, so on
  GrapheneOS Train is Bluetooth LE only.
- **The Polar Loop** (Sept 2025, no screen, no GPS) is no Bluetooth standard: it speaks
  Polar's own protocol, and doesn't broadcast its heart rate as a `0x180D` sensor (Polar:
  it "cannot be used as a heart rate sensor for other devices"). Polar's BLE SDK
  (Android and iOS, its own permissive licence, source-available) covers it: first-time
  setup from one's own app (`doFirstTimeUse()`, no Polar account named anywhere), live
  HR, acceleration, PPI, skin temperature and PPG, and the export of sleep, sessions,
  daily activity, 24/7 HR and PPI and Nightly Recharge. It pairs with one peer only;
  re-pairing needs a factory reset. Firmware updates fetch from Polar (a check sending
  the device's UUID, firmware version and hardware code, no health data). Polar's cloud
  (AccessLink, OAuth) gives much the same through Polar Flow, free for now and revocable.
- **Rowing machines**: none of the five brands publishes an API. WaterRower documents
  its S4/S5 monitor's USB serial protocol and its ComModule speaks FTMS; BH's current
  i.Concept range speaks FTMS; Kettler's Coach 2 very probably, Christopeit's and
  Tunturi's unconfirmed. Concept2's PM5 is documented and speaks FTMS. Older machines on
  Classic Bluetooth (SPP) are reachable neither on iOS nor well from Flutter.
- **The Christopeit WP 5000** (manual of 6 Nov 2025): Bluetooth 4.0, a built-in Bluetooth
  heart-rate receiver, water with a 16-level magnetic brake the console moves itself (its
  12 programmes change it), mains-powered. Its manual names Kinomap, picked by brand, not
  as a generic FTMS device; an owner reports (mid-2025) that neither a Garmin's FTMS
  rowing profile nor EXR could pair. Christopeit's own app, Christopeit Connect (Vescape,
  `com.vescape.christopeit`), supports it, declares no data collected (App Store), and
  writes workouts to Health Connect or Apple Health. Whether the console speaks FTMS is
  unknown until it is scanned.
- **Health Connect** is part of Android since 14, so GrapheneOS has it without Google Play;
  data stays on the device and each app is granted each type. Gadgetbridge (Garmin,
  Huawei, Colmi rings, Amazfit and others, no account) and Christopeit Connect write to
  it; OpenTracks writes sessions, distance, speed and routes there, but no heart rate.
- **OpenTracks** is Apache-2.0, so its logic may be taken into Train (AGPL) with its
  notice; the open projects talking to rowers and trainers (qdomyos-zwift,
  OpenRowingMonitor, pirowflo) are GPL, references for the protocols only.
- **Music by tempo**: Spotify closed its tempo data (audio features) to new apps on 27 Nov
  2024; Qobuz grants its API to commercial partners only and refuses open-source
  integrations. Another Android app's title can be read from its media session and its
  tempo looked up (Deezer's `bpm`, often missing; AcousticBrainz, frozen since 2022), but
  only shown, never chosen by. Jellyfin's API is open but has no tempo field.

## Decisions (27 Oct 2026, the developer's)

- **D1 What Train does**: running and cycling with "advanced features", rowing machines,
  the Polar Loop. For running at least what OpenTracks does (GPS tracking, pausing by
  itself), structured intervals (HIIT), music matching the pace, and planning routes as
  on Strava.
- **D2 In this order** (recommended): recording first (GPS, auto-pause, a heart-rate
  strap), then intervals, then route planning, then music. Recording is what everything
  else builds on; route planning is the largest piece (maps and a router); music needs
  the intervals' targets to follow.
- **D3 Train takes over the Polar Loop** through Polar's BLE SDK, with no Polar Flow and no
  Polar account (recommended, over AccessLink, Polar's cloud, as the main way in): the
  SDK gives nearly all AccessLink would, on the device and without Polar in between. Since
  the Loop pairs with one peer, the band is Train's alone; the phone holding it syncs it,
  and other devices see its data through Train's server, as the SDK runs on Android and
  iOS only.
- **D4 Music comes from Jellyfin, with Train as the player** (recommended, over reading
  what Spotify or Qobuz play, which the developer judged "a rather stale feature"): Train
  chooses the next track by the cadence or pace an interval asks for, may speed a track
  up or slow it down a few per cent, and downloads the chosen tracks before a run for
  playing offline. Each track's tempo is worked out by Train from its audio, once, and
  kept (Jellyfin has no tempo field; its MusicTags plugin only passes on a BPM tag files
  already have). A metronome at the target cadence works where no library is set.

- **D5 Devices connect to phones only, through `flutter_reactive_ble`** (BSD-3, Android
  and iOS, Philips Hue's; recommended once the desktop was out, over `universal_ble`,
  BSD-3 on every platform but younger and less used, and over `flutter_blue_plus`, whose
  own licence asks a fee for commercial use, which an AGPL app can't pass on): "I find it
  exceedingly unlikely one owns or purchases a fitness band or smartwatch without
  intending or even owning a smartphone", and "the open license is also of paramount
  importance". Train talks to its own small interface (scan, connect, subscribe, write),
  the profiles' parsers are pure Dart, tested on recorded frames, so another library can
  take its place behind it.

- **D6 Health Connect both ways** (recommended): Train writes its workouts there (the
  session with its type, route, heart rate, distance, speed and calories), off until the
  person turns it on, its own store staying the source of truth; and reads from there
  other apps' workout sessions (their heart rate, distance, power) and the person's weight
  and resting heart rate, each source turned on by itself. Sources Train can't speak to
  (Garmin and Colmi through Gadgetbridge, the WP 5000 through Christopeit Connect) come
  in so; Train tells its own records by their origin and takes a session another app
  wrote of a workout it recorded itself for the same one. Through the `health` package
  (MIT, CARP's, Android and iOS; recommended was a Kotlin channel of our own): it has no
  types for power or cadence, which Train therefore neither writes nor reads there, but
  records from the sensors directly; they can be added upstream or by a channel of our
  own when a source needs them.
- **D7 Train's data is sealed, always** (recommended): the payloads of its rows (sessions,
  routes, every series, what is imported) are ADR 0017's sealed content under the
  person's data key, as ADR 0019 seals a private studyset, so the instance syncs what it
  can't read; imported data syncs as Train's own does. Health data is a special category
  of the GDPR (Art. 9), and nothing on the server needs to read it. The instance still
  sees that a row exists, its size, and when it changed; what Train works out (training
  load, zones, records) is worked out on the device; a route shared later is a copy made
  readable on purpose, as publishing is in Learn.

- **D8 The WP 5000 is an FTMS rower, reached by Train's FTMS client and nothing of its
  own** (found on the developer's machine, 27 Oct, with nRF Connect): its console
  (`MRK-950R-8021`) advertises the Fitness Machine service, with its service data saying
  "rower, available" (`0x1826`, `01 10 00`), and offers Fitness Machine Feature, Status,
  Training Status, the Control Point (write, indicate), Rower Data (notify) and Supported
  Resistance Level Range. Its features: cadence, total distance, pace, resistance level,
  stroke count, energy, heart rate, elapsed time, power, and setting a resistance target.
  A frame while rowing: 25 strokes a minute, 6 strokes, 18 m, pace 439 s, 100 W,
  resistance 1, 1 kcal, heart rate 0 (no strap), 17 s. The forum's report that Garmin and
  EXR couldn't pair isn't explained by the machine as it is now (an older firmware, or a
  console already connected elsewhere, which most allow one of, may be). Its other
  services (`0xFFF0`, `0xF100`, `0xAE00`, and one whose UUID spells "YULU" and "MERACH",
  presumably the console's maker's own protocol, which Kinomap and Christopeit Connect
  may use) are left alone; Health Connect and working out a protocol aren't needed.

- **D9 No WaterRower USB driver for now** (recommended): it would serve S4 monitors
  without the ComModule, on Android only (iOS allows no USB serial device outside Apple's
  accessory programme), through an OTG cable during the workout, as a second transport
  beside Bluetooth, with no WaterRower at hand to test on; a WaterRower with the
  ComModule speaks FTMS. Its protocol is published, so a driver can be added behind
  Train's sensor interface once someone asks for it with one at hand.

- **D10 The Loop's firmware: from Polar directly by default, or through the instance**
  (the developer's choice, after option 1 alone was recommended for phones always on a
  VPN; over updating never). Polar's SDK checks with Polar's Firmware Management API
  (sending a UUID made from the band's ID, its firmware version, its hardware code and
  a fixed client name, no health data) and flashes either what it fetched or a file on
  the phone (`updateFirmware(identifier, firmwareUrl)` takes `file://`, read in its
  source, 27 Oct); the band's settings are backed up over Bluetooth into the phone's
  memory and written back, never sent anywhere. So:
  - **Directly** (the default): the phone asks Polar and fetches the file. Polar learns
    the phone's address, a VPN's exit if one is always on (on GrapheneOS with "Block
    connections without VPN").
  - **Through the instance** (a setting): the phone sends the band's UUID, firmware
    version and hardware code to Train's server, which asks Polar, fetches the file once
    for every band of that hardware and hands it to the phone, which flashes it from a
    file. Polar learns the instance's address, which on a small instance points at its
    operator nearly as well as a home's does.
  Train says both where the setting is, and names what Polar learns where it offers an
  update. An update never starts by itself: Train first syncs what is on the band and
  deletes it there, says the band is erased and set up again, and asks.

- **D11 Maps and routes** (recommended, after looking at how open apps do it; 27 Oct):
  - **The map widget is `flutter_map`** (BSD-3, Fleaflet's; the developer's choice, over
    `maplibre`, BSD-3 and rendering vector tiles natively but at 0.3): drawing tracks,
    heart rate's colours and waypoints over the map is ordinary Flutter. Vector tiles
    come through `vector_map_tiles` and its PMTiles plugin, tested on the developer's
    phone with a region's file before route planning is built; should it stutter,
    `maplibre` takes the map screens' place.
  - **The map's source is the instance's, one of three** (an operator's setting): a PMTiles
    file of its region as a sidecar (recommended, seen by no one else); OpenFreeMap (no
    key, no contract; the default without a sidecar); or a provider of the operator's
    choice with its address and key (MapTiler, JawgMaps and the European others), whose
    key is then readable by anyone using the app. OpenStreetMap's own tile servers forbid
    such use. The app names the source in use and what it sees.
  - **The router is BRouter, as a sidecar** (MIT; recommended over GraphHopper, which needs
    gigabytes to import a country, and Valhalla, with fewer sport profiles): its own image,
    128–512 MB, the 5°×5° segments of the operator's region downloaded from BRouter's
    project (built from OSM with elevation) and refreshed monthly, its profiles text files
    a running one can be written for. As LanguageTool: no site of its own, reached by
    Train's server only, for people signed in, limited, no waypoint kept or logged. A
    route saved is Train's data, sealed (D7). Searching for places (geocoding) is
    decided when planning is built; until then a route is planned by tapping the map.
  - **Offline**: recording, following a route (synced beforehand with BRouter's turn
    hints), noticing a detour and guiding back to the route, music. Maps offline along a
    route (a corridor of a few kilometres downloaded with it), whole regions optional:
    a phone with CoMaps holds them already. Planning, rerouting and searching need the
    instance; BRouter on Android (its app, or its engine inside Train) may come later,
    never on iOS (no Java).
  - **With CoMaps** (Apache-2.0, OSM's data; read in its source at `63a53413`, 27 Oct, by
    an agent, nothing tried on a device): its maps (MWM, its own format, in its private
    storage) can't be read by another app, so the apps work together instead:
    1. a route planned in Train goes to CoMaps as GPX (VIEW or SEND, a new list each;
       tracks, routes, waypoints, elevation, times, colours kept; no GeoJSON), to be
       seen there offline;
    2. turn hints come from Train, which announces them with the workout's own, since
       CoMaps navigates along no imported track (its issue #251, open);
    3. optional: a route to a point is handed to CoMaps (`cm://route?sll=…&saddr=…&dll=…
       &daddr=…&type=pedestrian|bicycle`), which builds it offline and the person starts;
       on Android, Train may read along through CoMaps' navigation provider (since mid
       2026; a runtime permission the person grants), for the route and what is left;
    4. tracks and routes from CoMaps reach Train as GPX when the person shares them.
    Train builds the `cm://` links itself: Organic Maps' API library and `om://` don't
    reach CoMaps on Android. On iOS (`iphone/Maps/CoMaps.plist`, `DeepLinkHandler.swift`,
    read 28 Oct): GPX, KML and KMZ open in CoMaps (1, 4), `cm://route` works with one
    point between start and finish (3), its `backurl` leads back to Train; picking a
    point (`crosshair`) is "not supported on iOS", and iOS has no navigation provider.

- **D12 Train's shell is Todo's** (recommended, 27 Oct; the icon the developer's choice,
  the rest taken without objection on 28 Oct):
  - **a server of its own**, `apps/train/` with `halkyon_train_server`, `_client`,
    `_flutter`, including halkyon_base (`trainBase()`), its own register when it runs
    alone and a member of the instance's where the operator names one (ADR 0016); over
    Train's endpoints in Learn's server, whose deploys, migrations and failures would
    then be Train's, and which an operator couldn't run Train without;
  - **synced with `serverpod_offline_sync`** (ADR 0001), its rows sealed (D7), since
    recording happens offline;
  - **ports of its own in development**, 8180–8191 where Learn and Todo share
    8080–8091, so that Train runs beside Learn's server as a member of its register;
  - **Android and iOS** record; **Linux, macOS and Windows** show and plan (D5); no web,
    as for Learn and Todo, whose keys (ADR 0017) need libsodium natively;
  - **Halkyon Train**, `app.halkyon.train`, in English and German alike;
  - **its icon is Lucide's `bike`**, in the family of [ADR 0031](0031-the-apps-icons.md)
    ("activity" and "heart-pulse" look like basic health apps, "a defibrillator or ADAC
    health application"; "this is more about training"; Lucide has no runner);
  - **deployed once recording works** and the developer wants to use it, not as an
    empty shell first.

## Recording (28 Oct 2026, the developer's)

- **D13 Positions are recorded by Train's own platform code** (recommended, over the
  `geolocator` package and paid background-tracking packages): on Android a foreground
  service of its own, as Learn's Pomodoro timer has, reading `LocationManager`'s GPS
  provider as OpenTracks does and appending every fix to a recording file in the app's
  folder, which the app follows while open and takes in when it opens again; on iOS
  CoreLocation with background location; both behind one interface (`TrackRecorder`),
  faked in tests. So a recording goes on with the app swiped away, and nothing of Google
  Play services is in the app: `geolocator`'s Android part depends on Play services'
  location library even where its `forceLocationManager` keeps it from being used, a
  non-free library that F-Droid's main repository refuses, and its positions arrive in
  Dart, which a swipe ends. The strap and the rower join the same service later.
- **D14 A workout and its series are kept apart** (recommended, over one row holding
  every sample and over sealed GPX or FIT files in the instance's storage): `workout`
  (type, start, end, name, notes, totals; changed when edited, the last writer winning, as
  a studyset's payload) and `workout_series` (a chunk of one series, positions, heart rate,
  strokes, pauses, of up to 10 minutes; written once as the workout is saved, never
  changed, deleted with its workout, as Learn's review log), every payload sealed (D7)
  and nothing readable in a column, not the type nor the start. Editing a workout touches
  one small row, two devices never conflict over samples, and every source is one more
  kind of series. A chunk's payload is JSON as every payload, each column (time,
  latitude, longitude, altitude, accuracy, …) a list of whole-number differences from the
  value before, gzipped before it is sealed, since sealed bytes compress no more. The
  recording file is the device's own and never synced. UUID v7 ids carry their creation
  time, which the sync engine's change times tell anyway (D7's list).
- **D15 Sealing: the envelope moves into halkyon_app, Train holds its key itself**
  (recommended, over moving all of Learn's key management and over a copy of the
  envelope in Train): Learn's `PayloadEnvelope` (a payload sealed for its table, row and
  column; readable payloads signed by the account; what doesn't open `Locked`) moves
  unchanged into halkyon_app, Learn importing it from there; Train has one key per person
  in a synced `content_key` row, its sealed ID saying it is the person's, and a device
  without the account's keys writes nothing, as in Learn. Learn's tree (private or
  readable studysets, their records, rewrites) stays Learn's. Publishing a workout later
  doesn't change it (asked by the developer): a published workout is a readable copy made
  on the device (D7), which may leave out the heart rate or the first and last few hundred
  metres, as Strava's privacy zones do, the sealed workout staying as it is; the envelope
  can sign the copy with the account's key.
- **D16 Auto-pause by each activity's best signal, never discarding samples**
  (recommended, over GPS alone and over manual pauses only): a pause, automatic or by
  hand, is a marker in a series of its own, and moving time, pace and the totals leave
  paused stretches out, so a misjudged pause is corrected from the data afterwards.
  Running and walking stop with the phone's step detector (Android's step sensor, iOS's
  pedometer; its permission asked for, GPS alone without it), confirmed by GPS; cycling
  by GPS speed, paused below 3 km/h for 5 s and going on above 5 km/h; rowing by the
  rower's stroke rate, no stroke for 5 s. On by default for running and cycling, the
  thresholds adjustable once Train has settings.
- **D17 Running, walking, hiking, cycling and indoor rowing first, in this order of
  parts** (recommended): 5a the envelope moved, Train's key holder, the two models and
  their migration, the chunk codec and the first synced repository; 5b recording on
  Android (the service, the file taken in after a swipe, the recording screen,
  auto-pause, saving); 5c the workouts and a workout's page with its track on a map
  (D11's `flutter_map` and the instance's tile source brought forward from route
  planning, the developer's choice: a run without its track looks unfinished), totals,
  splits and charts; 5d recording on iOS; 5e the heart-rate strap; 5f the WP 5000 over
  FTMS, from frames recorded with nRF Connect. Health Connect (D6) and the Loop (D3)
  follow recording. Each type maps onto one of Health Connect's exercise types, and is a
  name in the sealed payload, so new ones need no migration.
- **D18 The recording file is text, one sample a line, appended** (recommended, over
  fixed-size binary records, smaller only while recording, as a saved workout is
  delta-coded JSON anyway, and over samples handed straight to Dart, lost when Android
  stops the Flutter engine while the service runs, against D13): each line a sample with
  its kind (a position, a heart rate, a step, a pause's start or end) and its time,
  flushed every few seconds. A line cut short is dropped as it is read; an unfinished
  file the app finds when it next opens is offered to be saved up to its last sample or
  thrown away. It lies in the person's folder, in the app's private storage, unsealed,
  as their local database does, until it is saved as a workout and sealed (D7).
- **D19 Permissions asked when first needed, never background location**
  (recommended, over background location, which would let a recording start with Train
  closed and go on after Android killed its process, but asks to be allowed all the time
  on a settings page of its own since Android 11 and a declaration on Google Play, more
  than Train's privacy fits; OpenTracks does without it too): precise location and, on
  Android 13 and later, notifications as an activity with GPS first starts, not at
  install or first launch (refused notifications hide the recording's notification and
  leave it running); physical activity only as running, walking or hiking first starts
  with auto-pause on, for the step detector (D16), auto-pause falling back to GPS speed
  for that workout where it is refused. The service is a foreground service of the types
  `location` and `health`, started from the app while it is open, which keeps its access
  to location with the screen off; a process Android kills leaves its file (D18) to be
  found when Train next opens.
- **D20 Every fix is kept with its accuracy, and filtered as it is read**
  (recommended, over dropping imprecise fixes in the service, which keeps the series
  smaller and the reading simpler but can't be undone for the workouts recorded under a
  threshold that proves wrong): the file and the saved series hold each fix the phone
  delivered with the accuracy it reported; distance, pace, splits and the track drawn
  leave out the fixes less accurate than a threshold, 20 m to start with (as OpenTracks'
  default), adjustable once Train has settings. A better filter later (smoothing, a
  Kalman filter) reads old workouts anew, as nothing was thrown away. An hour at a fix a
  second is 3,600 fixes, tens of kilobytes delta-coded and gzipped.
- **D21 The recording screen** (proposed, the developer's): home's Record button opens a
  sheet of the activities, the last one recorded chosen, with auto-pause's switch as D16
  sets it; Start asks the permissions (D19). Until the first fix precise enough the
  screen says it waits for GPS, showing the accuracy, while it records already, the time
  from Start. Recording, it shows the moving time, the distance and the pace now (min/km
  running, walking and hiking, km/h cycling; rowing time alone until 5f; heart rate once
  a strap is there, 5e), an automatic pause told apart from one by hand, no map until 5c.
  Stop is a long press, so that no pocket ends a run (over a tap and a confirmation,
  quicker and easier to hit by mistake), then Save or Discard, Discard asked again. The
  notification shows time and distance with Pause or Resume and opens the app; stopping
  only in the app, so that no stray touch on the lock screen ends a workout. A recording
  found cut off as Train opens (D18) is offered to be saved up to its last sample or
  discarded.

  Built on 28 Oct with three choices of the implementation's, for the developer to look
  at: rowing indoors isn't offered yet, as a foreground service on Android 14 and later
  needs a type its permissions allow and rowing without the rower has none (5f's
  Bluetooth gives it `connectedDevice`); on foot without the step detector auto-pause
  goes by GPS speed below 2 km/h for 5 s, going on above 3 km/h (D16 names only the
  bike's thresholds); and Stop's hint shows above the button for three seconds rather
  than in a snack bar, which would cover the button it speaks of. The service holds a
  partial wake lock while it records (at most 12 hours), so that its clock and file run
  with the screen off where no GPS fix wakes the processor.
- **D22 OpenFreeMap first, the PMTiles sidecar with route planning** (recommended, over
  the sidecar now, which no one else ever sees the areas viewed through but needs Train's
  server and the sidecar deployed on learn.halkyon.eu before anything else of Train is
  ready, and a region chosen, gigabytes on the server's disk; and over a source fixed in
  the app, which skips D11's operator's setting and would be undone): Train's server names
  the instance's map source, as its operator sets it, OpenFreeMap where nothing is set
  (D11), so 5c needs no deploy; a workout's page names the source drawing it and what it
  sees (the device's address and the area viewed, not the track). The sidecar comes when
  route planning gives Train's server a reason to run on the instance: the region's
  PMTiles file served as ordinary tiles (D23).
- **D23 `flutter_map` 8 with `vector_map_tiles` 9's beta** (recommended; 28 Oct): D11's
  combination doesn't exist as released, as `vector_map_tiles`' stable line (8) needs
  `flutter_map` 7 and its PMTiles plugin (1.5.0, October 2024) `vector_map_tiles` 8,
  while the 9 line, beta since November 2024 (beta.13 on 1 September 2026), is where its
  maintainer works and needs `flutter_map` 8. Over the stable line, all three packages
  of 2024 and held there until 9 is out, and over `maplibre` (0.3.7), native and smooth
  but drawing tracks and their colours as its layers rather than Flutter's widgets, the
  reason for D11's choice. The beta is pinned exactly and moves to 9.0.0 once released;
  the sidecar serves the region's PMTiles file as ordinary vector tiles with Protomaps'
  `pmtiles serve` (go-pmtiles, BSD-3-Clause) instead of the app reading it, so the app
  holds nothing of PMTiles'.
- **D24 The workouts and a workout's page** (the developer's; 28 Oct): home lists
  the workouts newest first under their months, each with its activity's icon, its name
  (else the time of day's and the activity's, "Morning run"), its day and start, distance,
  moving time and pace (km/h cycling); one this device can't open shows locked; Record
  stays. A workout's page: the map on its upper part, the track in cobalt, its pauses
  gaps rather than straight jumps, start and end marked, the source's credit on it, fit
  to the track (heart rate's colours with the strap, 5e); its totals (distance, moving
  time, pace or speed, the whole time, the elevation gained, counting climbs above 3 m
  of the fixes precise enough, as GPS's altitude is noisy); its splits, per kilometre on
  foot, per 5 km cycling; charts of pace or speed and of elevation over the distance,
  drawn by the app's own small painter (over `fl_chart`, MIT, larger than two lines
  need and one more dependency to follow); under the map, which source draws it and what
  that sees, with its page where the operator names one (D22). Its name and notes are
  edited; deleting asks first and offers an undo, as Learn's deletions do. Without GPS no
  map and no splits.

  Built so (28 Oct), with choices of mine for the developer to look at: the day's parts
  start at 5, 12, 17 and 22 on the device's clock; locked workouts come last under no
  month, as when they were is sealed too; the map stays fixed above the page as it
  scrolls, pans and zooms but doesn't turn, and folds the source's credit into
  flutter_map's attribution button, the credit being wider than a phone; start green,
  end cerise, the track cobalt in either theme (the map's colours are the source's);
  the charts show their lowest and highest value beside them and nothing else; where
  the instance can't be asked for its map source, the page says so and shows the rest;
  the elevation counts a climb once it passes 3 m above where it last turned, so a
  steady climb's last metres short of the next 3 are not counted (a 20 m hill reads as
  18 or 19 m): the simple filter, to be compared with recorded runs.
- **D25 iOS recording waits for a Mac** (the developer's; 28 Oct): 5e and 5f come before
  5d, which moves to the end of step 5. iOS's side (CoreLocation with background
  location, the pedometer, a Live Activity for the notification) needs Xcode, which runs
  on macOS alone, and this machine has none; written unbuilt it would look done and not
  compile, and CI's runner has no Xcode either. The heart-rate strap and the WP 5000 are
  built and tested on Android here meanwhile.
- **D26 The recorder service talks to the strap itself** (recommended, the developer's;
  28 Oct; over `flutter_reactive_ble` for finding the strap beside it, two stacks touching
  one strap for one screen's sake, and over D5's Dart alone, whose heart rate would stop
  where a swipe stops the Flutter engine, D18, while GPS records on): Android's own
  Bluetooth API in Kotlin, as OpenTracks has it, connects during a recording and writes
  the `h` lines, so heart rate is recorded as long as positions are; finding and choosing
  a strap goes through the recorder's channel too, so Train has one Bluetooth stack and
  no Bluetooth package. This revises D5: its open licence holds, as nothing third-party
  is left, and the platform's API sits behind `RecorderPlatform` as the library would
  have behind Train's own interface. The profiles' parsers (Heart Rate Measurement now,
  FTMS's Rower Data in 5f) are Kotlin, tested on the JVM on recorded frames, and iOS (5d)
  needs them again in Swift.
- **D27 The strap, from choosing it to the workout's page** (the developer's; 28 Oct): the
  Record sheet's *Heart rate* row names the strap or "No strap" and opens a sheet that
  looks for devices advertising the Heart Rate Service (0x180D), the nearest first;
  the one chosen is remembered on the device, in the person's folder (`strap.json`, its
  address and name, never synced: each phone finds it anew), *Forget strap* letting it
  go. Nearby devices are asked for as the sheet first looks (D19), declared never to
  locate (Android 12 on; before, scanning needs location). No bonding: the service
  connects by address. A remembered strap records with every workout unless switched
  off on the sheet for one; the recording doesn't wait for it. The recording screen
  shows the heart rate ("–" while connecting), and "Strap lost" where GPS's state shows
  while it is gone; the notification stays time and distance, as a rate changing each
  second would repost it each second. A lost strap is asked for again in the background
  for as long as the recording runs; the minutes without it stay a gap. Only the rate is
  kept (`h` lines), not the R-R intervals, until something uses them. The workout's
  page: the heart's average and highest rate while moving among the totals, each
  split's average, and its chart over the distance (over the moving time without a
  track). The track is coloured by heart-rate zones once Train's settings hold the
  person's maximum rate, as colouring by each workout's own range paints an easy run
  as red as a race.

  Built so (28 Oct), with choices of mine for the developer to look at: the file gains
  `strap <time> connecting|connected|lost` lines (the format's version stays 1, as a
  reader before skipped lines it didn't know); a rate older than 5 s shows as "–"; the
  first attempt connects directly, falling back on Android's background connection
  once it fails and after every loss; a sighting stronger than −65 dBm is "close by";
  the heart-rate chart averages the rates of each stretch (as the pace chart's
  stretches, or 10 s, at least, over the time). Not yet tried with a real strap.
- **D28 The rower, from choosing it to the workout's page** (the developer's; 28 Oct): with
  indoor rowing chosen, the Record sheet's *Rowing machine* row opens a sheet as the
  strap's, looking for the Fitness Machine Service (0x1826) and listing the machines
  whose advertisement says "rower"; the one chosen is remembered in the person's folder
  (`rower.json`). Indoor rowing is offered again, Start asking for a rower. Train reads
  Rower Data and takes no control of the console (its resistance through the Control
  Point waits for a reason, as training plans); a console held by another app (one
  connection at a time) is said so. The foreground service gets the `connectedDevice`
  type, which nearby devices allow. Each frame becomes a line `r <time> <stroke rate>
  <strokes> <metres> <watts>` and a `rowing` series; the console's running totals count
  from its first frame, and its own heart rate is kept only without a strap.
  Auto-pause: no new stroke for 5 s pauses (D16), the next one goes on, on by default.
  The recording screen: moving time, metres, the split per 500 m of the last 30 s,
  the stroke rate, watts, the heart rate with a strap. The list shows the split per
  500 m; the page: distance, moving time, the average split, stroke rate and watts,
  the highest watts, strokes, heart rate; splits per 500 m with their split, stroke rate
  and watts; charts of the split, the stroke rate and watts over the distance; no map.
  Written from FTMS 1.0 and D8's frame, checked on the developer's machine later (an
  nRF Connect log of Rower Data and Fitness Machine Status, or a first row).

  Built so (28 Oct), with choices of mine for the developer to look at: a machine whose
  advertisement carries no Fitness Machine data is listed too, as it can't tell, and only
  one that says it is something else is left out; a console reset to zero mid-row counts
  on from there; metres rowed while paused count for nothing, as GPS's do (D16); the
  strap's sheet and the rower's are one sheet, the same "close by" (−65 dBm); the
  strap and the rower share one connection class (`NotifyLink.kt`), and a row asks for
  nearby devices as it starts, as a run asks for location, since Android needs it for
  the service's type; Fitness Machine Status (the console's own pause and reset) is
  not read yet, the strokes deciding the pause; the notification shows a row's
  distance in kilometres, as the others'. Not yet tried on the WP 5000.

## After recording (28 Oct 2026, the developer's)

- **D29 Health Connect next, the Loop after it** (recommended, the developer's; over the
  Loop first, and over neither before the intervals): Health Connect is the smaller
  piece and serves every workout Train records already; writing gives them to every
  other app on the phone, reading brings what Train can't speak to itself (Gadgetbridge's
  watches and rings, Christopeit Connect's rows), and the Loop's sessions can later go
  there the same way. The Loop needs Polar's SDK, its first-time setup and D10's
  firmware, and new kinds of data (daily activity, sleep, 24/7 heart rate, Nightly
  Recharge) with synced models of their own; it is tried only with the band at hand.
  The intervals (D2) need neither.
- **D30 Health Connect through Kotlin of our own** (recommended, the developer's; revises
  D6's `health` package, as D26 revised D5): `androidx.health.connect:connect-client`
  (Apache-2.0, AndroidX, no Play services) behind a channel of Train's, as the strap's.
  The `health` package (13.3.2) lacks power, cadence, elevation gained, distance and
  speed as records of their own, ties no heart rate to its session, has no changes API
  (so each read would be a whole window read again) and sets no client record ID (so a
  workout written again would be a second copy, not a replacement). Our own gets every
  record type, the changes API, Train's workout ID as the client record ID (written
  again replaces, deleted deletes) and each record's origin. Its logic is tested on the
  JVM, the Dart side on a fake platform; Apple Health needs Swift of its own later,
  as iOS waits for a Mac anyway (D25).
- **D31 Health Connect, from turning it on to the workouts** (the developer's; 28 Oct):
  - *Turning it on*: a *Health Connect* row in the account sheet (Train has no settings
    yet) opens a screen with two switches, both off: *Write my workouts* and *Read from
    other apps*. They are this phone's, as Health Connect is, kept per person in their
    folder (`health_connect.json`, with what was written and the changes token), not
    synced. Each permission is asked as its switch is turned on. Built into Android from
    14; below, the row says it needs the Health Connect app. `connect-client` 1.1.0, the
    stable one. The rationale Health Connect asks for is a short screen of Train's,
    linked from the system's permission dialog.
  - *Writing*: every workout of the person this phone holds, synced and earlier ones too;
    Train's workout ID is the record's client ID and a counter its version, so an edit
    replaces and a deletion deletes; imported ones are never written back. As a workout
    is saved, edited or deleted, and as the app opens (what a sync brought). The session
    (type, name, notes, its pauses as pause segments), its route with altitude and
    accuracy, heart rate, speed and the rower's watts as samples, distance and elevation
    gained as totals. No calories (no weight, no energy model yet); stroke rate has no
    type there.
  - *Reading*: the screen lists each app that wrote workouts there (Gadgetbridge,
    Christopeit Connect), each with its own switch, off, new ones appearing as they
    write; Train's permissions are per type, the filtering by app its own. As the app
    opens and comes to the front, through the changes token; nothing in the background.
    Turning it on asks for the history too, so older than 30 days comes in; refused, the
    last 30 days only. Each session becomes a workout of Train's own store (D7: sealed,
    synced), named by its app, with the samples Health Connect holds for it (heart rate,
    distance, power). Another app's route asks consent per session, so none comes by
    itself: an imported workout's page offers *Bring in the track*. An imported one's
    name and notes can be edited, and it can be deleted, remembered on this phone so it
    doesn't come back. Doubles: a session mostly overlapping one of Train's own of the
    same kind isn't added; where Train's has no heart rate, the other's is added to it,
    otherwise it is left out. Weight and resting heart rate wait for what uses them
    (calories, zones), with the synced model they need.
  - No migration: an imported workout is a workout, its origin one more field of the
    sealed, versioned payload.

  Writing built so (28 Oct), with choices of mine for the developer to look at: turning
  writing off leaves what was written there; a session's title is the workout's name
  only where the person gave one (Health Connect names it by the app otherwise); the
  route is the track as the page draws it (the precise fixes while moving), the speed
  a sample at each of its fixes after a piece's first; steps aren't written, as Train
  keeps none (the step detector only decides auto-pause); a workout is written again
  only where its payload's fingerprint changed; a write Health Connect refuses later
  (the permission taken back there) turns writing off and the screen says so; Android
  names only the apps the manifest lists (Gadgetbridge, Christopeit Connect,
  OpenTracks), any other shows by its package; the rationale opens only while someone
  is signed in. Train needs Android 8 (API 26) now, as `connect-client` 1.1.0 does.
  Not yet tried on a phone.

  Reading built so (28 Oct), with choices of mine for the developer to look at: a
  session another app deletes there stays in Train (Train's store is what counts); a
  treadmill's run is a run and a stationary bike's ride a ride, any other kind stays out;
  a double is the same kind overlapping for at least half the shorter of the two, and a
  session doubling one brought in from another app is left out too (two apps writing
  the same row); a ride's power isn't kept, having no series of its own yet (only the
  rower's has); an app turned off later keeps what it brought; a route location
  without its accuracy counts as precise; *Bring in the track* shows on every imported
  workout without one but rows, as whether a route is there isn't kept. Reading runs as
  the app opens and comes to the front, and an app turned on brings its earlier
  sessions at once. Not yet tried on a phone.

## The Polar Loop (28 Oct 2026, the developer's)

- **D32 The band's own days first** (recommended, the developer's; over its live heart
  rate first, and over its setup and sessions alone): 7a setting up and keeping the band
  (first-time use from Train, its battery, its clock, its firmware as D10 has it); 7b its
  daily data (steps, active time, calories, 24/7 heart rate, sleep, Nightly Recharge,
  skin temperature), new synced, sealed models and a first screen of days and nights; 7c
  its training sessions as workouts; 7d the Loop as live heart rate in a recording
  without a strap. All-day tracking is what the Loop is for, and its memory must be read
  regularly or it fills; a strap gives better heart rate in a workout than the wrist.
  Through Polar's BLE SDK (8.3.0, its notice kept; from JitPack), which covers the Loop
  with the Polar 360. Its licence isn't permissive, as this said first: D36.
- **D33 The body's figures in a sealed, synced profile** (recommended, the developer's;
  over this phone alone, and over keeping them nowhere): one synced model, `profile`,
  its payload sealed under the person's key as a workout's (D7), holding what the Loop's
  first-time use needs (sex as Polar's formulas take it, birth date, height, weight,
  resting and highest heart rate, VO2max, training background, typical day, sleep goal),
  each optional; a *Your body* screen edits them, the band's setup asks only for what is
  missing and offers Polar's defaults (60, 220 less the age, its VO2max table) to the
  band alone, a blank staying blank in Train; a change reaches the band at its next
  connection. The same figures serve calories and heart-rate zones later (D31), on every
  device of the person's. Weight is one current value for now.
- **D34 Setting up and keeping the Loop** (the developer's; 28 Oct): `profile` one row
  per person, its fields as D33's (sex female or male, birth date, height in cm, weight
  in kg, resting and highest heart rate, VO2max, Polar's six training backgrounds, typical
  day sitting, standing or moving, sleep goal 5 to 11 h), each optional, edited on *Your
  body* from the account sheet. A *Polar Loop* row there (Android) opens the band's
  screen; *Set up a Loop* opens a sheet looking for Loops nearby, nearest first; the one
  chosen is connected through Polar's SDK (Android's pairing dialog may show; nearby
  devices asked first where not granted), set up with the profile (a short form asks
  for what is missing, Polar's defaults offered to the band alone), its clock set, and
  remembered in the person's folder (`loop.json`, not synced: one phone holds it, D3).
  The band's screen: its name, battery and charging, firmware version, connected or when
  last seen; *Set up again*; *Let the band go* (a factory reset after asking, the only way
  to pair it elsewhere). The SDK connects while Train is open and the band in reach, each
  connection setting its clock and sending what changed in the profile; background
  syncing is decided with 7b. Firmware moves after 7b, as D10 syncs the band before an
  update erases it. A Kotlin class of `MainActivity`'s on `app.halkyon.train/loop`, the
  SDK at 8.3.0 from JitPack (a repository for Polar's package alone, its checksums
  verified), Polar's notice in the licence list.

- **D35 A changed profile reaches the band when the person says** (recommended, the
  developer's; 28 Oct; over sending it at each connection, as D34 had it, and over only
  at setup): in Polar's SDK 8.3.0 only the first-time use writes the body's figures
  (`/U/0/S/PHYSDATA.BPB`), and it writes a new user identifier each time too
  (`/U/0/USERID.BPB`, made from the time); no public call writes the figures alone. What
  a new identifier does to what the band holds isn't known until it is tried on a real
  Loop, so the band's screen offers *Send to the band* once the profile differs from
  what the band was set up with, and that runs the first-time use again; should it
  prove harmless, sending can become automatic.
- **D36 Polar's SDK under its own licence, in the one build** (the developer's; 28 Oct;
  over a second Android flavour with the SDK alone, a protocol of Train's own worked
  out from the band's traffic, and setting the Loop aside): D32 called the SDK's
  licence permissive, wrongly. `Polar_SDK_License.txt` (its 8.3.0 tag) is Polar's
  *Software Development Kit Limited License Agreement*: the object code may be
  distributed only together with an app and only to move data between Polar's devices
  and it (3.1); Polar may end it with 90 days' notice (10.2); neither Polar's name nor
  its products' may be used in marketing, nor any association implied, without its
  written consent (4.4); Finnish law. Train's Android app links it all the same, under
  an added permission of section 7 of the AGPL for that library
  (`apps/train/halkyon_train_flutter/LICENSE-polar-exception.md`; `LICENSE.md` names it);
  F-Droid's main repository would refuse Train so, and a listing of Train in the
  project's own repository is to declare `NonFreeDep`. Its sources on GitHub are under
  the same agreement, so a protocol of Train's own couldn't be written from them.
  Polar's licence and the notices of what the SDK bundles
  (`ThirdPartySoftwareListing.txt`) are in the licence list, on Android.

  7a built so (28 Oct), with choices of mine for the developer to look at: the band's
  defaults for a VO2max not given are the middle of the Moderate class of Polar's
  fitness tables (its manuals', after Shvartz and Reibold 1990), by sex and five-year
  age rows from 20 to 65, the first row below 20 and the last above 65, since Polar
  publishes no default of its own; a height above Polar's 240 cm goes to the band as
  240 (Your body takes up to 250); the setup asks for sex, birth date, height, weight,
  training background, typical day and sleep goal where the profile lacks them, each
  needed, on a page over the sheet, and keeps the answers in Your body; Loops are found
  by a name beginning *Polar Loop*; the SDK is made only once a person's app opens on
  Android, reconnects by itself while the band is remembered, and never asks for its
  telemetry feature; the clock is set as each connection is ready (and with each
  setup), a failure only logged; *Let the band go* keeps the band where it isn't in
  reach and says so; a band chosen that doesn't connect within 60 s, or refuses, isn't
  remembered and the search goes on. The SDK's files are pinned by SHA-256 in Gradle's
  `verification-metadata.xml` as JitPack served them on 28 Oct, every other group
  trusted as before. Not yet tried with a Loop.

- **D37 The Loop's days and nights** (recommended, the developer's; 28 Oct): what Polar's
  SDK reads of each day (its summary: steps, distance, calories of activity, training
  and rest, time in each activity class; its 24/7 heart rate; its sleep with phases,
  cycles, rating and skin temperature; Nightly Recharge; skin temperature) kept as
  Train's own models, as workouts are (over the band's raw files sealed as they come,
  which other devices could read only with Polar's protobuf definitions, under Polar's
  licence, and over summaries alone): one sealed, synced `day` per date (summary,
  Nightly Recharge, skin temperature), one `night` per sleep (its phases), and the day's
  heart rate in chunks (`day_series`, as a workout's series), each versioned JSON,
  gzipped and sealed. The band is read while Train is open and also in the background
  (over only while open): a WorkManager job every few hours connects and reads the days
  since the last read into a plain file of the person's folder (app-private, as
  `recording.txt`), which the app seals and syncs as it opens, since only the Dart side
  holds the person's key. Train deletes nothing on the band (over deleting the days
  kept), whose memory is taken to roll over by itself until a real band shows. Home
  gains *Days*: each day's steps and active time with the night before's sleep and
  Nightly Recharge, a day's page its heart rate as a chart, the night's phases and its
  skin temperature. Steps, sleep and heart rate to Health Connect wait (D31 writes
  workouts). D35 stays: the SDK's low-level `writeFile` could write the figures alone
  (`PHYSDATA.BPB`) without a new user identifier, to be weighed once a real band shows
  what a new identifier does.

- **D38 What the band holds is deleted once Train keeps it** (recommended, the
  developer's; 28 Oct; revises D37's third point, over deleting a day once it is in the
  phone's file, and over never deleting): Polar's own sync guideline (the SDK's
  `documentation/SyncImplementationGuideline.md`, 8.3.0) asks apps to remove what they
  synced, as soon as possible, to free the device's memory and not lose data, sensors
  such as the Polar 360 holding less than watches; D37 had assumed the band's memory
  rolls over by itself. So once the app has sealed a day (yesterday or earlier) into
  the person's database, the next read deletes it on the band, type by type (activity,
  daily summary, 24/7 samples, sleep and its score, Nightly Recharge, skin temperature
  and skin contact); the background job reads into the file alone and deletes only
  what the app confirmed kept; today is never deleted. Each read is framed by the
  SDK's sync start and stop calls, its calls to the band one after the other, as the
  guideline asks. The VO2max defaults of 7a are Polar's table too: the SDK's
  `documentation/FirstTimeUse.md` gives the same tables, without saying which class
  to take.

  7b built so (28 Oct), with choices of mine for the developer to look at: home has two
  tabs, *Workouts* and *Days*, on every device, *Days* saying where days come from
  while there are none; the first read of a band goes four weeks back, later ones from
  the day after those kept; the band is read as each connection is ready, at most every
  half hour, and when the person pulls the list; a later read replaces what it brought
  of a date (its summary, Recharge, skin temperature, heart rate, night) and keeps the
  rest, and rewrites nothing unchanged; a day's heart rate goes in chunks by the three
  hours each measurement began in, every row's ID made from the person and the date
  (`https://halkyon.app/train/day/<person>/<date>`, `…/night/…`, `…/heart/<index>`); a
  row that doesn't open on a device is left out of *Days*, as its date is sealed with
  it; what is kept goes up to the day before the latest read's (`keptUntil`, in
  `loop.json` with the last read's time), kept over a new setup of the same band and
  forgotten for another; the background job runs every four hours while the battery
  isn't low and the open app doesn't hold the band, and reads from the same day until
  the app has kept what it wrote, so the file can hold a day more than once, the last
  line winning; the app moves the file aside before reading it and deletes it once
  sealed, so lines written meanwhile wait; the schedule is the phone's, for the last
  person whose app opened with a band, and *Let the band go* stops it. Polar's
  NONREM12 and NONREM3 are shown as light and deep sleep; the chart draws each 24/7
  measurement's mean at its start; a night's times show in the phone's time zone. Not
  yet tried with a Loop: whether the SDK's deletion includes its `until` day, and
  whether the job connects to the band in the background, are for a real band.

- **D39 The Loop's training sessions are workouts** (recommended, the developer's; 28
  Oct): the band records a session by itself when it detects one (at least ten minutes
  of raised heart rate and movement, saved as *Other indoor* unless the person picks a
  sport; Polar's manual, *Automatic training detection*), which Polar's SDK reads from
  the band (`PolarTrainingSessionApi`: a summary with start, duration, sport,
  calories, distance and ascent, and samples at an interval: heart rate, and speed,
  distance and cadence where the band had them). Each becomes a sealed, synced
  workout, its heart rate its series, its origin *Polar Loop* with the band's
  reference of the session, as Health Connect's imports are (D31), so there is one
  list and one page, written to Health Connect with the rest (over a model of its own
  under *Days*). Train gains an activity *Other workout* for what it doesn't know,
  which the person can change on the workout's page; Polar's running (sport 1),
  cycling (2) and other outdoor (16, as *Other*) are the IDs the SDK itself names,
  every other sport comes as *Other* until a band shows its IDs (over leaving out
  what Train doesn't know, most of the band's sessions). A session overlapping a
  workout Train recorded for at least half the shorter, whatever the sports, is that
  workout's double: the recorded one stays and gains the band's heart rate where it
  has none (over keeping both). Sessions are read with the days, by the open app and
  the background job, and deleted on the band once sealed by the same rule (D38); one
  deleted in Train is remembered, so it isn't brought back.

  7c built so (28 Oct), with choices of mine for the developer to look at: a
  multisport session is a workout per exercise, each with the band's reference and its
  index as its ID; the workout's moving time is the session's length, its distance the
  band's sum, its series the heart rate alone (speed and cadence samples are read but
  not kept, as Train has no series for them yet, and a band without GPS has no
  track); a heart-rate sample of 0 is none; every session the band holds is listed at
  each read (they are few, as the kept ones go), those from the read's first day on
  read and those up to the day kept deleted; the sessions taken in are remembered in
  `loop.json` by ID with their date, forgotten a week after that date is kept, and a
  session already kept as a workout (`loop.json` lost) isn't brought again either; a
  double is looked for among the workouts Train recorded alone, not those brought in
  from Health Connect; *Other workout* can't be recorded on the phone (the record sheet
  offers it not), and the edit sheet changes any workout's activity. Polar's training
  sessions are protobuf messages of the SDK's, so the app names protobuf-javalite
  3.20.0, the SDK's own, for compiling (`compileOnly`), adding nothing to the APK.
  Health Connect's writing takes the Loop's workouts as Train's own, its *bring in the
  track* stays for Health Connect's. Not yet tried with a Loop: Polar's other sport
  IDs, and whether the band keeps samples of a detected session, are for a real band.

- **D40 How D10's firmware updates go** (recommended, the developer's; 28 Oct): Polar's
  SDK asks Polar's firmware service (`https://firmware-management.polar.com`, `POST
  /api/v1/firmware-update/check` with a client name, the band's UUID made from its ID,
  its firmware version and hardware code; read in the SDK's classes, 8.3.0) and gets a
  version and a file's address. Train asks once a day, on the day's first connection,
  and when the person taps *Check for updates* (over only when asked); an update found
  shows on the Loop screen with its version and what Polar learns, and never starts by
  itself. Through the instance, Train's server asks Polar for the person and fetches
  the file, keeping the latest per hardware and version in memory for a day (over a
  folder on disk, which the deploy would need a volume for); it offers this by default,
  for the people signed in, and its operator turns it off with
  `LOOP_FIRMWARE_RELAY=off` (over off by default, and over always on), the app then
  offering the direct way alone. Before an update Train reads and keeps what is on the
  band; after it, Train sets the band up again at once with the figures it had and sets
  its clock (over asking first), the band getting a new user identifier as with every
  setup (D35).

## Consequences

- Train's sensors are one pattern: open profiles read by the app itself, on the phone;
  the Loop through Polar's SDK, also on the phone.
- The recording runs in an Android foreground service from its first version, so that it
  survives the screen going off.
- The Loop can't be used with Polar Flow while Train holds it; going back needs a factory
  reset of the band.
- The desktop connects to no device and records nothing; it shows and plans, with what
  the phones recorded coming through the server.