station-master/docs/architecture/lobby-and-sessions.md

# Lobby and Sessions

Everything outside the rules engine: creating and joining games, seating, reconnection, and
persistence. Target is **small self-hosted** scale — a handful of concurrent games among people who
know each other.

Written against [`../rules/rules-v0.2.md`](../rules/rules-v0.2.md).

---

## 1. Identity

No accounts. Access to the server is a **join secret** (§2); identity within a game is a **display
name** plus a **session token** the server issues on join and the client stores locally.

```
Session
  token       : opaque, unguessable
  gameId
  player      : PlayerIndex
  displayName
```

**It names the PLAYER, not the seat.** Those became different things in v0.4.0, and the difference is
exactly the case this field has to survive: under Employee Rotation (§4) a player changes chairs while
their Revenue and their identity stay with them. A token naming a seat would sit a returning player
down in someone else's Office. The seat is derived with `seatOf(state, player)` whenever it is needed,
which is one lookup and always current.

The token is what makes reconnection work: it proves "I am the player who was in this game," which is
the only identity claim the game needs. Keep it out of URLs so it is not shoulder-surfed or pasted
into a chat.

**Tokens are per-origin.** The server may be reached by more than one address — `stationmaster.<domain>`
and `<ip>:<port>` are both expected — and browser storage is scoped to the origin. A player who joined
at one address must come back to that address, or they are a stranger with no token. Say so in the
UI at join time rather than letting someone discover it when they cannot get back in.

Real accounts can be layered on later without touching the rules engine, which is exactly why
[`overview.md`](overview.md) keeps that boundary sharp.

---

## 2. Creating and joining

```
Lobby.Create { secret, config }            →  { gameId, gameCode, token }
Lobby.Join   { secret, gameCode, displayName }  →  { token, player }
```

**These four messages are the one family with no types behind them**, because the engine has no
concept of a lobby — `intents.ts` starts at `localOps.choose`. Everywhere else, the code is the
protocol ([`protocol.md`](protocol.md)); here the prose is, until the server exists.

**The join secret gates the door.** Server-wide, set by environment variable, passed out of band by
whoever runs the server (`multiplayer.md` D14). Anyone holding it may create a game and may join any
created game that has not started. It is not per-game and it is not an account — it is the cheapest
thing that stops a clearnet-reachable box from being someone else's game server. Typed once and kept
per-origin, alongside the token.

A **game code** — short, human-speakable, e.g. `RAIL-4471` — is the discovery mechanism *inside* the
door. No matchmaking, no browsing, no public game list. Players are already talking to each other; the
code just needs to survive being read aloud.

**One game at a time per person is expected usage and is deliberately not enforced** (`multiplayer.md`
§10). Enforcing it needs cross-game state whose only job is deciding when to release someone, and
getting that wrong locks a player out of their own server.

The creating player is the **host**: they set the config (§3) and start the game. **If the host leaves
before `Lobby.Start`, host rights pass to the earliest-joined remaining player.** No lobby should ever
be stuck waiting on somebody who closed a tab, and the alternative — everyone leaves and someone makes
a new game — throws away the seating they had already agreed. Host rights end at `Lobby.Start`
regardless: the rules give no player special standing during play, and the Superintendent rotates
independently (§4).

**Player counts.** Solitaire is 1. Competitive and Co-op are **2 to 4**. Nothing in the engine enforces
a limit, so the lobby is where it is enforced — and the number is set to what is actually exercised
(`test/multiplayer.test.ts` plays 2, 3 and 4 to a finish) rather than to a guess. The cost of more is
real: the Division grows by one Office and one Mainline card per player (§4.3), and every added player
lengthens both the Superintendent's rotation and the phase-major waiting. Raise the cap when somebody
has played a bigger game and reported back, not before.

This is a lobby judgment, **not a rules limit** — §7 of the rules describes what happens at five or
more players (the New Train round ends mid-pass when the consist fills) and the engine implements it.
The cap is about what has been played, not about what the game can do.

---

## 3. Configuration, and when it locks

Set before start, immutable after:

```
mode          : solitaire | competitive | coop
victory       : firstToTarget | highestAfterDays
length        : short | standard | campaign
optionalRules : { reducedVisibility, sisterTrains, employeeRotation, emergencyToolbox }
```

These must lock at `Lobby.Start`. Changing `length` mid-game would move the finish line; changing
`mode` would switch which failure floors apply (§3.4, §3.5). Neither has a coherent meaning
mid-game, so the server should refuse rather than try.

`mode` and the seat count are checked together at start: solitaire is exactly 1, competitive and
co-op are 2 to 4 (§2). The engine will happily build a 7-player Division, so this is the only place
the limit exists.

---

## 4. Seating

**Seating order is not cosmetic.** The players form a physical west-to-east chain of Offices (§4.3),
and seat order determines three separate things:

1. Which Office is adjacent to which — and therefore where each player's trains arrive from.
2. The Superintendent rotation, which passes one seat left every three Stages (§5).
3. Acting order within a phase, which starts at the Superintendent and proceeds left (Gap 1).

So seating must be settled before the opening D12 rolls, and the lobby should show the chain
visually — a player should see whose Office lies east and west of theirs before the game begins.

**The opening rolls** (§4.4, §4.5) happen server-side at `Lobby.Start`, from the game seed: highest
D12 takes the Eastern Division Point, and a second roll picks the opening Superintendent.

**Both are implemented.** The Division roll was drawn and discarded until v0.4.1 — `void
divisionRolls`, with seating fixed by array order — so §4.4 decided nothing and the seat/player split
was untested at runtime. It orders the chain now: `seating[seat] = player`, ascending by roll from
seat 0 (west, beside the Western Division Point) to the last seat (east, "highest is the Eastern
Division Point").

The rule names only those two ends, because at a table the players are already sitting in a chain and
the roll only says which way round it is. There is no physical table here, so the roll orders
everybody — it uses a number every player is already told to roll, and it makes the roll matter to
more than the winner. Ties break toward the lower player index sitting further east, the same
first-max-wins convention the Superintendent roll uses.

Both rolls are kept on the state as `openingRolls`, indexed by player, **so the lobby can show the
rolls rather than only their outcome** — it is the game's first moment of drama and there is no reason
to hide it. Show the chain forming.

**Solitaire is unchanged and must stay so:** one player is one seat, seating is trivially `[0]`, and
every published replay depends on that.

**Employee Rotation** (Appendix B), if enabled, moves every player one seat left at the end of each
Day, carrying their Revenue and the Fedora with them. **The model supports this as of v0.4.0**:
offices and districts are keyed by seat, hands and Revenue and the Fedora by player, and `seating[]`
maps between them, so rotating is a one-line operation and every `areaOf(state, player)` caller
follows without changing. The *rule* is still unimplemented — nothing rotates `seating` yet — but the
structure it needs is in place and tested, which was the point of doing it early.

---

## 5. Disconnection and reconnection

Being turn-based makes this far easier than it would be in a real-time game. (It is no longer "one
actor at a time" — v0.4.0 gave every player their own `TurnState` precisely so that players can act in
parallel where the rules allow it, `multiplayer.md` D19. Nothing about disconnection changes: the game
still waits on whoever it is waiting on.)

**On disconnect:** keep the seat. Do not remove the player, do not auto-play. The game simply waits
if it was their turn. Broadcast a disconnect notice so everyone else can see why nothing is happening
— silence with no explanation is the worst version of this. There is no such event type today; it is
server-layer news about a *connection*, not about the game, so it belongs with the transport rather
than in `GameEvent`, which must stay replayable from a seed.

**On reconnect:** the client presents its token and the server replies with a **full current view**.
Not an event tail — a returning client needs the position, not the history of how it got there, and
the server can always produce the position because it holds the game
([`protocol.md`](protocol.md) §3). This is why catching up is not a special case.

**On a player who does not come back:** the honest options at this scale are to wait, or to let the
host end the game.

**Nothing moves on an absent player's behalf.** No forcing timer, no bot stepping in. Jesse's call,
and the reasoning is the same one that keeps bots out of running games generally: the Superintendent's
clearance decision materially affects *other* players' scores, so anything that answers it
automatically changes the game rather than preserving it. Bots fill empty seats **at game start
only** (`multiplayer.md` D8). Two ways out of a permanently-halted game are recorded in `TODO.md` and
deliberately not designed here — a bot playing minimally-damaging defensive moves for someone who
stepped away, and a player *consenting* to resign their railroad to a bot. Both need care; neither is
a timer.

### The turn clock — a stopwatch, not a shot clock

**Record how long each player takes on each turn.** Not to enforce anything: to find out where the
game actually goes. "A 4-player game takes an evening" is currently a guess, and the useful version of
that answer is per-phase — whether the wait is Local Operations, or making up a train, or one player
thinking about a clearance while three others watch. That is what tells you which part is worth
speeding up, and it is the one measurement no amount of bot simulation can produce, because the bot
does not think.

Where it lives matters:

- **Outside the rules engine.** The engine has no clock and must not acquire one — it is pure and
  deterministic given a seed, which is what makes every replay and every test work.
- **Outside the canonical record.** Timings are stored beside `{ engineVersion, seed, config, history }`,
  never inside it. A replay must reproduce a game from decisions alone; if it depended on wall-clock
  data it would no longer be reproducible from a seed.
- **In the session host**, which already sees each intent arrive and already knows whose turn it is.
  Wall-clock at the start of a turn, wall-clock at the intent that ends it, per player per phase.

Show the current turn's elapsed time in the UI if it is unobtrusive — a table can self-regulate on
information alone, which is the polite version of a shot clock and costs nothing.

---

## 6. Persistence

Persist the **intents**, not a state snapshot and not the event log. This section said "event log"
until v0.4.0, on the strength of a `state = fold(events)` claim that was never true
([`protocol.md`](protocol.md) §3) — folding the log rebuilds card plays and switching, and not one
train movement. The intents genuinely do reconstruct a game, they are smaller still, and `fromSave`
already replays them, so a mid-game server restart is a replay rather than a recovery.

```
games        : gameId → { engineVersion, config, seed, status, createdAt, gameCode }
history      : gameId → ordered Intent list
sessions     : token  → { gameId, player, displayName }
```

`engineVersion` is stored beside the seed because a replay is only faithful under the rules it was
recorded with. A version mismatch on load must be refused explicitly rather than replayed and quietly
diverged (`multiplayer.md` Phase 3).

Requirements are modest enough that the storage choice is genuinely open — SQLite on a single node
covers this comfortably, and so would flat files with an append-only list per game. The constraint
worth honouring is that the **history is append-only**: rewriting it breaks the one property that
makes replay trustworthy. (Undo is not an exception — it replays the history *without* the last
intent, and a server does not offer it at all.)

**Snapshotting** is an optimisation, not a requirement. If a Campaign game's history grows long
enough that replay feels slow, periodically store a state snapshot with its intent count and replay
forward from there. Do not build this until it is needed. Measured today: a finished solitaire game
is ~350 intents and a few hundred bytes.

**Retention.** Finished games **keep their history, seed and config**, because post-game replay is a
wanted capability (see [`overview.md`](overview.md#post-game-replay--a-desired-future-capability))
and those three are the only things it needs — the replay viewer on the site already reads exactly
that file. Do not delete finished games by default.

Retention is an operator setting rather than an application decision — a self-hosted instance among
friends may as well keep everything, since a finished game's log is small. If a cap is wanted, expire
by age or by total count and say so plainly in the UI, because a replay that silently stops existing
is worse than one that was never offered.

**Turn timings** (§5) are stored per game beside the history, never inside it, and are retained on the
same terms — they are only interesting in aggregate, and only across enough games to see a pattern.

Session tokens are the exception: those can expire on a short window once their game has finished.

---

## 7. What this deliberately omits

At small self-hosted scale these are not needed, and building them early costs more than it returns:

- **Matchmaking or a public game browser.** The game code covers discovery.
- **Ranking, ladders, persistent profiles.** §3's timed modes produce comparable scores, so a
  high-score table would be a natural first addition — but it is a feature, not infrastructure.
- **Spectators.** Straightforward to add later: a spectator is a `Frame` with no hand and no ability
  to send intents. `snapshot` already takes a viewing seat, so the only new thing is a viewer that
  belongs to no seat — the same shape post-game replay already uses.
- **Horizontal scaling and cross-process coordination.** One process holds every active game
  in memory.