The collaborative listening party. Guests queue tracks, the crowd votes them up or down, and the highest-scored track plays next. Modelled as three small aggregates — Fissa, Track, Wallet — that coordinate by domain events rather than a shared transaction.
Fissa:
A live listening party, root pin. A small aggregate: owner, the currentlyPlaying
pointer, expectedEndTime, and the member roster. References its Tracks and Wallets by
id — it does not contain them. Owns: the pointer aims at a real Track or nothing;
lifecycle; owner-only actions.
Avoid: Room, session, "the whole party object".
Track:
A queued song, root (pin, trackId), owning its Votes and Score. The voting
consistency boundary — a vote locks one Track, not the party. Owns: re-vote replaces the
prior vote (delta new − previous); a guest never earns from voting on their own track.
Avoid: Song, queue item.
Wallet (= Membership):
A guest's participation in one Fissa, root (pin, userId), holding their Points
balance. Owns the points >= 0 on spend invariant — the reason it's a separate
aggregate. Starts at 50.
Avoid: Player row, participant, account.
Vote:
A guest's up (+1) / down (−1) reaction to a Track, unique per (pin, trackId, user).
Lives inside the Track aggregate.
Score:
A Track's current net votes, driving queue order (highest plays next); resets to 0 when
the track plays. totalScore is the lifetime net and never resets.
Avoid: Rank, weight.
Points: A spendable per-Fissa currency held by a Wallet. Governed by one principle: earning is eventual, spending is transactional (see below). Avoid: Score (that belongs to a Track), coins, credits.
Badge:
A lifetime, cross-Fissa counter per (user, name). A projection, not an aggregate:
it folds domain events into counts and protects no invariant. Some badges debounce
(max once per 24h).
Avoid: Achievement object, points aggregate.
Playback:
The engine that advances a Fissa's currentlyPlaying pointer — it decides what plays
next and when, drives the Spotify player, tops up the queue with recommendations, and
rewards the played Track's owner. Distinct from the Fissa aggregate, which owns the
pointer's lifecycle and the owner-only commands (skip, restart, pause) that ask Playback
to act. Playback reads/writes through repositories; it holds no invariant of its own.
Avoid: Player (that's Spotify's device), scheduler (that's the background sync loop).
Earning is eventual, spending is transactional:
Vote-credit, play-reward, and skip-penalty raise PointsAwarded events that the Wallet
folds shortly after the cause commits (a ~50ms balance lag is accepted). Spending is a
synchronous Wallet command that checks points >= 0 against committed state, so the
floor always holds.
Outbox: Domain events are written to an outbox table in the same transaction as their cause and drained by a worker — at-least-once delivery. Folds must be idempotent (dedupe by event id) so redelivery never double-credits.
Domain events:
PointsAwarded {eventId, walletPin, userId, amount, reason} ·
VoteCast {byUser, forUser, direction} · TrackAdded {userId, count} ·
FissaCreated {userId} · MemberJoined {userId}. Raised by an aggregate, drained
from the outbox into the Badge projection and Wallets.
- Score is overloaded: a Track's net votes vs. a Badge's counter. Always qualify ("track score", "badge score").
- Points floor: hard clamp at 0 everywhere, or floor only on explicit spend while skip penalties may go negative? Undecided — resolve when the first spend command lands.
Dev: A vote comes in on someone else's track. What's the consistency boundary? Domain expert: Just that one Track. Its score moves and the vote is recorded atomically — the party isn't locked. The owner's reward isn't part of that transaction; the Track raises a
PointsAwardedevent. Dev: So when does the owner's balance go up? Domain expert: A beat later — the worker drains the outbox and the owner's Wallet folds the event. Earning is eventual. But if that owner tries to spend, that's a synchronous command on their Wallet that checks the balance can't go below zero. Spending never waits on an event.