# Runtime and Components

The diagram below focuses on the deployed `galaxy/backend` process and
its runtime dependencies. Every component is wired in
`backend/cmd/backend/main.go`.

```mermaid
flowchart LR
    subgraph Inbound
        Gateway["Gateway<br/>HTTP + gRPC push subscriber"]
        Probes["Liveness / readiness<br/>probes"]
    end

    subgraph BackendProcess["Backend process"]
        HTTP["HTTP listener<br/>:8080<br/>/api/v1/{public,user,internal,admin}"]
        Push["gRPC push listener<br/>:8081<br/>Push.SubscribePush"]
        Metrics["Optional Prometheus<br/>metrics listener"]
        AuthSvc["auth.Service"]
        UserSvc["user.Service"]
        AdminSvc["admin.Service"]
        LobbySvc["lobby.Service"]
        RuntimeSvc["runtime.Service"]
        MailSvc["mail.Service"]
        NotifSvc["notification.Service"]
        GeoSvc["geo.Service"]
        PushSvc["push.Service<br/>(ring buffer + cursor)"]
        Caches["Write-through caches<br/>auth / user / admin /<br/>lobby / runtime"]
        MailWorker["mail worker"]
        NotifWorker["notification worker"]
        Sweeper["lobby sweeper"]
        RuntimeWorkers["runtime worker pool +<br/>scheduler + reconciler"]
        Telemetry["zap + OpenTelemetry"]
    end

    Postgres[(Postgres<br/>backend schema)]
    Docker[(Docker daemon)]
    SMTP[(SMTP relay)]
    GeoDB[(GeoLite2 mmdb)]
    Game[(galaxy-game-{id}<br/>engine containers)]

    Gateway --> HTTP
    Gateway --> Push
    Probes --> HTTP

    HTTP --> AuthSvc & UserSvc & AdminSvc & LobbySvc & RuntimeSvc & MailSvc & NotifSvc & GeoSvc
    Push --> PushSvc

    AuthSvc & UserSvc & AdminSvc & LobbySvc & RuntimeSvc & MailSvc & NotifSvc --> Caches
    AuthSvc & UserSvc & AdminSvc & LobbySvc & RuntimeSvc & MailSvc & NotifSvc & GeoSvc --> Postgres

    MailWorker --> Postgres
    MailWorker --> SMTP
    NotifWorker --> Postgres
    NotifWorker --> MailSvc & PushSvc
    Sweeper --> LobbySvc
    RuntimeWorkers --> Docker
    RuntimeWorkers --> Game
    RuntimeWorkers --> RuntimeSvc

    GeoSvc --> GeoDB

    HTTP & Push & MailWorker & NotifWorker & Sweeper & RuntimeWorkers --> Telemetry
```

## Process lifecycle

`internal/app.App` orchestrates startup and shutdown. The start order
is fixed:

1. Load configuration with `internal/config.LoadFromEnv` and validate.
2. Build the zap logger and OpenTelemetry runtime.
3. Open the Postgres pool through `internal/postgres.Open`.
4. Apply embedded migrations with `pressly/goose/v3` before any
   listener binds.
5. Build the push service (no listener yet) so domain modules can be
   given a real publisher.
6. Build domain services in dependency order: geo → user (uses geo)
   → mail → auth (uses user, mail, push) → admin → lobby (uses runtime
   adapter, notification adapter, user-entitlement adapter) → runtime
   (uses lobby consumer) → notification (uses mail, push, accounts).
7. Warm every cache (`auth`, `user`, `admin`, `lobby`, `runtime`).
   Each cache exposes `Ready()`; `/readyz` waits on every flag.
8. Wire HTTP handlers and the gin engine.
9. Start the HTTP server, the gRPC push server, the mail worker, the
   notification worker, the lobby sweeper, the runtime worker pool,
   the runtime scheduler, and the reconciler. The optional
   Prometheus metrics server is added only when configured.

`app.New` accepts a `shutdownTimeout` (`BACKEND_SHUTDOWN_TIMEOUT`,
default `30s`). On `SIGINT`/`SIGTERM`, components are stopped in
reverse order:

1. Refuse new HTTP and gRPC traffic.
2. Drain in-flight requests (`BACKEND_HTTP_SHUTDOWN_TIMEOUT`,
   `BACKEND_GRPC_PUSH_SHUTDOWN_TIMEOUT`).
3. Flush the mail worker's currently-running attempt; pending rows
   stay in the database for the next process to pick up.
4. Flush push events that already left domain services to the gateway
   buffer.
5. Drain pending geo counter goroutines.
6. Close the Docker client and the runtime engine HTTP client.
7. Close the Postgres pool.
8. Shut down telemetry, flushing any buffered traces.

The smaller of `BACKEND_SHUTDOWN_TIMEOUT` and the per-component
deadline always wins.

## Cyclic dependency adapters

Several domain pairs are mutually dependent (auth↔user for session
revoke on permanent block; lobby↔runtime for start/stop calls and
snapshot push-back; user/lobby/runtime↔notification for fan-out
publishers). The wiring code in `cmd/backend/main.go` constructs a
small adapter struct first, then patches its inner pointer once the
real service exists. The adapters live next to the wiring code and
never grow domain logic; they are pure forwarders that fall back to a
no-op when the inner pointer is still `nil` (the initial state during
boot).

## Worker pools

- **Mail worker** (`internal/mail.Worker`) — single goroutine that
  scans `mail_deliveries` with `SELECT ... FOR UPDATE SKIP LOCKED`,
  sends through SMTP, records the attempt, and either marks `sent` or
  schedules `next_attempt_at` with backoff plus jitter. Drains pending
  and retrying rows on startup.
- **Notification worker** (`internal/notification.Worker`) — same
  pattern over `notification_routes`: pulls a route, dispatches push
  or email, writes the outcome, and either marks delivered or moves
  the route into `notification_dead_letters` after the configured
  attempt budget.
- **Lobby sweeper** (`internal/lobby.Sweeper`) — `pkg/cronutil` job
  that releases `pending_registration` Race Name Directory entries
  past `BACKEND_LOBBY_PENDING_REGISTRATION_TTL` and auto-closes
  enrollment-expired games whose `approved_count >= min_players`.
- **Runtime worker pool** (`internal/runtime.Workers`) — bounded
  concurrency (`BACKEND_RUNTIME_WORKER_POOL_SIZE`) over a buffered
  channel (`BACKEND_RUNTIME_JOB_QUEUE_SIZE`). Long-running pulls and
  starts execute here; the calling path returns as soon as the job is
  queued. After Docker reports the container running, the worker
  polls the engine `/healthz` until the listener is bound (Docker
  marks a container running as soon as the entrypoint starts; the
  Go binary inside takes a moment to bind its TCP port). Only after
  `/healthz` succeeds does the worker call `/admin/init`.
- **Runtime scheduler** (`internal/runtime.SchedulerComponent`) —
  `pkg/cronutil` schedule per running game; each tick invokes the
  engine `admin/turn`. Force-next-turn flips a one-shot skip flag in
  `runtime_records`; the next scheduled tick observes the flag and
  consumes it.
- **Runtime reconciler** (`internal/runtime.Reconciler`) — periodic
  list of containers labelled `galaxy.backend=1`, matched against
  `runtime_records`. Adopts unrecorded labelled containers, marks
  recorded but missing as `removed`, and emits
  `lobby.OnRuntimeJobResult` for the latter.

## Telemetry

Tracing covers `HTTP request → domain operation → Postgres call →
external client (SMTP, Docker, engine)`. zap injects `otel_trace_id`
and `otel_span_id` into every log entry written inside a request
scope. OTel exporters honour `BACKEND_OTEL_TRACES_EXPORTER` and
`BACKEND_OTEL_METRICS_EXPORTER`; both default to `otlp` and accept
`none`, `stdout`, and (for metrics) `prometheus`.

`TraceFieldsFromContext(ctx)` is exposed by
`internal/telemetry.Runtime` rather than the logger package because
the helper is used by middleware and depends on the OTel runtime, not
the logger configuration. Keeping it next to the runtime keeps
`server → telemetry` import direction one-way.