MobilityOps/backend/app/services/dispatcher.py

from __future__ import annotations

import logging
import threading
import uuid
from datetime import UTC, datetime, timedelta

import httpx
from sqlalchemy import select

from app.core.config import get_settings
from app.core.db import SessionLocal
from app.models.outbox import OutboxEvent

logger = logging.getLogger("mobilityops.dispatcher")
settings = get_settings()

_stop_event = threading.Event()


def _backoff_seconds(attempts: int) -> int:
    return min(2**attempts, 60)


def _reclaim_stale_deliveries(batch_size: int = 10) -> int:
    """Recover events stuck in 'delivering' because the process that claimed them died
    before recording an outcome. Only leases whose deadline has passed are touched, so an
    in-flight delivery from a still-alive worker is never disturbed or double-processed;
    `attempts` is preserved so the count reflects true history."""
    db = SessionLocal()
    try:
        now = datetime.now(UTC)
        rows = db.scalars(
            select(OutboxEvent)
            .where(
                OutboxEvent.delivery_status == "delivering",
                OutboxEvent.next_attempt_at.is_not(None),
                OutboxEvent.next_attempt_at <= now,
            )
            .limit(batch_size)
            .with_for_update(skip_locked=True)
        ).all()
        for row in rows:
            row.delivery_status = "pending"
            row.next_attempt_at = None
            row.last_error = (
                "Recovered from a stale 'delivering' lease "
                f"(no outcome recorded within {settings.n8n_delivery_lease_seconds:.0f}s; "
                f"the process likely crashed mid-delivery). attempts preserved at {row.attempts}."
            )[:2000]
        db.commit()
        return len(rows)
    finally:
        db.close()


def _claim_due_events(batch_size: int = 5) -> list[uuid.UUID]:
    """Claim a batch of due events with a short-lived transaction (no network I/O held open).
    Each claimed row gets a lease deadline (next_attempt_at) so a crash between this claim
    and the outcome being recorded is recoverable by _reclaim_stale_deliveries."""
    db = SessionLocal()
    try:
        now = datetime.now(UTC)
        rows = db.scalars(
            select(OutboxEvent)
            .where(
                OutboxEvent.delivery_status == "pending",
                (OutboxEvent.next_attempt_at.is_(None)) | (OutboxEvent.next_attempt_at <= now),
            )
            .order_by(OutboxEvent.occurred_at.asc())
            .limit(batch_size)
            .with_for_update(skip_locked=True)
        ).all()
        claimed_ids = [row.event_id for row in rows]
        lease_deadline = now + timedelta(seconds=settings.n8n_delivery_lease_seconds)
        for row in rows:
            row.delivery_status = "delivering"
            row.next_attempt_at = lease_deadline
        db.commit()
        return claimed_ids
    finally:
        db.close()


def _deliver_one(event_id: uuid.UUID) -> None:
    db = SessionLocal()
    try:
        event = db.get(OutboxEvent, event_id)
        if event is None:
            return
        # Reconstruct the wire envelope from contracts/events.schema.json: only the fields
        # the schema declares (additionalProperties: false), sourced from real columns where
        # possible. `payload_json` also carries an internal `aggregate_ref` convenience field
        # for our own dashboard/audit reads, which must not be forwarded to n8n.
        try:
            wire_event = {
                "event_id": str(event.event_id),
                "event_type": event.event_type,
                "occurred_at": event.occurred_at.isoformat(),
                "correlation_id": event.payload_json["correlation_id"],
                "aggregate": event.payload_json["aggregate"],
                "data": event.payload_json["data"],
            }
            payload_error: str | None = None
        except KeyError as exc:
            # A malformed payload must still resolve the claimed "delivering" row to a
            # terminal-or-retryable state below, rather than leaving it stuck forever.
            wire_event = None
            payload_error = f"Malformed outbox payload, missing key {exc}"
        attempts = event.attempts
    finally:
        db.close()

    if wire_event is None:
        success, error, body = False, payload_error, None
    else:
        try:
            response = httpx.post(
                settings.n8n_webhook_url,
                json=wire_event,
                timeout=settings.n8n_http_timeout_seconds,
            )
            response.raise_for_status()
            body = response.json()
            success = bool(body.get("ok", True))
            error = None if success else f"n8n reported failure: {body}"
        except httpx.HTTPError as exc:
            success = False
            error = f"{type(exc).__name__}: {exc}"
            body = None

    db = SessionLocal()
    try:
        event = db.get(OutboxEvent, event_id)
        if event is None:
            return
        event.attempts = attempts + 1
        if success:
            event.delivery_status = "succeeded"
            event.last_error = None
            event.next_attempt_at = None
            event.external_run_id = str((body or {}).get("event_id", event_id))
        else:
            event.last_error = (error or "delivery failed")[:2000]
            if event.attempts >= settings.n8n_max_attempts:
                event.delivery_status = "failed"
                event.next_attempt_at = None
            else:
                event.delivery_status = "pending"
                event.next_attempt_at = datetime.now(UTC) + timedelta(
                    seconds=_backoff_seconds(event.attempts)
                )
        db.commit()
    finally:
        db.close()


def run_dispatch_cycle() -> int:
    """Run one reclaim+claim+deliver cycle. Returns the number of events processed."""
    _reclaim_stale_deliveries()
    claimed = _claim_due_events()
    for event_id in claimed:
        _deliver_one(event_id)
    return len(claimed)


def _loop() -> None:
    while not _stop_event.is_set():
        try:
            run_dispatch_cycle()
        except Exception:  # noqa: BLE001
            logger.exception("Outbox dispatch cycle failed")
        _stop_event.wait(settings.n8n_dispatch_interval_seconds)


def start_background_dispatcher() -> None:
    if not settings.n8n_dispatch_enabled:
        return
    _stop_event.clear()
    thread = threading.Thread(target=_loop, name="outbox-dispatcher", daemon=True)
    thread.start()


def stop_background_dispatcher() -> None:
    _stop_event.set()