languard-servers-manager/THREADING.md

Languard Servers Manager — Threading & Concurrency Design

Overview

The system uses a hybrid concurrency model:

• FastAPI (asyncio) handles HTTP requests and WebSocket connections
• Python threads (threading.Thread) handle long-running background work per server
• Queue bridges the thread world → asyncio world for WebSocket broadcasting
• SQLAlchemy sync sessions are used in threads (thread-local connections)

The key change for multi-game support: core threads are game-agnostic and receive game-specific behavior (log parsers, remote admin clients) via dependency injection from the adapter.

---

Thread Map

Main Process (FastAPI / asyncio event loop)
│
├── [uvicorn] HTTP/WS event loop                     (asyncio)
│     ├── REST request handlers                       (async def / plain def)
│     └── WebSocket handlers                          (async def)
│
├── BroadcastThread                                   (daemon thread, 1 global)
│     └── Reads from broadcast_queue (thread-safe)
│         Calls asyncio.run_coroutine_threadsafe()
│         → ConnectionManager.broadcast()
│
└── Per-running-server thread group (started when server starts, stopped when server stops):
      ├── ProcessMonitorThread    (1 per server, 1s interval)       — CORE
      ├── LogTailThread           (1 per server, 100ms interval)    — CORE + adapter LogParser
      ├── MetricsCollectorThread  (1 per server, 5s interval)      — CORE
      └── RemoteAdminPollerThread (1 per server, 10s interval)    — CORE + adapter RemoteAdmin

For N running servers, there are:

• 4*N background threads + 1 BroadcastThread = 4N+1 background threads total
• (If adapter has no remote_admin, RemoteAdminPollerThread is skipped → 3*N+1)

---

Adapter Injection into Threads

The ThreadRegistry resolves the adapter at thread creation time and injects game-specific components into the generic core threads:

class ThreadRegistry:
    @classmethod
    def start_server_threads(cls, server_id: int, db: Connection) -> None:
        server = ServerRepository(db).get_by_id(server_id)
        adapter = GameAdapterRegistry.get(server["game_type"])

        threads: dict[str, BaseServerThread] = {}

        # Core threads — always present
        threads["process_monitor"] = ProcessMonitorThread(server_id)
        threads["metrics_collector"] = MetricsCollectorThread(server_id)

        # Core thread with adapter's log parser injected
        log_parser = adapter.get_log_parser()
        threads["log_tail"] = LogTailThread(
            server_id,
            parser=log_parser,
            log_file_resolver=log_parser.get_log_file_resolver(server_id),
        )

        # Core thread with adapter's remote admin injected (if supported)
        remote_admin = adapter.get_remote_admin()
        if remote_admin is not None:
            threads["remote_admin_poller"] = RemoteAdminPollerThread(
                server_id,
                remote_admin_factory=lambda: remote_admin.create_client(
                    host="127.0.0.1",
                    port=server["rcon_port"],
                    password=_get_remote_admin_password(server_id, db),
                ),
            )

        # Adapter-declared custom threads (for game-specific background work)
        for thread_factory in adapter.get_custom_thread_factories():
            thread = thread_factory(server_id, db)
            threads[thread.name_key] = thread

        with cls._lock:
            cls._threads[server_id] = threads

        for thread in threads.values():
            thread.start()

---

Thread Safety Rules

Resource	Access Pattern	Protection
ProcessManager._processes	read/write from multiple threads	threading.Lock
ThreadRegistry._threads	read/write from main + shutdown	threading.Lock
broadcast_queue	multi-writer, single reader	queue.Queue (thread-safe built-in)
ConnectionManager._connections	async, single event loop	asyncio.Lock
SQLite connections	one connection per thread	Thread-local via threading.local()
Config files on disk	write on start, read-only during run	No lock needed (regenerated before start)
Adapter objects	read-only after registration	No lock needed (registered once at startup)
RemoteAdminClient calls	called from RemoteAdminPollerThread only	Core wraps with per-server threading.Lock (see below)

RemoteAdminClient Thread Safety

Adapters do NOT need to make their RemoteAdminClient implementations thread-safe. The core wraps every RemoteAdminClient call with a per-server threading.Lock so only one call executes at a time against a given server's admin client.

# In RemoteAdminPollerThread
class RemoteAdminPollerThread(BaseServerThread):
    def __init__(self, server_id: int,
                 remote_admin_factory: Callable[[], "RemoteAdminClient"]):
        super().__init__(server_id, self.interval)
        self._client_factory = remote_admin_factory
        self._client: RemoteAdminClient | None = None
        self._connected = False
        self._call_lock = threading.Lock()  # per-server lock

    def _call(self, method, *args, **kwargs):
        """All RemoteAdminClient calls go through this to serialize access."""
        with self._call_lock:
            return method(*args, **kwargs)

    # In tick(), replace direct self._client.get_players() with:
    #   players = self._call(self._client.get_players)

This means:

• Adapter authors write simple, non-thread-safe clients
• Core guarantees no concurrent calls to the same client
• Different servers' clients can call concurrently (different locks)

SQLite Thread Safety

# Each background thread creates its own SQLAlchemy connection
# from the same engine (WAL mode allows concurrent reads)
# PRAGMA busy_timeout=5000 prevents "database is locked" errors
#
# If busy_timeout is exhausted (5s), the write fails with
# OperationalError. Background threads retry with exponential
# backoff: 1s, 2s, 4s — then log and skip the tick.
# API request handlers retry up to 2 times with 1s backoff,
# then return 503 "database temporarily unavailable".

class BaseServerThread(threading.Thread):
    _db_retry_delays = [1.0, 2.0, 4.0]  # seconds, exponential backoff

    def run(self):
        engine = get_engine()
        self._db = engine.connect()
        try:
            self.setup()
            while not self._stop_event.is_set():
                try:
                    self.tick()
                except OperationalError as e:
                    if "database is locked" in str(e):
                        retried = self._retry_db_write(self.tick)
                        if not retried:
                            logger.warning(f"{self.name}: DB locked after all retries, skipping tick")
                    else:
                        self.on_error(e)
                except Exception as e:
                    self.on_error(e)
                self._stop_event.wait(self.interval)
        except Exception as e:
            logger.error(f"{self.name} setup error: {e}")
        finally:
            self.teardown()
            self._db.close()

    def _retry_db_write(self, fn, max_retries=3):
        for i, delay in enumerate(self._db_retry_delays[:max_retries]):
            self._stop_event.wait(delay)
            if self._stop_event.is_set():
                return False
            try:
                fn()
                return True
            except OperationalError:
                continue
        return False

---

BroadcastThread — Asyncio Bridge

This is the critical bridge between background threads and the asyncio WebSocket layer. Game-agnostic.

Background Thread                         Asyncio Event Loop
─────────────────                         ──────────────────
Any background thread                     uvicorn runs here
  │
  ▼
BroadcastThread.enqueue(                  loop = asyncio.get_running_loop()
  server_id=1,                            (stored at app startup)
  msg_type='log',
  data={...}
)
  │
  ▼
broadcast_queue.put({                     asyncio.run_coroutine_threadsafe(
  'server_id': 1,                             connection_manager.broadcast(
  'type': 'log',                                  server_id=1,
  'data': {...}                                   message={type, data}
)                                             ),
  │                                           loop=self._loop
  ▼                                       )
BroadcastThread.run() ──────────────────►
  while True:
    msg = queue.get()
    fut = run_coroutine_threadsafe(
              broadcast_coro,
              self._loop
          )
    fut.result(timeout=5)

Implementation Sketch

# core/websocket/broadcaster.py
import asyncio
import queue
import threading

_broadcast_queue: queue.Queue = queue.Queue(maxsize=10000)
_event_loop: asyncio.AbstractEventLoop | None = None

class BroadcastThread(threading.Thread):
    daemon = True

    def __init__(self, loop: asyncio.AbstractEventLoop, manager):
        super().__init__(name="BroadcastThread")
        self._loop = loop
        self._manager = manager
        self._running = True

    def run(self):
        while self._running:
            try:
                msg = _broadcast_queue.get(timeout=1.0)
                server_id = msg['server_id']
                outgoing = {
                    'type': msg['type'],
                    'server_id': server_id,
                    'data': msg['data'],
                }
                future = asyncio.run_coroutine_threadsafe(
                    self._manager.broadcast(str(server_id), outgoing, channel=msg['type']),
                    self._loop
                )
                try:
                    future.result(timeout=5.0)
                except TimeoutError:
                    logger.warning(f"Broadcast timeout for server {server_id} msg type {msg['type']}")
            except queue.Empty:
                continue
            except Exception as e:
                logger.error(f"BroadcastThread error: {e}")

    def stop(self):
        self._running = False

    @staticmethod
    def enqueue(server_id: int, msg_type: str, data: dict):
        """Thread-safe. Called from any background thread."""
        try:
            _broadcast_queue.put_nowait({
                'server_id': server_id,
                'type': msg_type,
                'data': data,
            })
        except queue.Full:
            logger.warning(f"Broadcast queue full, dropping {msg_type} for server {server_id}")

---

ProcessMonitorThread — Crash Detection & Auto-Restart

Game-agnostic. This thread only checks OS-level process status and updates the core servers table.

class ProcessMonitorThread(BaseServerThread):
    interval = 1.0

    def tick(self):
        proc = ProcessManager.get().get_process(self.server_id)
        if proc is None:
            self.stop()
            return

        exit_code = proc.poll()
        if exit_code is not None:
            self._handle_process_exit(exit_code)
            self.stop()

    def _handle_process_exit(self, exit_code: int):
        is_crash = (exit_code != 0)
        status = 'crashed' if is_crash else 'stopped'

        server = ServerRepository(self._db).get_by_id(self.server_id)
        ServerRepository(self._db).update_status(
            self.server_id, status, pid=None,
            stopped_at=datetime.utcnow().isoformat()
        )
        PlayerRepository(self._db).clear(self.server_id)
        ServerEventRepository(self._db).insert(
            self.server_id, status,
            actor='system',
            detail={'exit_code': exit_code}
        )

        BroadcastThread.enqueue(self.server_id, 'status', {'status': status})
        BroadcastThread.enqueue(self.server_id, 'event', {
            'event_type': status,
            'detail': {'exit_code': exit_code}
        })

        # Stop other threads for this server via daemon cleanup thread
        # (avoids thread joining itself)
        import threading as _threading

        def _cleanup_and_maybe_restart():
            try:
                ThreadRegistry.get().stop_server_threads(self.server_id)
                if is_crash and server.get('auto_restart'):
                    self._schedule_auto_restart(server)
            except Exception as e:
                logger.error(f"Cleanup/restart failed for server {self.server_id}: {e}")
                BroadcastThread.enqueue(self.server_id, 'event', {
                    'event_type': 'auto_restart_failed',
                    'detail': {'error': str(e)}
                })

        _threading.Thread(
            target=_cleanup_and_maybe_restart,
            daemon=True,
            name=f"StopCleanup-{self.server_id}"
        ).start()

    def _schedule_auto_restart(self, server: dict):
        # IMPORTANT: Runs in daemon cleanup thread, NOT ProcessMonitorThread.
        # Must create its own DB connection.
        from database import get_thread_db
        db = get_thread_db()

        restart_count = server['restart_count']
        max_restarts = server['max_restarts']
        window = server['restart_window_seconds']
        last_restart = server.get('last_restart_at')

        if last_restart:
            last_dt = datetime.fromisoformat(last_restart)
            elapsed = (datetime.utcnow() - last_dt).total_seconds()
            if elapsed > window:
                ServerRepository(db).reset_restart_count(self.server_id)
                restart_count = 0

        if restart_count < max_restarts:
            delay = min(10 * (restart_count + 1), 60)  # exponential backoff
            logger.info(f"Auto-restarting server {self.server_id} in {delay}s (attempt {restart_count+1}/{max_restarts})")
            threading.Timer(delay, self._auto_restart).start()
        else:
            logger.warning(f"Server {self.server_id} exceeded max auto-restarts ({max_restarts})")
            BroadcastThread.enqueue(self.server_id, 'event', {
                'event_type': 'max_restarts_exceeded',
                'detail': {'restart_count': restart_count}
            })

    def _auto_restart(self):
        from core.servers.service import ServerService
        try:
            ServerService().start(self.server_id)
        except Exception as e:
            logger.error(f"Auto-restart failed for server {self.server_id}: {e}")

---

LogTailThread — Generic File Tailing with Adapter Parser

Core thread that takes an adapter-provided LogParser for game-specific log line parsing and file discovery.

class LogTailThread(BaseServerThread):
    interval = 0.1  # 100ms

    def __init__(self, server_id: int, log_parser: "LogParser",
                 log_file_resolver: Callable[[Path], Path | None]):
        super().__init__(server_id, self.interval)
        self._parser = log_parser
        self._log_file_resolver = log_file_resolver
        self._file: TextIO | None = None
        self._current_path: Path | None = None
        self._last_size: int = 0

    def setup(self):
        self._open_latest_log()

    def _open_latest_log(self):
        """
        Uses the adapter-provided log_file_resolver to find the current log file.
        Opens it and seeks to end (tail behavior).

        NOTE: Do NOT use os.stat().st_ino for rotation detection — on Windows/NTFS
        st_ino is always 0. Instead, track filename and file size.
        """
        server_dir = get_server_dir(self.server_id)
        log_path = self._log_file_resolver(server_dir)
        if log_path is None:
            return  # Server hasn't created log yet; retry in next tick

        try:
            self._file = open(log_path, 'r', encoding='utf-8', errors='replace')
            self._file.seek(0, 2)  # seek to end
            self._current_path = log_path
            self._last_size = self._file.tell()
        except OSError:
            self._file = None

    def tick(self):
        if self._file is None:
            self._open_latest_log()
            return

        # Rotation detection: only re-check every 5 seconds
        now = time.monotonic()
        if now - getattr(self, '_last_glob_time', 0) > 5.0:
            self._last_glob_time = now
            server_dir = get_server_dir(self.server_id)
            log_path = self._log_file_resolver(server_dir)
            if log_path is not None and log_path != self._current_path:
                self._file.close()
                self._open_latest_log()
                return

        try:
            current_size = self._current_path.stat().st_size
        except OSError:
            return

        if current_size < self._last_size:
            self._file.close()
            self._open_latest_log()
            return

        # Read new lines and parse using adapter's parser
        while True:
            line = self._file.readline()
            if not line:
                break
            self._last_size = self._file.tell()
            line = line.rstrip('\n')
            if not line:
                continue

            # Adapter parses the line — game-specific format
            entry = self._parser.parse_line(line)
            if entry:
                LogRepository(self._db).insert(self.server_id, entry)
                BroadcastThread.enqueue(self.server_id, 'log', entry)

    def teardown(self):
        if self._file is not None:
            try:
                self._file.close()
            except OSError:
                pass
            self._file = None

---

MetricsCollectorThread — Game-Agnostic Resource Monitoring

Fully game-agnostic. Uses psutil to monitor any process.

class MetricsCollectorThread(BaseServerThread):
    interval = 5.0

    def tick(self):
        pid = ProcessManager.get().get_pid(self.server_id)
        if pid is None:
            return

        try:
            proc = psutil.Process(pid)
            cpu = proc.cpu_percent(interval=0.5)
            ram = proc.memory_info().rss / (1024 * 1024)  # MB
        except (psutil.NoSuchProcess, psutil.AccessDenied):
            return

        player_count = PlayerRepository(self._db).count(self.server_id)

        MetricsRepository(self._db).insert(self.server_id, cpu, ram, player_count)
        BroadcastThread.enqueue(self.server_id, 'metrics', {
            'cpu_percent': cpu,
            'ram_mb': ram,
            'player_count': player_count,
        })

---

RemoteAdminPollerThread — Generic Polling with Adapter Client

Core thread that takes an adapter-provided RemoteAdmin factory for game-specific admin protocol communication. Skipped entirely if adapter has no remote_admin capability.

class RemoteAdminPollerThread(BaseServerThread):
    interval = 10.0
    STARTUP_DELAY = 30.0

    def __init__(self, server_id: int,
                 remote_admin_factory: Callable[[], "RemoteAdminClient"]):
        super().__init__(server_id, self.interval)
        self._client_factory = remote_admin_factory
        self._client: RemoteAdminClient | None = None
        self._connected = False

    def setup(self):
        # Wait for server to start up before attempting connection
        # Uses _stop_event.wait() instead of time.sleep() for immediate shutdown
        startup_delay = self._get_startup_delay()
        if self._stop_event.wait(startup_delay):
            return  # stop was requested during wait
        self._connect()

    def _get_startup_delay(self) -> float:
        # Default delay; adapter may override via RemoteAdmin.get_startup_delay()
        return self.STARTUP_DELAY

    def _connect(self):
        try:
            self._client = self._client_factory()
            self._connected = True
        except Exception as e:
            logger.warning(f"Remote admin connection failed for server {self.server_id}: {e}")
            self._connected = False

    def tick(self):
        if not self._connected:
            self._reconnect_attempts = getattr(self, '_reconnect_attempts', 0) + 1
            delay = min(10 * 2 ** self._reconnect_attempts, 120)  # exponential backoff
            if self._reconnect_attempts > 1:
                logger.info(f"Remote admin reconnect attempt {self._reconnect_attempts} for server {self.server_id}")
                if self._stop_event.wait(delay):
                    return
            self._connect()
            if not self._connected:
                return
            self._reconnect_attempts = 0

        try:
            players = self._call(self._client.get_players)
            PlayerService(self._db).update_from_remote_admin(self.server_id, players)
            BroadcastThread.enqueue(self.server_id, 'players', {
                'players': [p for p in players],
                'count': len(players),
            })
        except ConnectionError:
            self._connected = False
            logger.warning(f"Remote admin connection lost for server {self.server_id}")
        except RemoteAdminError as e:
            logger.error(f"Remote admin adapter error for server {self.server_id}: {e}")
            self._connected = False

    def teardown(self):
        if self._client is not None:
            try:
                self._client.disconnect()
            except Exception:
                pass
            self._client = None

---

Thread Lifecycle

Start Server Flow

POST /servers/{id}/start
  │
  ├── ServerService.start()
  │     ├── adapter = GameAdapterRegistry.get(server.game_type)
  │     ├── check_server_ports_available(server_id)
  │     │     └── For ALL running servers, resolve each adapter,
  │     │         get port conventions, check full derived port set
  │     │         (cross-game: Arma 3 game+steam query + other games' ports)
  │     ├── adapter.config_generator.write_configs()
  │     │     └── Atomic write: write to .tmp files first, then os.replace()
  │     │         On failure: .tmp files cleaned up, originals untouched
  │     ├── launch_args = adapter.config_generator.build_launch_args()
  │     ├── ProcessManager.start()     ← creates subprocess.Popen
  │     └── ThreadRegistry.start_server_threads(id, db)
  │           ├── ProcessMonitorThread(id)              ← core, always
  │           ├── LogTailThread(id, adapter.log_parser)  ← core + adapter
  │           ├── MetricsCollectorThread(id)             ← core, always
  │           └── RemoteAdminPollerThread(id, adapter.remote_admin)
  │                                                       ← core + adapter (if available)
  │
  └── BroadcastThread.enqueue(id, 'status', {status: 'starting'})

Error paths on start:
  ├── ConfigWriteError → rollback .tmp files, return 500 to client
  ├── ConfigValidationError → return 422 with validation details
  ├── LaunchArgsError → return 400 with invalid arg info
  ├── ExeNotAllowedError → return 403 with executable name
  └── PortInUseError → return 409 with conflicting port info

Stop Server Flow

POST /servers/{id}/stop
  │
  ├── adapter.remote_admin.shutdown()    ← if adapter has remote_admin
  ├── Wait up to 30s for process exit (ProcessManager.stop(timeout=30))
  ├── If still running: ProcessManager.kill()
  ├── ThreadRegistry.stop_server_threads(id)
  │     ├── ProcessMonitorThread.stop()
  │     ├── LogTailThread.stop()
  │     ├── MetricsCollectorThread.stop()
  │     └── RemoteAdminPollerThread.stop()  ← if present
  │     └── Thread.join(timeout=5) for each
  │
  └── BroadcastThread.enqueue(id, 'status', {status: 'stopped'})

App Shutdown Flow

FastAPI shutdown event
  │
  ├── ThreadRegistry.stop_all()    ← stop all threads for all servers
  ├── BroadcastThread.stop()
  ├── ConnectionManager.close_all()
  └── database engine dispose

---

Stop Event Pattern

All background threads use a threading.Event for graceful shutdown:

class BaseServerThread(threading.Thread):
    def __init__(self, server_id: int, interval: float):
        super().__init__(name=f"{self.__class__.__name__}-{server_id}", daemon=True)
        self.server_id = server_id
        self.interval = interval
        self._stop_event = threading.Event()

    def stop(self):
        self._stop_event.set()

    def is_stopped(self) -> bool:
        return self._stop_event.is_set()

    def teardown(self):
        """Override to release resources (close files, sockets) after the loop ends."""
        pass

    def run(self):
        try:
            self.setup()
        except Exception as e:
            logger.error(f"{self.name} setup error: {e}")
            return  # setup failed completely

        try:
            while not self._stop_event.is_set():
                try:
                    self.tick()
                except Exception as e:
                    self.on_error(e)
                self._stop_event.wait(self.interval)
        finally:
            self.teardown()

    def on_error(self, error: Exception):
        """Default error handler. Adapter exceptions are typed for specific handling."""
        if isinstance(error, RemoteAdminError):
            logger.error(f"{self.name} remote admin error: {error}")
            # RemoteAdminPollerThread overrides to set _connected = False
        elif isinstance(error, ConfigWriteError):
            logger.critical(f"{self.name} config write error (atomic write failed): {error}")
        elif isinstance(error, ConfigValidationError):
            logger.error(f"{self.name} config validation error: {error}")
        else:
            logger.error(f"{self.name} unhandled error: {error}")

---

WebSocket Connection Manager (asyncio)

Game-agnostic. No changes from single-game design.

# core/websocket/manager.py
class ConnectionManager:
    def __init__(self):
        self._connections: dict[str, set[WebSocket]] = defaultdict(set)
        self._channel_subs: dict[WebSocket, set[str]] = defaultdict(set)
        self._lock = asyncio.Lock()

    async def connect(self, ws: WebSocket, server_id: str):
        await ws.accept()
        async with self._lock:
            self._connections[server_id].add(ws)
            self._channel_subs[ws].add('status')
            if server_id == 'all':
                self._connections['all'].add(ws)

    async def disconnect(self, ws: WebSocket, server_id: str):
        async with self._lock:
            self._connections[server_id].discard(ws)
            self._connections['all'].discard(ws)
            self._channel_subs.pop(ws, None)

    async def subscribe(self, ws: WebSocket, channels: list[str]):
        async with self._lock:
            self._channel_subs[ws].update(channels)

    async def unsubscribe(self, ws: WebSocket, channels: list[str]):
        async with self._lock:
            self._channel_subs[ws].difference_update(channels)

    async def broadcast(self, server_id: str, message: dict, channel: str = None):
        targets: set[WebSocket] = set()
        async with self._lock:
            server_clients = self._connections.get(server_id, set())
            all_clients = self._connections.get('all', set())
            candidates = server_clients | all_clients

            if channel:
                targets = {ws for ws in candidates
                           if channel in self._channel_subs.get(ws, set())}
            else:
                targets = candidates

        dead = []
        for ws in targets:
            try:
                await ws.send_json(message)
            except Exception:
                dead.append(ws)

        if dead:
            async with self._lock:
                for ws in dead:
                    for bucket in self._connections.values():
                        bucket.discard(ws)
                    self._channel_subs.pop(ws, None)

---

Memory & Performance Considerations

Thread	Memory Impact	CPU Impact
ProcessMonitorThread	Minimal (one os.kill check)	Negligible
LogTailThread	Buffer for unread log lines	Low (file I/O + adapter parsing)
MetricsCollectorThread	psutil subprocess scan	Low-Medium
RemoteAdminPollerThread	Adapter client socket + buffer	Low (varies by adapter protocol)
BroadcastThread	Queue buffer (max 10000 entries)	Low

Recommendations

• Set all threads as daemon=True — they die automatically if main process exits
• broadcast_queue.maxsize=10000 — backpressure; drop on Full (log warning)
• LogTailThread buffers max ~100 lines per tick before writing to DB in batch
• MetricsCollectorThread uses psutil.Process.cpu_percent(interval=0.5) — blocks 500ms, acceptable at 5s interval
• For N=10 servers: 31-41 background threads — well within Python's thread limits
• Games without remote admin skip the RemoteAdminPollerThread entirely