[PERFORMANCE]: Implement SSE Backpressure and Slow Client Handling

[crivetimihai]

Implement SSE backpressure to prevent memory exhaustion from slow SSE clients.

Problem

Location: mcpgateway/transports/sse_transport.py line 112

class SSETransport:
    def __init__(self, base_url: str = None):
        self._base_url = base_url or f"http://{settings.host}:{settings.port}"
        self._connected = False
        self._message_queue = asyncio.Queue()  # Unbounded - no maxsize!
        self._client_gone = asyncio.Event()
        self._session_id = str(uuid.uuid4())

    # Line 215: Message sending
    async def send_message(self, message: dict) -> None:
        await self._message_queue.put(message)  # Always succeeds, never blocks

Issues

1. Unbounded queue - No limit on queued messages
2. No slow client detection - Can't identify problematic clients
3. No backpressure - Producers never slowed
4. Memory exhaustion - Slow clients accumulate messages indefinitely

Memory Impact

Slow Clients	Messages/Client	Memory Growth
1	10,000	~10MB
10	10,000	~100MB
100	10,000	~1GB

Proposed Solution

Bounded Queue with Backpressure

class SSETransport:
    def __init__(self, max_queue_size: int = 1000):
        self._message_queue = asyncio.Queue(maxsize=max_queue_size)
        self._client_slow = False

    async def send_message(self, message, timeout: float = 5.0):
        try:
            await asyncio.wait_for(
                self._message_queue.put(message),
                timeout=timeout
            )
        except asyncio.TimeoutError:
            self._client_slow = True
            raise SlowClientError(f"Client queue full after {timeout}s")

Configuration

SSE_MAX_QUEUE_SIZE=1000
SSE_QUEUE_TIMEOUT=5.0
SSE_SLOW_CLIENT_DISCONNECT=true

Acceptance Criteria

• Message queue bounded with configurable size
• Slow clients detected and optionally disconnected
• Backpressure prevents unbounded memory growth
• Metrics expose slow client detection
• All SSE tests pass
• Passes make verify