App: Limit rate of requests to http queue

src/lightning/app/core/constants.py

@@ -53,6 +53,7 @@ def get_lightning_cloud_url() -> str:
 HTTP_QUEUE_URL = os.getenv("LIGHTNING_HTTP_QUEUE_URL", "http://localhost:9801")
 HTTP_QUEUE_REFRESH_INTERVAL = float(os.getenv("LIGHTNING_HTTP_QUEUE_REFRESH_INTERVAL", "1"))
 HTTP_QUEUE_TOKEN = os.getenv("LIGHTNING_HTTP_QUEUE_TOKEN", None)
+HTTP_QUEUE_REQUESTS_PER_SECOND = float(os.getenv("LIGHTNING_HTTP_QUEUE_REQUESTS_PER_SECOND", "1"))
 
 USER_ID = os.getenv("USER_ID", "1234")
 FRONTEND_DIR = str(Path(__file__).parent.parent / "ui")

src/lightning/app/core/queues.py

@@ -29,6 +29,7 @@
 
 from lightning.app.core.constants import (
     HTTP_QUEUE_REFRESH_INTERVAL,
+    HTTP_QUEUE_REQUESTS_PER_SECOND,
     HTTP_QUEUE_TOKEN,
     HTTP_QUEUE_URL,
     LIGHTNING_DIR,
@@ -77,7 +78,9 @@ def get_queue(self, queue_name: str) -> "BaseQueue":
             return MultiProcessQueue(queue_name, default_timeout=STATE_UPDATE_TIMEOUT)
         if self == QueuingSystem.REDIS:
             return RedisQueue(queue_name, default_timeout=REDIS_QUEUES_READ_DEFAULT_TIMEOUT)
-        return HTTPQueue(queue_name, default_timeout=STATE_UPDATE_TIMEOUT)
+        return RateLimitedQueue(
+            HTTPQueue(queue_name, default_timeout=STATE_UPDATE_TIMEOUT), HTTP_QUEUE_REQUESTS_PER_SECOND
+        )
 
     def get_api_response_queue(self, queue_id: Optional[str] = None) -> "BaseQueue":
         queue_name = f"{queue_id}_{API_RESPONSE_QUEUE_CONSTANT}" if queue_id else API_RESPONSE_QUEUE_CONSTANT
@@ -347,6 +350,45 @@ def from_dict(cls, state: dict) -> "RedisQueue":
         return cls(**state)
 
 
+class RateLimitedQueue(BaseQueue):
+    def __init__(self, queue: BaseQueue, requests_per_second: float):
+        """This is a queue wrapper that will block on get or put calls if they are made too quickly.
+
+        Args:
+            queue: The queue to wrap.
+            requests_per_second: The target number of get or put requests per second.
+
+        """
+        self.name = queue.name
+        self.default_timeout = queue.default_timeout
+
+        self._queue = queue
+        self._seconds_per_request = 1 / requests_per_second
+
+        self._last_get = 0.0
+        self._last_put = 0.0
+
+    @property
+    def is_running(self) -> bool:
+        return self._queue.is_running
+
+    def _wait_until_allowed(self, last_time: float) -> None:
+        t = time.time()
+        diff = t - last_time
+        if diff < self._seconds_per_request:
+            time.sleep(self._seconds_per_request - diff)
+
+    def get(self, timeout: Optional[float] = None) -> Any:
+        self._wait_until_allowed(self._last_get)
+        self._last_get = time.time()
+        return self._queue.get(timeout=timeout)
+
+    def put(self, item: Any) -> None:
+        self._wait_until_allowed(self._last_put)
+        self._last_put = time.time()
+        return self._queue.put(item)
+
+
 class HTTPQueue(BaseQueue):
     def __init__(self, name: str, default_timeout: float) -> None:
         """

tests/tests_app/core/test_queues.py

@@ -8,8 +8,15 @@
 import requests_mock
 from lightning.app import LightningFlow
 from lightning.app.core import queues
-from lightning.app.core.constants import HTTP_QUEUE_URL
-from lightning.app.core.queues import READINESS_QUEUE_CONSTANT, BaseQueue, QueuingSystem, RedisQueue
+from lightning.app.core.constants import HTTP_QUEUE_URL, STATE_UPDATE_TIMEOUT
+from lightning.app.core.queues import (
+    READINESS_QUEUE_CONSTANT,
+    BaseQueue,
+    HTTPQueue,
+    QueuingSystem,
+    RateLimitedQueue,
+    RedisQueue,
+)
 from lightning.app.utilities.imports import _is_redis_available
 from lightning.app.utilities.redis import check_if_redis_running
 
@@ -162,7 +169,7 @@ def test_redis_raises_error_if_failing(redis_mock):
 
 class TestHTTPQueue:
     def test_http_queue_failure_on_queue_name(self):
-        test_queue = QueuingSystem.HTTP.get_queue(queue_name="test")
+        test_queue = HTTPQueue("test", STATE_UPDATE_TIMEOUT)
         with pytest.raises(ValueError, match="App ID couldn't be extracted"):
             test_queue.put("test")
 
@@ -174,7 +181,7 @@ def test_http_queue_failure_on_queue_name(self):
 
     def test_http_queue_put(self, monkeypatch):
         monkeypatch.setattr(queues, "HTTP_QUEUE_TOKEN", "test-token")
-        test_queue = QueuingSystem.HTTP.get_queue(queue_name="test_http_queue")
+        test_queue = HTTPQueue("test_http_queue", STATE_UPDATE_TIMEOUT)
         test_obj = LightningFlow()
 
         # mocking requests and responses
@@ -200,8 +207,7 @@ def test_http_queue_put(self, monkeypatch):
 
     def test_http_queue_get(self, monkeypatch):
         monkeypatch.setattr(queues, "HTTP_QUEUE_TOKEN", "test-token")
-        test_queue = QueuingSystem.HTTP.get_queue(queue_name="test_http_queue")
-
+        test_queue = HTTPQueue("test_http_queue", STATE_UPDATE_TIMEOUT)
         adapter = requests_mock.Adapter()
         test_queue.client.session.mount("http://", adapter)
 
@@ -218,7 +224,7 @@ def test_http_queue_get(self, monkeypatch):
 def test_unreachable_queue(monkeypatch):
     monkeypatch.setattr(queues, "HTTP_QUEUE_TOKEN", "test-token")
 
-    test_queue = QueuingSystem.HTTP.get_queue(queue_name="test_http_queue")
+    test_queue = HTTPQueue("test_http_queue", STATE_UPDATE_TIMEOUT)
 
     resp1 = mock.MagicMock()
     resp1.status_code = 204
@@ -235,3 +241,23 @@ def test_unreachable_queue(monkeypatch):
     # Test backoff on queue.put
     test_queue.put("foo")
     assert test_queue.client.post.call_count == 3
+
+
+def test_rate_limited_queue():
+    mock_queue = mock.MagicMock()
+
+    mock_queue.name = "inner_queue"
+    mock_queue.default_timeout = 10.0
+
+    rate_limited_queue = RateLimitedQueue(mock_queue, requests_per_second=1)
+
+    assert rate_limited_queue.name == "inner_queue"
+    assert rate_limited_queue.default_timeout == 10.0
+
+    timeout = time.perf_counter() + 1
+    while time.perf_counter() < timeout:
+        rate_limited_queue.get()
+
+    assert (
+        mock_queue.get.call_count == 2
+    ), f"the inner queue should have been called exactly twice but was called {mock_queue.get.call_count} times"