[serve] Enable deployment of functions/classes that take no parameters (

#19708)

python/ray/serve/replica.py

@@ -287,9 +287,15 @@ async def invoke_single(self, request_item: Query) -> Any:
         start = time.time()
         method_to_call = None
         try:
-            method_to_call = sync_to_async(
-                self.get_runner_method(request_item))
-            result = await method_to_call(*args, **kwargs)
+            runner_method = self.get_runner_method(request_item)
+            method_to_call = sync_to_async(runner_method)
+            result = None
+            if len(inspect.signature(runner_method).parameters) > 0:
+                result = await method_to_call(*args, **kwargs)
+            else:
+                # The method doesn't take in anything, including the request
+                # information, so we pass nothing into it
+                result = await method_to_call()
 
             result = await self.ensure_serializable_response(result)
             self.request_counter.inc()

python/ray/serve/tests/test_api.py

@@ -85,6 +85,73 @@ async def slow_numbers():
     assert resp.status_code == 418
 
 
+def test_deploy_sync_function_no_params(serve_instance):
+    @serve.deployment()
+    def sync_d():
+        return "sync!"
+
+    serve.start()
+
+    sync_d.deploy()
+    assert requests.get("http://localhost:8000/sync_d").text == "sync!"
+    assert ray.get(sync_d.get_handle().remote()) == "sync!"
+
+
+def test_deploy_async_function_no_params(serve_instance):
+    @serve.deployment()
+    async def async_d():
+        await asyncio.sleep(5)
+        return "async!"
+
+    serve.start()
+
+    async_d.deploy()
+    assert requests.get("http://localhost:8000/async_d").text == "async!"
+    assert ray.get(async_d.get_handle().remote()) == "async!"
+
+
+def test_deploy_sync_class_no_params(serve_instance):
+    @serve.deployment
+    class Counter:
+        def __init__(self):
+            self.count = 0
+
+        def __call__(self):
+            self.count += 1
+            return {"count": self.count}
+
+    serve.start()
+    Counter.deploy()
+
+    assert requests.get("http://127.0.0.1:8000/Counter").json() == {"count": 1}
+    assert requests.get("http://127.0.0.1:8000/Counter").json() == {"count": 2}
+    assert ray.get(Counter.get_handle().remote()) == {"count": 3}
+
+
+def test_deploy_async_class_no_params(serve_instance):
+    @serve.deployment
+    class AsyncCounter:
+        async def __init__(self):
+            await asyncio.sleep(5)
+            self.count = 0
+
+        async def __call__(self):
+            self.count += 1
+            await asyncio.sleep(5)
+            return {"count": self.count}
+
+    serve.start()
+    AsyncCounter.deploy()
+
+    assert requests.get("http://127.0.0.1:8000/AsyncCounter").json() == {
+        "count": 1
+    }
+    assert requests.get("http://127.0.0.1:8000/AsyncCounter").json() == {
+        "count": 2
+    }
+    assert ray.get(AsyncCounter.get_handle().remote()) == {"count": 3}
+
+
 def test_user_config(serve_instance):
     @serve.deployment(
         "counter", num_replicas=2, user_config={