[tune/serve] Fix tune/serve integration script broken by serve API change (#10586)

doc/source/tune/_tutorials/tune-serve-integration-mnist.py

@@ -98,6 +98,7 @@ import torch.nn as nn
 import torch.nn.functional as F
 import torch.optim as optim
 from ray import tune, serve
+from ray.serve.exceptions import RayServeException
 from ray.tune import CLIReporter
 from ray.tune.schedulers import ASHAScheduler
 
@@ -449,28 +450,34 @@ def serve_new_model(model_dir, checkpoint, config, metrics, day, gpu=False):
     checkpoint_path = _move_checkpoint_to_model_dir(model_dir, checkpoint,
                                                     config, metrics)
 
-    serve.init()
+    try:
+        # Try to connect to an existing cluster.
+        client = serve.connect()
+    except RayServeException:
+        # If this is the first run, need to start the cluster.
+        client = serve.start(detached=True)
+
     backend_name = "mnist:day_{}".format(day)
 
-    serve.create_backend(backend_name, MNISTBackend, checkpoint_path, config,
-                         metrics, gpu)
+    client.create_backend(backend_name, MNISTBackend, checkpoint_path, config,
+                          metrics, gpu)
 
-    if "mnist" not in serve.list_endpoints():
+    if "mnist" not in client.list_endpoints():
         # First time we serve a model - create endpoint
-        serve.create_endpoint(
+        client.create_endpoint(
             "mnist", backend=backend_name, route="/mnist", methods=["POST"])
     else:
         # The endpoint already exists, route all traffic to the new model
         # Here you could also implement an incremental rollout, where only
         # a part of the traffic is sent to the new backend and the
         # rest is sent to the existing backends.
-        serve.set_traffic("mnist", {backend_name: 1.0})
+        client.set_traffic("mnist", {backend_name: 1.0})
 
     # Delete previous existing backends
-    for existing_backend in serve.list_backends():
+    for existing_backend in client.list_backends():
         if existing_backend.startswith("mnist:day") and \
            existing_backend != backend_name:
-            serve.delete_backend(existing_backend)
+            client.delete_backend(existing_backend)
 
     return True