Introduce constant for ID_SIZE in python code. (#2517)

python/ray/monitor.py

@@ -16,14 +16,14 @@ from ray.autoscaler.autoscaler import LoadMetrics, StandardAutoscaler
 import ray.cloudpickle as pickle
 import ray.gcs_utils
 import ray.utils
+import ray.ray_constants as ray_constants
 from ray.services import get_ip_address, get_port
 from ray.utils import binary_to_hex, binary_to_object_id, hex_to_binary
 from ray.worker import NIL_ACTOR_ID
 
 # These variables must be kept in sync with the C codebase.
 # common/common.h
-DB_CLIENT_ID_SIZE = 20
-NIL_ID = b"\xff" * DB_CLIENT_ID_SIZE
+NIL_ID = b"\xff" * ray_constants.ID_SIZE
 
 # common/task.h
 TASK_STATUS_LOST = 32
@@ -348,8 +348,8 @@ class Monitor(object):
         This resets the number of heartbeats that we've missed from this plasma
         manager.
         """
-        # The first DB_CLIENT_ID_SIZE characters are the client ID.
-        db_client_id = data[:DB_CLIENT_ID_SIZE]
+        # The first ray_constants.ID_SIZE characters are the client ID.
+        db_client_id = data[:ray_constants.ID_SIZE]
         # Reset the number of heartbeats that we've missed from this plasma
         # manager.
         self.live_plasma_managers[db_client_id] = 0