[Serve] Require traffic weights to sum more closely to 1. (#8476)

python/ray/serve/master.py

@@ -458,14 +458,20 @@ async def set_traffic(self, endpoint_name, traffic_policy_dictionary):
  dict), "Traffic policy must be dictionary"
  prob = 0
  for backend, weight in traffic_policy_dictionary.items():
+ if weight < 0:
+ raise ValueError(
+ "Attempted to assign a weight of {} to backend '{}'. "
+ "Weights cannot be negative.".format(weight, backend))
  prob += weight
  if backend not in self.backends:
  raise ValueError(
  "Attempted to assign traffic to a backend '{}' that "
  "is not registered.".format(backend))
 
+ # These weights will later be plugged into np.random.choice, which
+ # uses a tolerance of 1e-8.
  assert np.isclose(
- prob, 1, atol=0.02
+ prob, 1, atol=1e-8
  ), "weights must sum to 1, currently they sum to {}".format(prob)
 
  self.traffic_policies[endpoint_name] = traffic_policy_dictionary