Managing Deployments¶

This section should help you:

  • create, query, update and configure deployments

  • configure resources of your deployments

  • specify different Python dependencies across different deployment using Runtime Environments


Get in touch with us if you’re using or considering using Ray Serve.

Updating a Deployment¶

Often you want to be able to update your code or configuration options for a deployment over time. Deployments can be updated simply by updating the code or configuration options and calling deploy() again.

@serve.deployment(name="my_deployment", num_replicas=1)
class SimpleDeployment:

# Creates one initial replica.

# Re-deploys, creating an additional replica.
# This could be the SAME Python script, modified and re-run.
@serve.deployment(name="my_deployment", num_replicas=2)
class SimpleDeployment:


# You can also use Deployment.options() to change options without redefining
# the class. This is useful for programmatically updating deployments.

By default, each call to .deploy() will cause a redeployment, even if the underlying code and options didn’t change. This could be detrimental if you have many deployments in a script and and only want to update one: if you re-run the script, all of the deployments will be redeployed, not just the one you updated. To prevent this, you may provide a version string for the deployment as a keyword argument in the decorator or Deployment.options(). If provided, the replicas will only be updated if the value of version is updated; if the value of version is unchanged, the call to .deploy() will be a no-op. When a redeployment happens, Serve will perform a rolling update, bringing down at most 20% of the replicas at any given time.

Configuring a Deployment¶

There are a number of things you’ll likely want to do with your serving application including scaling out or configuring the maximum number of in-flight requests for a deployment. All of these options can be specified either in @serve.deployment or in Deployment.options().

To update the config options for a running deployment, simply redeploy it with the new options set.

Scaling Out¶

To scale out a deployment to many processes, simply configure the number of replicas.

# Create with a single replica.
def func(*args):


# Scale up to 10 replicas.

# Scale back down to 1 replica.


Serve also has experimental support for a demand-based replica autoscaler. It reacts to traffic spikes via observing queue sizes and making scaling decisions. To configure it, you can set the _autoscaling field in deployment options.


The API is experimental and subject to change. We welcome you to test it out and leave us feedback through Github Issues or our discussion forum!

        "min_replicas": 1,
        "max_replicas": 5,
        "target_num_ongoing_requests_per_replica": 10,
def func(_):
    return ""

func.deploy() # The func deployment will now autoscale based on requests demand.

The min_replicas and max_replicas fields configure the range of replicas which the Serve autoscaler chooses from. Deployments will start with min_replicas initially.

The target_num_ongoing_requests_per_replica configuration specifies how aggressively the autoscaler should react to traffic. Serve will try to make sure that each replica has roughly that number of requests being processed and waiting in the queue. For example, if your processing time is 10ms and the latency constraint is 100ms, you can have at most 10 requests ongoing per replica so the last requests can finish within the latency constraint. We recommend you benchmark your application code and set this number based on end to end latency objective.


The Ray Serve Autoscaler is an application-level autoscaler that sits on top of the Ray Autoscaler. Concretely, this means that the Ray Serve autoscaler asks Ray to start a number of replica actors based on the request demand. If the Ray Autoscaler determines there aren’t enough available CPUs to place these actors, it responds by adding more nodes. Similarly, when Ray Serve scales down and terminates some replica actors, it may result in some nodes being empty, at which point the Ray autoscaler will remove those nodes.

Resource Management (CPUs, GPUs)¶

To assign hardware resources per replica, you can pass resource requirements to ray_actor_options. By default, each replica requires one CPU. To learn about options to pass in, take a look at Resources with Actor guide.

For example, to create a deployment where each replica uses a single GPU, you can do the following:

@serve.deployment(ray_actor_options={"num_gpus": 1})
def func(*args):
    return do_something_with_my_gpu()

Fractional Resources¶

The resources specified in ray_actor_options can also be fractional. This allows you to flexibly share resources between replicas. For example, if you have two models and each doesn’t fully saturate a GPU, you might want to have them share a GPU by allocating 0.5 GPUs each. The same could be done to multiplex over CPUs.

@serve.deployment(name="deployment1", ray_actor_options={"num_gpus": 0.5})
def func(*args):
    return do_something_with_my_gpu()

@serve.deployment(name="deployment2", ray_actor_options={"num_gpus": 0.5})
def func(*args):
    return do_something_with_my_gpu()

Configuring Parallelism with OMP_NUM_THREADS¶

Deep learning models like PyTorch and Tensorflow often use multithreading when performing inference. The number of CPUs they use is controlled by the OMP_NUM_THREADS environment variable. To avoid contention, Ray sets OMP_NUM_THREADS=1 by default because Ray workers and actors use a single CPU by default. If you do want to enable this parallelism in your Serve deployment, just set OMP_NUM_THREADS to the desired value either when starting Ray or in your function/class definition:

OMP_NUM_THREADS=12 ray start --head
OMP_NUM_THREADS=12 ray start --address=$HEAD_NODE_ADDRESS
class MyDeployment:
    def __init__(self, parallelism):
        os.environ["OMP_NUM_THREADS"] = parallelism
        # Download model weights, initialize model, etc.



Some other libraries may not respect OMP_NUM_THREADS and have their own way to configure parallelism. For example, if you’re using OpenCV, you’ll need to manually set the number of threads using cv2.setNumThreads(num_threads) (set to 0 to disable multi-threading). You can check the configuration using cv2.getNumThreads() and cv2.getNumberOfCPUs().

User Configuration (Experimental)¶

Suppose you want to update a parameter in your model without needing to restart the replicas in your deployment. You can do this by writing a reconfigure method for the class underlying your deployment. At runtime, you can then pass in your new parameters by setting the user_config option.

The following simple example will make the usage clear:

import requests
import random

from ray import serve


class Threshold:
    def __init__(self):
        # self.model won't be changed by reconfigure.
        self.model = random.Random()  # Imagine this is some heavyweight model.

    def reconfigure(self, config):
        # This will be called when the class is created and when
        # the user_config is updated.
        self.threshold = config["threshold"]

    def __call__(self, request):
        return self.model.random() > self.threshold

Threshold.options(user_config={"threshold": 0.01}).deploy()
print(requests.get("").text)  # true, probably

Threshold.options(user_config={"threshold": 0.99}).deploy()
print(requests.get("").text)  # false, probably

The reconfigure method is called when the class is created if user_config is set. In particular, it’s also called when new replicas are created in the future if scale up your deployment later. The reconfigure method is also called each time user_config is updated.