Deploy services to a swarm

Estimated reading time: 21 minutes

When you are running Docker Engine in swarm mode, you run docker service create to deploy your application in the swarm. The swarm manager accepts the service description as the desired state for your application. The built-in swarm orchestrator and scheduler deploy your application to nodes in your swarm to achieve and maintain the desired state.

For an overview of how services work, refer to How services work.

This guide assumes you are working with the Docker Engine running in swarm mode. You must run all docker service commands from a manager node.

If you haven’t already, read through Swarm mode key concepts and How services work.

Create a service

To create the simplest type of service in a swarm, you only need to supply a container image:

$ docker service create <IMAGE>

The swarm orchestrator schedules one task on an available node. The task invokes a container based upon the image. For example, you could run the following command to create a service of one instance of an nginx web server:

$ docker service create --name my_web nginx

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In this example the --name flag names the service my_web.

To list the service, run docker service ls from a manager node:

$ docker service ls

ID            NAME    REPLICAS  IMAGE  COMMAND
anixjtol6wdf  my_web  1/1       nginx

To make the web server accessible from outside the swarm, you need to publish the port where the swarm listens for web requests.

You can include a command to run inside containers after the image:

$ docker service create <IMAGE> <COMMAND>

For example to start an alpine image that runs ping docker.com:

$ docker service create --name helloworld alpine ping docker.com

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Configure services

When you create a service, you can specify many different configuration options and constraints. See the output of docker service create --help for a full listing of them. Some common configuration options are described below.

Created services do not always run right away. A service can be in a pending state if its image is unavailable, no node meets the requirements you configure for the service, or other reasons. See Pending services for more information.

Configure the runtime environment

You can configure the following options for the runtime environment in the container:

  • environment variables using the --env flag
  • the working directory inside the container using the --workdir flag
  • the username or UID using the --user flag

For example:

$ docker service create --name helloworld \
  --env MYVAR=myvalue \
  --workdir /tmp \
  --user my_user \
  alpine ping docker.com

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Grant a service access to secrets

To create a service with access to Docker-managed secrets, use the --secret flag. For more information, see Manage sensitive strings (secrets) for Docker services

Specify the image version the service should use

When you create a service without specifying any details about the version of the image to use, the service uses the version tagged with the latest tag. You can force the service to use a specific version of the image in a few different ways, depending on your desired outcome.

An image version can be expressed in several different ways:

  • If you specify a tag, the manager (or the Docker client, if you use content trust) resolves that tag to a digest. When the request to create a container task is received on a worker node, the worker node only sees the digest, not the tag.

    $ docker service create --name="myservice" ubuntu:16.04
    

    Some tags represent discrete releases, such as ubuntu:16.04. Tags like this will almost always resolve to a stable digest over time. It is recommended that you use this kind of tag when possible.

    Other types of tags, such as latest or nightly, may resolve to a new digest often, depending on how often an image’s author updates the tag. It is not recommended to run services using a tag which is updated frequently, to prevent different service replica tasks from using different image versions.

  • If you don’t specify a version at all, by convention the image’s latest tag is resolved to a digest. Workers use the image at this digest when creating the service task.

    Thus, the following two commands are equivalent:

    $ docker service create --name="myservice" ubuntu
    
    $ docker service create --name="myservice" ubuntu:latest
    
  • If you specify a digest directly, that exact version of the image is always used when creating service tasks.

    $ docker service create \
        --name="myservice" \
        ubuntu:16.04@sha256:35bc48a1ca97c3971611dc4662d08d131869daa692acb281c7e9e052924e38b1
    

When you create a service, the image’s tag is resolved to the specific digest the tag points to at the time of service creation. Worker nodes for that service will use that specific digest forever unless the service is explicitly updated. This feature is particularly important if you do use often-changing tags such as latest, because it ensures that all service tasks use the same version of the image.

Note: If content trust is enabled, the client actually resolves the image’s tag to a digest before contacting the swarm manager, in order to verify that the image is signed. Thus, if you use content trust, the swarm manager receives the request pre-resolved. In this case, if the client cannot resolve the image to a digest, the request fails.

If the manager is not able to resolve the tag to a digest, each worker node is responsible for resolving the tag to a digest, and different nodes may use different versions of the image. If this happens, a warning like the following will be logged, substituting the placeholders for real information.

unable to pin image <IMAGE-NAME> to digest: <REASON>

To see an image’s current digest, issue the command docker inspect <IMAGE>:<TAG> and look for the RepoDigests line. The following is the current digest for ubuntu:latest at the time this content was written. The output is truncated for clarity.

$ docker inspect ubuntu:latest
"RepoDigests": [
    "ubuntu@sha256:35bc48a1ca97c3971611dc4662d08d131869daa692acb281c7e9e052924e38b1"
],

After you create a service, its image is never updated unless you explicitly run docker service update with the --image flag as described below. Other update operations such as scaling the service, adding or removing networks or volumes, renaming the service, or any other type of update operation do not update the service’s image.

Update a service’s image after creation

Each tag represents a digest, similar to a Git hash. Some tags, such as latest, are updated often to point to a new digest. Others, such as ubuntu:16.04, represent a released software version and are not expected to update to point to a new digest often if at all. In Docker 1.13 and higher, when you create a service, it is constrained to create tasks using a specific digest of an image until you update the service using service update with the --image flag. If you use an older version of Docker Engine, you must remove and re-create the service to update its image.

When you run service update with the --image flag, the swarm manager queries Docker Hub or your private Docker registry for the digest the tag currently points to and updates the service tasks to use that digest.

Note: If you use content trust, the Docker client resolves image and the swarm manager receives the image and digest, rather than a tag.

Usually, the manager is able to resolve the tag to a new digest and the service updates, redeploying each task to use the new image. If the manager is unable to resolve the tag or some other problem occurs, the next two sections outline what to expect.

If the manager resolves the tag

If the swarm manager can resolve the image tag to a digest, it instructs the worker nodes to redeploy the tasks and use the image at that digest.

  • If a worker has cached the image at that digest, it uses it.

  • If not, it attempts to pull the image from Docker Hub or the private registry.

    • If it succeeds, the task is deployed using the new image.

    • If the worker fails to pull the image, the service fails to deploy on that worker node. Docker tries again to deploy the task, possibly on a different worker node.

If the manager cannot resolve the tag

If the swarm manager cannot resolve the image to a digest, all is not lost:

  • The manager instructs the worker nodes to redeploy the tasks using the image at that tag.

  • If the worker has a locally cached image that resolves to that tag, it uses that image.

  • If the worker does not have a locally cached image that resolves to the tag, the worker tries to connect to Docker Hub or the private registry to pull the image at that tag.

    • If this succeeds, the worker uses that image.

    • If this fails, the task fails to deploy and the manager tries again to deploy the task, possibly on a different worker node.

Control service scale and placement

Swarm mode has two types of services, replicated and global. For replicated services, you specify the number of replica tasks for the swarm manager to schedule onto available nodes. For global services, the scheduler places one task on each available node.

You control the type of service using the --mode flag. If you don’t specify a mode, the service defaults to replicated. For replicated services, you specify the number of replica tasks you want to start using the --replicas flag. For example, to start a replicated nginx service with 3 replica tasks:

$ docker service create \
  --name my_web \
  --replicas 3 \
  nginx

To start a global service on each available node, pass --mode global to docker service create. Every time a new node becomes available, the scheduler places a task for the global service on the new node. For example to start a service that runs alpine on every node in the swarm:

$ docker service create \
  --name myservice \
  --mode global \
  alpine top

Service constraints let you set criteria for a node to meet before the scheduler deploys a service to the node. You can apply constraints to the service based upon node attributes and metadata or engine metadata. For more information on constraints, refer to the docker service create CLI reference.

Reserving memory or number of CPUs for a service

To reserve a given amount of memory or number of CPUs for a service, use the --reserve-memory or --reserve-cpu flags. If no available nodes can satisfy the requirement (for instance, if you request 4 CPUs and no node in the swarm has 4 CPUs), the service remains in a pending state until a node is available to run its tasks.

Configure service networking options

Swarm mode lets you network services in a couple of ways:

  • publish ports externally to the swarm using ingress networking or directly on each swarm node
  • connect services and tasks within the swarm using overlay networks

Publish ports

When you create a swarm service, you can publish that service’s ports to hosts outside the swarm in two ways:

  • You can rely on the routing mesh. When you publish a service port, the swarm makes the service accessible at the target port on every node, regardless of whether there is a task for the service running on that node or not. This is less complex and is the right choice for many types of services.

  • You can publish a service task’s port directly on the swarm node where that service is running. This feature is available in Docker 1.13 and higher. This bypasses the routing mesh and provides the maximum flexibility, including the ability for you to develop your own routing framework. However, you are responsible for keeping track of where each task is running and routing requests to the tasks, and load-balancing across the nodes.

Keep reading for more information and use cases for each of these methods.

Publish a service’s ports using the routing mesh

To publish a service’s ports externally to the swarm, use the --publish <TARGET-PORT>:<SERVICE-PORT> flag. The swarm makes the service accessible at the target port on every swarm node. If an external host connects to that port on any swarm node, the routing mesh routes it to a task. The external host does not need to know the IP addresses or internally-used ports of the service tasks to interact with the service. When a user or process connects to a service, any worker node running a service task may respond.

Example: Run a three-task Nginx service on 10-node swarm

Imagine that you have a 10-node swarm, and you deploy an Nginx service running three tasks on a 10-node swarm:

$ docker service create --name my_web \
                        --replicas 3 \
                        --publish 8080:80 \
                        nginx

Three tasks will run on up to three nodes. You don’t need to know which nodes are running the tasks; connecting to port 8080 on any of the 10 nodes will connect you to one of the three nginx tasks. You can test this using curl (the HTML output is truncated):

$ curl localhost:8080

<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
...truncated...
</html>

Subsequent connections may be routed to the same swarm node or a different one.

Publish a service’s ports directly on the swarm node

Using the routing mesh may not be the right choice for your application if you need to make routing decisions based on application state or you need total control of the process for routing requests to your service’s tasks. To publish a service’s port directly on the node where it is running, use the mode=host option to the --publish flag.

Note: If you publish a service’s ports directly on the swarm node using mode=host and also set published=<PORT> this creates an implicit limitation that you can only run one task for that service on a given swarm node. In addition, if you use mode=host and you do not use the --mode=global flag on docker service create, it will be difficult to know which nodes are running the service in order to route work to them.

Example: Run a cadvisor monitoring service on every swarm node

Google cAdvisor is a tool for monitoring Linux hosts which run containers. Typically, cAdvisor is run as a stand-alone container, because it is designed to monitor a given Docker Engine instance. If you run cAdvisor as a service using the routing mesh, connecting to the cAdvisor port on any swarm node will show you the statistics for (effectively) a random swarm node running the service. This is probably not what you want.

The following example runs cAdvisor as a service on each node in your swarm and exposes cAdvisor port locally on each swarm node. Connecting to the cAdvisor port on a given node will show you that node’s statistics. In practice, this is similar to running a single stand-alone cAdvisor container on each node, but without the need to manually administer those containers.

$ docker service create \
  --mode global \
  --mount type=bind,source=/,destination=/rootfs,ro=1 \
  --mount type=bind,source=/var/run,destination=/var/run \
  --mount type=bind,source=/sys,destination=/sys,ro=1 \
  --mount type=bind,source=/var/lib/docker/,destination=/var/lib/docker,ro=1 \
  --publish mode=host,target=8080,published=8080 \
  --name=cadvisor \
  google/cadvisor:latest

You can reach cAdvisor on port 8080 of every swarm node. If you add a node to the swarm, a cAdvisor task will be started on it. You cannot start another service or container on any swarm node which binds to port 8080.

Note: This is a naive example that works well for system monitoring applications and similar types of software. Creating an application-layer routing framework for a multi-tiered service is complex and out of scope for this topic.

Add an overlay network

Use overlay networks to connect one or more services within the swarm.

First, create an overlay network on a manager node the docker network create command:

$ docker network create --driver overlay my-network

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After you create an overlay network in swarm mode, all manager nodes have access to the network.

When you create a service and pass the --network flag to attach the service to the overlay network:

$ docker service create \
  --replicas 3 \
  --network my-network \
  --name my-web \
  nginx

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The swarm extends my-network to each node running the service.

For more information on overlay networking and service discovery, refer to Attach services to an overlay network. See also Docker swarm mode overlay network security model.

Configure update behavior

When you create a service, you can specify a rolling update behavior for how the swarm should apply changes to the service when you run docker service update. You can also specify these flags as part of the update, as arguments to docker service update.

The --update-delay flag configures the time delay between updates to a service task or sets of tasks. You can describe the time T as a combination of the number of seconds Ts, minutes Tm, or hours Th. So 10m30s indicates a 10 minute 30 second delay.

By default the scheduler updates 1 task at a time. You can pass the --update-parallelism flag to configure the maximum number of service tasks that the scheduler updates simultaneously.

When an update to an individual task returns a state of RUNNING, the scheduler continues the update by continuing to another task until all tasks are updated. If, at any time during an update a task returns FAILED, the scheduler pauses the update. You can control the behavior using the --update-failure-action flag for docker service create or docker service update.

In the example service below, the scheduler applies updates to a maximum of 2 replicas at a time. When an updated task returns either RUNNING or FAILED, the scheduler waits 10 seconds before stopping the next task to update:

$ docker service create \
  --replicas 10 \
  --name my_web \
  --update-delay 10s \
  --update-parallelism 2 \
  --update-failure-action continue \
  alpine

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The --update-max-failure-ratio flag controls what fraction of tasks can fail during an update before the update as a whole is considered to have failed. For example, with --update-max-failure-ratio 0.1 --update-failure-action pause, after 10% of the tasks being updated fail, the update will be paused.

An individual task update is considered to have failed if the task doesn’t start up, or if it stops running within the monitoring period specified with the --update-monitor flag. The default value for --update-monitor is 30 seconds, which means that a task failing in the first 30 seconds after its started counts towards the service update failure threshold, and a failure after that is not counted.

Roll back to the previous version of a service

In case the updated version of a service doesn’t function as expected, it’s possible to roll back to the previous version of the service using docker service update’s --rollback flag. This will revert the service to the configuration that was in place before the most recent docker service update command.

Other options can be combined with --rollback; for example, --update-delay 0s to execute the rollback without a delay between tasks:

$ docker service update \
  --rollback \
  --update-delay 0s
  my_web

my_web

Configure mounts

You can create two types of mounts for services in a swarm, volume mounts or bind mounts. You pass the --mount flag when you create a service. The default is a volume mount if you don’t specify a type.

  • Volumes are storage that remain alive after a container for a task has been removed. The preferred method to mount volumes is to leverage an existing volume:
$ docker service create \
  --mount src=<VOLUME-NAME>,dst=<CONTAINER-PATH> \
  --name myservice \
  <IMAGE>

For more information on how to create a volume, see the volume create CLI reference.

The following method creates the volume at deployment time when the scheduler dispatches a task, just before starting the container:

$ docker service create \
  --mount type=volume,src=<VOLUME-NAME>,dst=<CONTAINER-PATH>,volume-driver=<DRIVER>,volume-opt=<KEY0>=<VALUE0>,volume-opt=<KEY1>=<VALUE1>
  --name myservice \
  <IMAGE>
  • Bind mounts are file system paths from the host where the scheduler deploys the container for the task. Docker mounts the path into the container. The file system path must exist before the swarm initializes the container for the task.

The following examples show bind mount syntax:

# Mount a read-write bind
$ docker service create \
  --mount type=bind,src=<HOST-PATH>,dst=<CONTAINER-PATH> \
  --name myservice \
  <IMAGE>

# Mount a read-only bind
$ docker service create \
  --mount type=bind,src=<HOST-PATH>,dst=<CONTAINER-PATH>,readonly \
  --name myservice \
  <IMAGE>

Important note: Bind mounts can be useful but they are also dangerous. In most cases, we recommend that you architect your application such that mounting paths from the host is unnecessary. The main risks include the following:

If you bind mount a host path into your service’s containers, the path must exist on every machine. The Docker swarm mode scheduler can schedule containers on any machine that meets resource availability requirements and satisfies all --constraints you specify.

The Docker swarm mode scheduler may reschedule your running service containers at any time if they become unhealthy or unreachable.

Host bind mounts are completely non-portable. When you use bind mounts, there is no guarantee that your application will run the same way in development as it does in production.

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