Welcome back! In the last post, we got Docker Swarm mode installed and up and running. Now, we're delving into creating services and getting them deployed on your swarm.
Creating a Service
With Docker Swarm Mode, a service is a long-running Docker container that can be deployed to any node worker. It’s something that either remote systems or other containers within the swarm can connect to and consume.
For this example, we’re going to deploy a Redis service.
Deploying a Replicated Service
A replicated service is a Docker Swarm service that has a specified number of replicas running. These replicas consist of multiple instances of the specified Docker container. In our case, each replica will be a unique Redis instance.
To create our new service, we’ll use the
docker command while specifying the
service create options. The following command will create a service named
redis that has
2 replicas and publishes port
6379 across the cluster.
root@swarm-01:~# docker service create --name redis --replicas 2 --publish 6379:6379 redis er238pvukeqdev10nfmh9q1kr
In addition to specifying the
service create options, we also used the
--name flag to name the service
redis and the
--replicas flag to specify that this service should run on
2 different nodes. We can validate that it is in fact running on both nodes by executing the
docker command with the
service ls options.
root@swarm-01:~# docker service ls ID NAME REPLICAS IMAGE COMMAND er238pvukeqd redis 2/2 redis
In the output, we can see there are
2 replicas currently running. If we want to see more details on these tasks, we can run the
docker command with the
service ps option.
root@swarm-01:~# docker service ps redis ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR 5lr10nbpy91csmc91cew5cul1 redis.1 redis swarm-02.example.com Running Running 40 minutes ago 1t77jsgo1qajxxdekbenl4pgk redis.2 redis swarm-01.example.com Running Running 40 minutes ago
service ps option will show the tasks (containers) for the specified service. In this example, we can see the
redis service has a task (container) running on both of our swarm nodes.
Connecting to the Redis Service
Since we have validated that the service is running, we can try to connect to this service from a remote system with the
vagrant@vagrant:~$ redis-cli -h swarm-01.example.com -p 6379 swarm-01.example.com:6379>
From the connection above, we were successful in connecting to the
redis service. This means our service is up and available.
How Docker Swarm Publishes Services
When we created the
redis service, we used the
--publish flag with the
docker service create command. This flag was used to tell Docker to publish port
6379 as an available port for the
When Docker publishes a port for a service, it does so by listening on that port across all nodes within the Swarm Cluster. When traffic arrives on that port, that traffic is then routed to a container running for that service. While this concept is pretty standard when all nodes are running a service’s container, this concept gets interesting when we have more nodes than we do replicas.
To see how this works, let’s add a third node worker to the Swarm Cluster.
Adding a Third Node Worker into the Mix
To add another node worker, we can simply repeat the installation and setup steps in the first part of this article. Since we already covered those steps, we’ll skip ahead to the point where we have a three-node Swarm Cluster. We can once again check the status of this cluster by running the
root@swarm-01:~# docker node ls ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS 13evr7hmiujjanbnu3n92dphk swarm-02.example.com Ready Active awwiap1z5vtxponawdqndl0e7 * swarm-01.example.com Ready Active Leader e4ymm89082ooms0gs3iyn8vtl swarm-03.example.com Ready Active
We can see that the cluster consists of three hosts:
When we created our service with two replicas, it created a task (container) on
swarm-02. Let’s see if this is still the case even though we added another node worker.
root@swarm-01:~# docker service ps redis ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR 5lr10nbpy91csmc91cew5cul1 redis.1 redis swarm-02.example.com Running Running 55 minutes ago 1t77jsgo1qajxxdekbenl4pgk redis.2 redis swarm-01.example.com Running Running 55 minutes ago
With replicated services, Docker Swarm’s goal is to ensure that there is a task (container) running for every replica specified. When we created the
redis service, we specified that there should be
2 replicas. This means that even though we have a third node, Docker has no reason to start a new task on that node.
At this point, we have an interesting situation: We have a service that’s running on
2 of the
3 Swarm nodes. In a non-Swarm world, that would mean the
redis service would be unavailable when connecting to our third Swarm node. With Swarm Mode however, that is not the case.
Connecting to a Service on a Non-Task-Tunning Worker
Earlier when I described how Docker publishes a service port, I mentioned that it does so by publishing that port across all nodes within the Swarm. What’s interesting about this is what happens when we connect to a node worker that isn’t running any containers (tasks) associated with our service.
Let’s take a look at what happens when we connect to
swarm-03 over the
redis published port.
vagrant@vagrant:~$ redis-cli -h swarm-03.example.com -p 6379 swarm-03.example.com:6379>
What’s interesting about this is that our connection was successful. It was successful despite the fact that
swarm-03 is not running any
redis containers. This works because internally Docker is rerouting our
redis service traffic to a node worker running a
Docker calls this ingress load balancing. The way it works is that all node workers listen for connections to published service ports. When that service is called by external systems, the receiving node will accept the traffic and internally load balance it using an internal DNS service that Docker maintains.
So even if we scaled out our Swarm cluster to 100 node workers, end users of our
redis service can simply connect to any node worker. They will then be redirected to one of the two Docker hosts running the service tasks (containers).
All of this rerouting and load balancing is completely transparent to the end user. It all happens within the Swarm Cluster.
Making Our Service Global
At this point, we have the
redis service set up to run with
2 replicas, meaning it’s running containers on
2 of the
If we wanted our
redis service to consist of an instance on every node worker, we could do that easily by modifying the service’s number of desired replicas from
3. This would mean however that with every node worker we add or subtract, we will need to adjust the number of replicas.
We could alternatively do this automatically by making our service a Global Service. A Global Service in Docker Swarm Mode is used to create a service that has a task running on every node worker automatically. This is useful for common services such as Redis that may be leveraged internally by other services.
To show this in action, let’s go ahead and recreate our
redis service as a Global Service.
root@swarm-01:~# docker service create --name redis --mode global --publish 6379:6379 redis 5o8m338zmsped0cmqe0guh2to
The command to create a Global Service is the same
docker service create command we used to create a replicated service. The only difference is the
--mode flag along with the value of
With the service now created, we can see how Docker distributed our tasks for this service by once again executing the
docker command with the
service ps options.
root@swarm-01:~# docker service ps redis ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR 27s6q5yvmyjvty8jvp5k067ul redis redis swarm-03.example.com Running Running 26 seconds ago 2xohhkqvlw7969qj6j0ca70xx \_ redis redis swarm-02.example.com Running Running 38 seconds ago 22wrdkun5f5t9lku6sbprqi1k \_ redis redis swarm-01.example.com Running Running 38 seconds ago
We can see that when the service was created as a Global Service, a task was then started on every node worker within our Swarm Cluster.
In this article, we not only installed Docker Engine, we also set up a Swarm Cluster, deployed a replicated service, and then created a Global Service.
In a recent article, I not only installed Kubernetes, I also created a Kubernetes service. In comparing the Docker Swarm Mode services with the Kubernetes services, I personally find that Swarm Mode services were easier to get set up and created. For someone who simply wishes to use the “services” features of Kubernetes and doesn’t need some of its other capabilities, Docker Swarm Mode may be an easier alternative.