A docker service is a group of containers running the same image, which can be scaled over multiple nodes in a swarm, updated with newer versions etc, thus easily distributed and managed.

We will assume that we have setup a docker swarm with three nodes (single manager node and two worker nodes).

docker node ls

ID                            HOSTNAME            STATUS              AVAILABILITY        MANAGER STATUS      ENGINE VERSION
ccn5ut84cgfrylf5wds7q6dcb     node2         Ready               Active                                  19.03.2
c19rzvk5c0l8vw5tlqj3vn2sr     node3         Ready               Active                                  19.03.2
lqljj1kqrc847x5j1h2jk49kl *   node1             Ready               Active              Leader              19.03.2

Creating a service

We will create a service that runs nginx version 1.16.1 and have two replicas of the same for load balancing purposes. We will name our service ‘webservice’. We will map the container port 80 to docker host’s port 1213.

Run the following on manager node to create a service named ‘webservice’.

docker service create --replicas 2 -p 1213:80 --name webservice nginx:1.16.1

overall progress: 2 out of 2 tasks
1/2: running   [==================================================>]
2/2: running   [==================================================>]
verify: Service converged

List and inspect services

Run the following on Manager node to list services.

docker service ls

ID                  NAME                MODE                REPLICAS            IMAGE              PORTS
qsjwqiuiqsnd        webservice                 replicated          2/2                 nginx:1.16.1        *:1213->80/tcp

docker service ps webservice

ID                  NAME                IMAGE               NODE                DESIRED STATE       CURRENT STATE            ERROR               PORTS
eb2x695knsl7        webservice.1               nginx:1.16.1        node1             Running             Running 21 minutes ago               
tz17eogzgciq        webservice.2               nginx:1.16.1        node1             Running             Running 21 minutes ago               

The above output shows that two replicas of nginx webservice that was created, is running on ‘node1’

To get all the details about the service, run the following command on Manager node.

docker service inspect webservice --pretty

ID:             qsjwqiuiqsndu88ltx0utzmbd
Name:           webservice
Service Mode:   Replicated
 Replicas:      2
Placement:
UpdateConfig:
 Parallelism:   1
 On failure:    pause
 Monitoring Period: 5s
 Max failure ratio: 0
 Update order:      stop-first
RollbackConfig:
 Parallelism:   1
 On failure:    pause
 Monitoring Period: 5s
 Max failure ratio: 0
 Rollback order:    stop-first
ContainerSpec:
 Image:         nginx:1.16.1@sha256:0d0af9bc6ca2db780b532a522a885bef7fcaddd52d11817fc4cb6a3ead3eacc0
 Init:          false
Resources:
Endpoint Mode:  vip
Ports:
 PublishedPort = 1213
  Protocol = tcp
  TargetPort = 80
  PublishMode = ingress

Scaling services

We will scale up the services to 5 replicas from 2 replicas.

Run the following command on manager node.

docker service scale web=5

webservice scaled to 5
overall progress: 5 out of 5 tasks
1/5: running   [==================================================>]
2/5: running   [==================================================>]
3/5: running   [==================================================>]
4/5: running   [==================================================>]
5/5: running   [==================================================>]
verify: Service converged

Run the following on Manager node to check the status of service.

docker service ps webservice

ID                  NAME                IMAGE               NODE                DESIRED STATE       CURRENT STATE            ERROR               PORTS
eb2x695knsl7        webservice.1               nginx:1.16.1        node1             Running             Running 2 hours ago                  
tz17eogzgciq        webservice.2               nginx:1.16.1        node1             Running             Running 2 hours ago                  
vcbs2t5kmjsv        webservice.3               nginx:1.16.1        node1             Running             Running 7 minutes ago                
vj0bkro1we0q         \_ webservice.3           nginx:1.16.1        node2         Shutdown            Shutdown 2 minutes ago               
dp7wj7r17wa5        webservice.4               nginx:1.16.1        node1             Running             Running 7 minutes ago                
5eypzevneakz         \_ webservice.4           nginx:1.16.1        node2         Shutdown            Shutdown 2 minutes ago               
23vdife1bvbo         \_ webservice.4           nginx:1.16.1        node3         Shutdown            Starting 9 minutes ago               
ja7bf8dumtc8        webservice.5               nginx:1.16.1        node1             Running             Running 7 minutes ago                
saxhoh6muh5e         \_ webservice.5           nginx:1.16.1        node2         Shutdown            Shutdown 2 minutes ago               
yxylpxzncfg6         \_ webservice.5           nginx:1.16.1        node3         Shutdown            Starting 9 minutes ago               

Note that the service shows up with running status with a count of 5 across the nodes.

Running updates on images deployed in swarm

Suppose we want to update the nginx version to 1.17.4, run the following command on Manager node.

 docker service update --image nginx:1.17.4 webservice

overall progress: 5 out of 5 tasks
1/5: running   [==================================================>]
2/5: running   [==================================================>]
3/5: running   [==================================================>]
4/5: running   [==================================================>]
5/5: running   [==================================================>]
verify: Service converged

Docker updates the versions one replica at a time and waits for a pre configured interval before updating the next replica. Upon failure, it pauses the update altogether.

Run the following on Manager node, to check the status of updates on all replicas.

docker service ps webservice

ID                  NAME                IMAGE               NODE                DESIRED STATE       CURRENT STATE             ERROR               PORTS
l2fakwiqc6vd        webservice.1               nginx:1.17.4        node2         Running             Running 4 minutes ago                
eb2x695knsl7         \_ webservice.1           nginx:1.16.1        node1             Shutdown            Shutdown 4 minutes ago               
t2yuti85kwnl        webservice.2               nginx:1.17.4        node1             Running             Running 3 minutes ago                
tz17eogzgciq         \_ webservice.2           nginx:1.16.1        node1             Shutdown            Shutdown 3 minutes ago               
ku2dkabpioxv        webservice.3               nginx:1.17.4        node1             Running             Running 3 minutes ago                
vcbs2t5kmjsv         \_ webservice.3           nginx:1.16.1        node1             Shutdown            Shutdown 3 minutes ago               
vj0bkro1we0q         \_ webservice.3           nginx:1.16.1        node3         Shutdown            Shutdown 33 minutes ago              
0mstkiea7aac        webservice.4               nginx:1.17.4        node3         Running             Running 4 minutes ago                
dp7wj7r17wa5         \_ webservice.4           nginx:1.16.1        node1             Shutdown            Shutdown 4 minutes ago               
5eypzevneakz         \_ webservice.4           nginx:1.16.1        node3         Shutdown            Shutdown 33 minutes ago              
23vdife1bvbo         \_ webservice.4           nginx:1.16.1        node2         Shutdown            Shutdown 28 minutes ago              
iz7nfjgj7jnz        webservice.5               nginx:1.17.4        node2         Running             Running 4 minutes ago                
ja7bf8dumtc8         \_ webservice.5           nginx:1.16.1        node1             Shutdown            Shutdown 4 minutes ago               
saxhoh6muh5e         \_ webservice.5           nginx:1.16.1        node3         Shutdown            Shutdown 33 minutes ago              
yxylpxzncfg6         \_ webservice.5           nginx:1.16.1        node2         Shutdown            Shutdown 28 minutes ago  

Deleting a service

Run the following on Manager node to delete a service.

docker service rm webservice

webservice

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