moby/docs/swarm/swarm-tutorial/scale-service.md

Scale the service in the Swarm

Once you have deployed a service to a Swarm, you are ready to use the Docker CLI to scale the number of service tasks in the Swarm.

1. If you haven't already, open a terminal and ssh into the machine where you run your manager node. For example, the tutorial uses a machine named manager1.

2. Run the following command to change the desired state of the service runing in the Swarm:

   $ docker service update --scale NUMBER-OF-TASKS SERVICE-ID
   

   The --scale flag indicates the number of tasks you want in the new desired state. For example:

   $ docker service update --scale 5 helloworld
   helloworld
   

3. Run docker service tasks SERVICE-ID to see the updated task list:

   $ docker service tasks helloworld
   
   ID                         NAME          SERVICE     IMAGE   DESIRED STATE  LAST STATE          NODE
   1n6wif51j0w840udalgw6hphg  helloworld.1  helloworld  alpine  RUNNING        RUNNING 2 minutes   manager1
   dfhsosk00wxfb7j0cazp3fmhy  helloworld.2  helloworld  alpine  RUNNING        RUNNING 15 seconds  worker2
   6cbedbeywo076zn54fnwc667a  helloworld.3  helloworld  alpine  RUNNING        RUNNING 15 seconds  worker1
   7w80cafrry7asls96lm2tmwkz  helloworld.4  helloworld  alpine  RUNNING        RUNNING 10 seconds  worker1
   bn67kh76crn6du22ve2enqg5j  helloworld.5  helloworld  alpine  RUNNING        RUNNING 10 seconds  manager1
   

   You can see that Swarm has created 4 new tasks to scale to a total of 5 running instances of Alpine Linux. The tasks are distributed between the three nodes of the Swarm. Two are running on manager1.

4. Run docker ps to see the containers running on the node where you're connected. The following example shows the tasks running on manager1:

   $ docker ps
   
   CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
   910669d5e188        alpine:latest       "ping docker.com"   10 seconds ago      Up 10 seconds                           helloworld.5.bn67kh76crn6du22ve2enqg5j
   a0b6c02868ca        alpine:latest       "ping docker.com"   2 minutes  ago      Up 2 minutes                            helloworld.1.1n6wif51j0w840udalgw6hphg
   

   If you want to see the containers running on other nodes, you can ssh into those nodes and run the docker ps command.

What's next?

At this point in the tutorial, you're finished with the helloworld service. The next step shows how to delete the service.