Docker Swarm Cheatsheet
Cluster Status
Use the following command to get the status of a cluster. This can only be run from a manager node.
docker node ls
- This node is the "primary" manager that actually manages the cluster. The other managers are replicas there for redundancy.
- This node is "drained" meaning that it is not running any services, nor will any services be deployed to it in future. This is can be good for managers as it means they are dedicated to managing.
- This node is unreachable, meaning it has likely gone offline.
- This star indicates the node that you ran the query on.
You may notice that one of the nodes has a blank under the manager status. This means that it is a worker node and not a manager.
Node Management
Add A Node
When you created the cluster, you should have been given the command that you need to give to other nodes in order to join the cluster. If you don't have this command anymore, you can retrieve it by executing:
docker swarm join-token worker
If you want to get the command to join nodes as managers, you can execute:
docker swarm join-token manager
The output of both of these commands will be similar to below:
To add a manager to this swarm, run the following command:
docker swarm join \
--token SWMTKN-1-154zu01lysyz6qbqxhm5i27b591gg8ffhce2jq438damwelgz6-2qve6pcf9y6j5t3d9dmqnl5kg \
10.1.0.48:2377
Scale the service in the swarm Now that you have the relevant command, simply execute it from the node that you wish to have join the swarm. Make sure not to have more than 7 managers, but you can have as many workers as you like.
Remove A Node
To gracefully remove a node from a cluster, use the following command on the node itself:
docker swarm leave
The node will still show in the list of nodes in the cluster, but its status will be Down and its availability will be Active.
To now remove the node from the cluster, run the following command on the manager.
docker node rm $NODE_ID
Inspect Node
docker node inspect --pretty $NODE_ID
Drain Node
If you want to gracefully remove any containers from a node, and prevent services from running on that node in future then you want to drain it. This could be a useful step to run before updating and rebooting nodes.
docker node update \
--availability drain \
$NODE_ID
To undo this change, execute:
docker node update --availability active $NODE_ID
Refer here for more info on draining nodes.
Demote Node
To demote a node from a manager to a "follower" node, use the command below:
docker node demote $NODE_ID
Promote Node
To promote a node from a worker to a manager, execute:
docker node promote $NODE_ID
Make sure to have an odd number of managers, and no more than 7. All managers should have minimal downtime and have static IPs.
Services
To deploy your application to a swarm cluster, you deploy it as a service. By being a service, it has the ability to:
- be deployed to any of the nodes.
- be automatically re-deployed if it dies.
- can be scaled to be run any number of times simultaneously across the cluster.
- have requests be load-balanced across running containers of the service.
Create Service
To deploy your application as a service run:
docker service create $IMAGE
Chances are that you probably want to give your service a name to reference it by, and you may want to specify the number of instances/replicas:
docker service create --name my_web --replicas 3 $IMAGE
Please refer here for the full list of optional parameters.
Remove Service
When you want to remove a service from your cluster, use the following command:
docker service rm $SERVICE_ID
List Services
To see which services are running on your cluster:
docker service ls
Scale Service
If you want to scale up/down after a service has already been deployed...
docker service scale $SERVICE_ID=$NUM_INSTANCES
For example
docker service scale whoami=3
List Service Processes
When there are multiple instances of a single service running, you can see them with:
docker service ps $SERVICE_ID
This will indicate which nodes the service's containers are running on.
ID NAME SERVICE IMAGE LAST STATE DESIRED STATE NODE
8p1vev3fq5zm0mi8g0as41w35 helloworld.1 helloworld alpine Running 7 minutes Running worker2
c7a7tcdq5s0uk3qr88mf8xco6 helloworld.2 helloworld alpine Running 24 seconds Running worker1
6crl09vdcalvtfehfh69ogfb1 helloworld.3 helloworld alpine Running 24 seconds Running worker1
auky6trawmdlcne8ad8phb0f1 helloworld.4 helloworld alpine Running 24 seconds Accepted manager1
ba19kca06l18zujfwxyc5lkyn helloworld.5 helloworld alpine Running 24 seconds Running worker2
Overlay Networks
Containers deployed on the same overlay network can communicate with each other, even when they are on different nodes. For more information pleas refer here.
Create Overlay Network
The command below will create an overlay network called my-network.
docker network create \
--driver overlay \
--subnet 10.0.9.0/24 \
--opt encrypted \
my-network
Remove Overlay Network
docker network rm $NETWORK_ID
Deploy Service To Specific Network
Use the --network [overlay network ID] option when creating a service if you wish to specify which network to join.
List Networks
docker network ls
Example output:
NETWORK ID NAME DRIVER SCOPE a924a6335935 bridge bridge local 0eb588929cb3 docker_gwbridge bridge local 6520f47d6e19 host host local 8puto62h939d ingress overlay swarm 064db85ed9e3 none null local 0fv9x8scsntd traefik-net overlay swarm
Inspect Network
docker network inspect $NETWORK_ID
References
First published: 16th August 2018