Node Availability


To view a list of nodes in the swarm run docker node ls from a manager node:

root@master:~# docker node ls
ID                            HOSTNAME   STATUS    AVAILABILITY   MANAGER STATUS   ENGINE VERSION
buc72h0hyo66gcilq7melh9pq *   master     Ready     Active         Leader           20.10.11
xvdyauu8t83r3udrqh6c5w8a8     worker01   Ready     Active                          20.10.11
xctiaq0n14aew1falekqybq53     worker02   Ready     Active                          20.10.11
root@master:~#


The AVAILABILITY column shows whether or not the scheduler can assign tasks to the node:


Active means that the scheduler can assign tasks to the node.

Pause means the scheduler doesn’t assign new tasks to the node, but existing tasks remain running.

Drain means the scheduler doesn’t assign new tasks to the node. The scheduler shuts down any existing tasks and schedules them on an available node.


Let’s see an example of each of these.

 

To check whether the master node is able to assign any task to the paused worker node or not, we will pause one of the nodes-

[email protected]master:~# docker node update --availability=pause worker02
worker02
[email protected]master:~#
[email protected]master:~# docker node ls
ID                            HOSTNAME   STATUS    AVAILABILITY   MANAGER STATUS   ENGINE VERSION
buc72h0hyo66gcilq7melh9pq *   master     Ready     Active         Leader           20.10.11
xvdyauu8t83r3udrqh6c5w8a8     worker01   Ready     Active                          20.10.11
xctiaq0n14aew1falekqybq53     worker02   Ready     Pause                           20.10.11
[email protected]master:~#



Now lets create service with 3 replicas using below command -

[email protected]master:~# docker service create -d --replicas 3 alpine ping 192.168.0.123                                                                                u9wpp4l87zrx46hqlz5egbpx6
[email protected]master:~# docker service ps u9
ID             NAME              IMAGE           NODE       DESIRED STATE   CURRENT STATE           ERROR     PORTS
w20byjbi1ihn   sleepy_galois.1   alpine:latest   master     Running         Running 8 seconds ago
wng1vz7j688n   sleepy_galois.2   alpine:latest   master     Running         Running 8 seconds ago
1ky4mbh8lf69   sleepy_galois.3   alpine:latest   worker01   Running         Running 8 seconds ago
[email protected]master:~#


Here you will notice that the manager node did not send any load on the worker node 2.

The master node will again start sending the load on node 2 once it is active.


Note that docker doesn’t do any rebalancing of the load onto the worker nodes.


Now, let's drain node 2 using below command:

[email protected]master:~# docker node update --availability=drain worker02
worker02
[email protected]master:~# docker node ls
ID                            HOSTNAME   STATUS    AVAILABILITY   MANAGER STATUS   ENGINE VERSION
buc72h0hyo66gcilq7melh9pq *   master     Ready     Active         Leader           20.10.11
xvdyauu8t83r3udrqh6c5w8a8     worker01   Ready     Active                          20.10.11
xctiaq0n14aew1falekqybq53     worker02   Ready     Drain                           20.10.11
[email protected]master:~#


Once worker 2 is drained, the containers which were on node 2 will be shifted to the other available nodes. In our case, it would be the master node and node 1.