Apply rolling updates to a service

In a previous step of the tutorial, you scaled the number of instances of a service. In this part of the tutorial, you deploy a service based on the Redis 7.4.0 container tag. Then you upgrade the service to use the Redis 7.4.1 container image using rolling updates.

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. Deploy your Redis tag to the swarm and configure the swarm with a 10 second update delay. Note that the following example shows an older Redis tag:

   $ docker service create \
     --replicas 3 \
     --name redis \
     --update-delay 10s \
     redis:7.4.0
   
   0u6a4s31ybk7yw2wyvtikmu50
   

   You configure the rolling update policy at service deployment time.

   The --update-delay flag configures the time delay between updates to a service task or sets of tasks. You can describe the time T as a combination of the number of seconds Ts, minutes Tm, or hours Th. So 10m30s indicates a 10 minute 30 second delay.

   By default the scheduler updates 1 task at a time. You can pass the --update-parallelism flag to configure the maximum number of service tasks that the scheduler updates simultaneously.

   By default, when an update to an individual task returns a state of RUNNING, the scheduler schedules another task to update until all tasks are updated. If at any time during an update a task returns FAILED, the scheduler pauses the update. You can control the behavior using the --update-failure-action flag for docker service create or docker service update.

3. Inspect the redis service:

   $ docker service inspect --pretty redis
   
   ID:             0u6a4s31ybk7yw2wyvtikmu50
   Name:           redis
   Service Mode:   Replicated
    Replicas:      3
   Placement:
    Strategy:	    Spread
   UpdateConfig:
    Parallelism:   1
    Delay:         10s
   ContainerSpec:
    Image:         redis:7.4.0
   Resources:
   Endpoint Mode:  vip
   

4. Now you can update the container image for redis. The swarm manager applies the update to nodes according to the UpdateConfig policy:

   $ docker service update --image redis:7.4.1 redis
   redis
   

   The scheduler applies rolling updates as follows by default:

   • Stop the first task.
   • Schedule update for the stopped task.
   • Start the container for the updated task.
   • If the update to a task returns RUNNING, wait for the specified delay period then start the next task.
   • If, at any time during the update, a task returns FAILED, pause the update.

5. Run docker service inspect --pretty redis to see the new image in the desired state:

   $ docker service inspect --pretty redis
   
   ID:             0u6a4s31ybk7yw2wyvtikmu50
   Name:           redis
   Service Mode:   Replicated
    Replicas:      3
   Placement:
    Strategy:	    Spread
   UpdateConfig:
    Parallelism:   1
    Delay:         10s
   ContainerSpec:
    Image:         redis:7.4.1
   Resources:
   Endpoint Mode:  vip
   

   The output of service inspect shows if your update paused due to failure:

   $ docker service inspect --pretty redis
   
   ID:             0u6a4s31ybk7yw2wyvtikmu50
   Name:           redis
   ...snip...
   Update status:
    State:      paused
    Started:    11 seconds ago
    Message:    update paused due to failure or early termination of task 9p7ith557h8ndf0ui9s0q951b
   ...snip...
   

   To restart a paused update run docker service update <SERVICE-ID>. For example:

   $ docker service update redis
   

   To avoid repeating certain update failures, you may need to reconfigure the service by passing flags to docker service update.

6. Run docker service ps <SERVICE-ID> to watch the rolling update:

   $ docker service ps redis
   
   NAME                                   IMAGE        NODE       DESIRED STATE  CURRENT STATE            ERROR
   redis.1.dos1zffgeofhagnve8w864fco      redis:7.4.1  worker1    Running        Running 37 seconds
    \_ redis.1.88rdo6pa52ki8oqx6dogf04fh  redis:7.4.0  worker2    Shutdown       Shutdown 56 seconds ago
   redis.2.9l3i4j85517skba5o7tn5m8g0      redis:7.4.1  worker2    Running        Running About a minute
    \_ redis.2.66k185wilg8ele7ntu8f6nj6i  redis:7.4.0  worker1    Shutdown       Shutdown 2 minutes ago
   redis.3.egiuiqpzrdbxks3wxgn8qib1g      redis:7.4.1  worker1    Running        Running 48 seconds
    \_ redis.3.ctzktfddb2tepkr45qcmqln04  redis:7.4.0  mmanager1  Shutdown       Shutdown 2 minutes ago
   

   Before Swarm updates all of the tasks, you can see that some are running redis:7.4.0 while others are running redis:7.4.1. The output above shows the state once the rolling updates are done.

Next steps

Next, you'll learn how to drain a node in the swarm.