Deploying services in Swarm in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Service replicas count after deployment

You deploy a service in Docker Swarm with the command:

docker service create --name webapp --replicas 3 nginx

What will be the output of docker service ls regarding the replicas column immediately after deployment?

A webapp nginx replicated 0/3 running

B webapp nginx global 3/3 running

C webapp nginx replicated 3/0 running

D webapp nginx replicated 3/3 running

💡 Hint

Think about how quickly replicas start after service creation.

Immediately after deployment, the service is created but replicas may not be running yet, so the output shows 0/3.

❓ Configuration

intermediate

Correct syntax for updating service replicas

You want to update an existing service named api to have 5 replicas. Which command is correct?

A docker service update --replicas=5 api

B docker service update api --replicas 5

C docker service scale api=5

D docker service update --replica 5 api

💡 Hint

Check the correct order and spelling of flags in docker service update.

The correct syntax places the service name before the flags and uses --replicas (plural) without equals sign.

❓ Troubleshoot

advanced

Service fails to deploy with port conflict

You try to deploy a service with this command:

docker service create --name web --publish 80:80 nginx

But it fails with an error about port binding. What is the most likely cause?

A Port 80 is already in use on one or more swarm nodes

B The nginx image does not expose port 80

C The service name 'web' is already used by another service

D Docker Swarm does not support publishing ports

💡 Hint

Consider what prevents binding to a port on the host.

If port 80 is already used by another process or service on any node, the new service cannot bind to it.

🔀 Workflow

advanced

Order of commands to deploy and scale a service

Arrange the steps in the correct order to deploy a service named db with 2 replicas and then scale it to 4 replicas.

A 1,4,2,3

B 2,1,3,4

C 1,3,2,4

D 1,2,4,3

💡 Hint

Think about deploying first, then checking status, then scaling.

First create the service, then list services, check tasks, then update replicas.

✅ Best Practice

expert

Ensuring zero downtime during service update

You want to update a service's image without downtime. Which option ensures rolling updates with minimal disruption?

A docker service update --image newimage:latest --force myservice

B docker service update --image newimage:latest --rollback myservice

C docker service update --image newimage:latest --update-parallelism 1 --update-delay 10s myservice

D docker service update --image newimage:latest --replicas 0 myservice

💡 Hint

Look for options controlling update speed and parallelism.

Using --update-parallelism and --update-delay allows rolling updates one task at a time, avoiding downtime.