Scaling services with replicas in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does scaling a service with replicas mean in Docker?

It means running multiple copies (replicas) of the same service to handle more work and improve availability.

beginner

Which Docker command is used to scale a service to a specific number of replicas?

The command is docker service scale <service_name>=<number_of_replicas>.

beginner

What happens if you scale a service down from 5 replicas to 2 replicas?

Docker stops 3 running containers of that service, leaving only 2 active replicas.

beginner

How can you check the current number of replicas running for a Docker service?

Use docker service ls to list services and see replicas, or docker service ps <service_name> for detailed tasks.

beginner

Why is scaling services with replicas useful in real life?

It helps handle more users or requests, keeps the service running if one replica fails, and improves speed by sharing the work.

Which command scales a Docker service named 'webapp' to 4 replicas?

A docker service scale webapp=4

B docker scale service webapp 4

C docker service update --replicas 4 webapp

D docker container scale webapp 4

The correct syntax to scale a service is docker service scale <service_name>=<number>.

What does increasing replicas of a service improve?

A Service availability and ability to handle more requests

B Only the speed of the Docker engine

C The size of the Docker image

D The number of Docker networks

More replicas mean more copies running, so the service can handle more users and stay available if some replicas fail.

If you run docker service scale myservice=0, what happens?

A Nothing changes

B One replica of 'myservice' runs

C The service is deleted

D All replicas of 'myservice' stop running

Setting replicas to 0 stops all containers for that service but does not delete the service.

How do you see the number of replicas currently running for a service?

A docker ps

B docker service ls

C docker images

D docker network ls

docker service ls shows services with their replica counts.

What is a replica in Docker service scaling?

A A storage volume

B A Docker image version

C A running container copy of the service

D A network bridge

A replica is one running container instance of the service.

Explain how to scale a Docker service and why you would do it.

Think about running more copies of the same service.

Describe what happens behind the scenes when you increase the number of replicas for a service.

Imagine adding more workers to share the work.

Practice

1. What does scaling a Docker service with replicas do?

easy

A. Runs multiple copies of the service to handle more work

B. Deletes the service completely

C. Changes the service's network settings

D. Updates the Docker engine version

Solution

1. Step 1: Understand the purpose of replicas

   Replicas are copies of a service running simultaneously to share the workload.

2. Step 2: Identify the effect of scaling replicas

   Scaling increases or decreases the number of these copies to improve performance and availability.

3. Final Answer:

   Runs multiple copies of the service to handle more work -> Option A

4. Quick Check:

   Scaling = multiple copies [OK]

Hint: Scaling means running more copies of your service [OK]

Common Mistakes:

• Thinking scaling deletes the service
• Confusing scaling with network changes
• Assuming scaling updates Docker itself

2. Which is the correct command to scale a Docker service named webapp to 5 replicas?

easy

A. docker service scale webapp -replicas 5

B. docker scale service webapp 5

C. docker service scale webapp --replicas=5

D. docker service scale webapp=5

Solution

1. Step 1: Recall the correct syntax for scaling

   The correct syntax is docker service scale SERVICE_NAME=NUMBER.

2. Step 2: Match the syntax with the options

   docker service scale webapp=5 matches the exact syntax: docker service scale webapp=5.

3. Final Answer:

   docker service scale webapp=5 -> Option D

4. Quick Check:

   Correct syntax = docker service scale NAME=NUM [OK]

Hint: Use SERVICE_NAME=NUMBER format with docker service scale [OK]

Common Mistakes:

• Using wrong command order
• Adding extra flags not needed
• Using spaces instead of '='

3. Given the command docker service scale api=3, what will be the number of running containers for the api service?

medium

A. 3 containers

B. 1 container

C. 0 containers

D. Depends on the Docker version

Solution

1. Step 1: Understand the scale command effect

   The command sets the number of replicas to 3, so Docker runs 3 containers for the service.

2. Step 2: Confirm the number of containers

   Scaling to 3 means exactly 3 containers will be running for api.

3. Final Answer:

   3 containers -> Option A

4. Quick Check:

   Scale api=3 means 3 containers [OK]

Hint: Scale number equals running containers count [OK]

Common Mistakes:

• Assuming only 1 container runs by default
• Thinking scaling depends on Docker version
• Confusing replicas with CPU cores

4. You run docker service scale backend=two but get an error. What is the problem?

medium

A. The service name cannot be 'backend'

B. The number of replicas must be a numeric value

C. The command should be run as root user

D. Docker service scale does not support scaling

Solution

1. Step 1: Check the scale command syntax

   The replicas count must be a number, not a word.

2. Step 2: Identify the error cause

   Using 'two' instead of '2' causes a syntax error.

3. Final Answer:

   The number of replicas must be a numeric value -> Option B

4. Quick Check:

   Replicas must be numeric [OK]

Hint: Use digits, not words, for replica counts [OK]

Common Mistakes:

• Using words instead of numbers
• Assuming service name is invalid
• Thinking root user is required

5. You have a service app running with 2 replicas. You want to update it to 4 replicas but keep the service running without downtime. Which command should you use?

hard

A. docker service scale app 4

B. docker container run app --replicas 4

C. docker service update --replicas 4 app

D. docker service restart app --replicas 4

Solution

1. Step 1: Understand how to update replicas without downtime

   Using docker service update --replicas 4 app changes replicas smoothly.

2. Step 2: Compare options for scaling

   docker service scale can scale but docker service update is preferred for zero downtime updates.

3. Step 3: Identify incorrect commands

   docker container run and docker service restart do not scale replicas properly.

4. Final Answer:

   docker service update --replicas 4 app -> Option C

5. Quick Check:

   Use service update for zero downtime scaling [OK]

Hint: Use 'docker service update --replicas' to avoid downtime [OK]

Common Mistakes:

• Using docker service scale causing brief downtime
• Trying to run containers manually
• Restarting service instead of updating replicas