Service scaling in Docker - Step-by-Step Execution

Process Flow - Service scaling

Start with 1 service instance

↓

Check desired scale number

↓

Compare current instances with desired

↓

Add more instances

↓

Update running services

↓

Scaling complete

This flow shows how Docker service scaling adjusts the number of running instances to match the desired count by adding or removing containers.

Execution Sample

Docker

docker service create --name webapp nginx

docker service scale webapp=3

docker service ps webapp

Create a service named 'webapp' with 1 instance, then scale it to 3 instances, and finally list the running instances.

Process Table

Step	Command	Current Instances	Desired Instances	Action	Result
1	docker service create --name webapp nginx	0	1 (default)	Create service with 1 instance	Service 'webapp' created with 1 instance
2	docker service ps webapp	1	1	List running instances	1 instance running
3	docker service scale webapp=3	1	3	Scale up by adding 2 instances	Service scaled to 3 instances
4	docker service ps webapp	3	3	List running instances	3 instances running
5	docker service scale webapp=2	3	2	Scale down by removing 1 instance	Service scaled to 2 instances
6	docker service ps webapp	2	2	List running instances	2 instances running
7	docker service scale webapp=2	2	2	No change needed	Scaling skipped, already at desired count

💡 Scaling stops when current instances equal desired instances.

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	Final
Current Instances	0	1	3	2	2
Desired Instances	1 (default)	1	3	2	2

Key Moments - 3 Insights

Why does the service start with 1 instance even if we don't specify a number?

By default, Docker creates a service with 1 instance as shown in step 1 of the execution_table where desired instances is 1.

What happens if we scale to the same number of instances as currently running?

No scaling action is taken, as shown in step 7 where current and desired instances are both 2, so scaling is skipped.

How does Docker decide whether to add or remove instances?

Docker compares current and desired instances (step 3 and 5). If desired is greater, it adds instances; if less, it removes instances.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. How many instances are added to scale the service?

A 3

B 1

C 2

D 0

💡 Hint

Check the 'Current Instances' and 'Desired Instances' columns at step 3.

At which step does the service scale down?

A Step 2

B Step 5

C Step 4

D Step 7

💡 Hint

Look for where 'Desired Instances' is less than 'Current Instances' in the execution_table.

If you run 'docker service scale webapp=4' after step 5, what will be the new 'Current Instances'?

A 4

B 3

C 2

D 5

💡 Hint

Scaling up increases instances to the desired number as shown in step 3.

Concept Snapshot

Service scaling in Docker adjusts the number of running containers.
Use 'docker service scale <service>=<count>' to set desired instances.
Docker adds or removes containers to match the desired count.
Default service creation starts with 1 instance.
Check running instances with 'docker service ps <service>'.

Full Transcript

Service scaling in Docker means changing how many copies of a service run. When you create a service, it starts with one instance by default. You can increase or decrease this number using the 'docker service scale' command. Docker compares the current number of instances with the desired number. If you want more, Docker adds instances. If you want fewer, Docker removes some. You can check how many instances are running with 'docker service ps'. This process helps manage load and availability easily.

Practice

1. What does the command docker service scale myservice=3 do?

easy

A. It stops the service named 'myservice'.

B. It runs 3 copies of the service named 'myservice'.

C. It updates the service 'myservice' to version 3.

D. It removes the service named 'myservice'.

Solution

1. Step 1: Understand the command structure

   The command docker service scale SERVICE_NAME=NUMBER sets the number of replicas for a service.

2. Step 2: Apply to the given command

   Here, myservice=3 means run 3 replicas of the service named 'myservice'.

3. Final Answer:

   It runs 3 copies of the service named 'myservice'. -> Option B

4. Quick Check:

   Scaling sets replicas = 3 [OK]

Hint: Scale sets number of service copies with SERVICE=NUMBER [OK]

Common Mistakes:

• Confusing scaling with stopping or removing services
• Thinking scale changes service version
• Using wrong syntax without '=' sign

2. Which of these is the correct syntax to scale a Docker service named 'webapp' to 5 replicas?

easy

A. docker service scale webapp 5

B. docker service scale -replicas=5 webapp

C. docker service scale webapp=5

D. docker scale service webapp=5

Solution

1. Step 1: Recall correct scale command syntax

   The correct syntax is docker service scale SERVICE_NAME=NUMBER with an equals sign.

2. Step 2: Match options to syntax

   docker service scale webapp=5 uses webapp=5 which matches the correct syntax.

3. Final Answer:

   docker service scale webapp=5 -> Option C

4. Quick Check:

   Correct syntax uses '=' sign [OK]

Hint: Use '=' between service name and replica count [OK]

Common Mistakes:

• Omitting '=' sign between service and number
• Using wrong command order
• Confusing 'docker scale' with 'docker service scale'

3. Given the command docker service scale backend=2 was run on a service with 1 replica, what will be the output of docker service ls for the 'backend' service's REPLICAS column?

medium

A. 2/2

B. 1/2

C. 2/1

D. 1/1

Solution

1. Step 1: Understand scaling effect on replicas

   Scaling to 2 means the service should have 2 running replicas out of 2 desired.

2. Step 2: Interpret REPLICAS column format

   The format is running/desired. After scaling, both running and desired should be 2.

3. Final Answer:

   2/2 -> Option A

4. Quick Check:

   Replicas show running/desired = 2/2 [OK]

Hint: Replicas show running/desired count after scaling [OK]

Common Mistakes:

• Mixing running and desired counts order
• Assuming replicas don't update immediately
• Confusing scale number with current replicas

4. You run docker service scale frontend=abc but get an error. What is the likely cause?

medium

A. The replica count must be a number, not 'abc'.

B. The service name 'frontend' is invalid.

C. You need to use a space instead of '='.

D. Scaling requires the service to be stopped first.

Solution

1. Step 1: Check the replica count format

   The replica count must be a positive integer number, not a string like 'abc'.

2. Step 2: Validate error cause

   Using 'abc' causes a syntax or validation error because replicas must be numeric.

3. Final Answer:

   The replica count must be a number, not 'abc'. -> Option A

4. Quick Check:

   Replica count must be numeric [OK]

Hint: Replica count must be a number, not letters [OK]

Common Mistakes:

• Using letters instead of numbers for replicas
• Thinking service must be stopped to scale
• Using space instead of '=' sign

5. You have a service 'api' currently running 4 replicas. You want to handle a sudden traffic spike by doubling the replicas temporarily. Which command correctly doubles the replicas?

hard

A. docker service scale api=4

B. docker service scale api=16

C. docker service scale api=2

D. docker service scale api=8

Solution

1. Step 1: Identify current replica count

   The service 'api' currently has 4 replicas running.

2. Step 2: Calculate double replicas

   Doubling 4 replicas means setting replicas to 8.

3. Step 3: Choose correct scale command

   The command docker service scale api=8 sets replicas to 8, doubling the current count.

4. Final Answer:

   docker service scale api=8 -> Option D

5. Quick Check:

   Double 4 replicas = 8 [OK]

Hint: Double replicas by setting scale to twice current count [OK]

Common Mistakes:

• Setting replicas to current count instead of double
• Setting replicas to half or quadruple by mistake
• Using wrong syntax for scaling