Service scaling in Docker - Step-by-Step Execution

Process Flow - Service scaling

Start with 1 service instance

↓

Check desired scale number

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Compare current instances with desired

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Add more instances

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Update running services

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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.