Scaling services with replicas in Docker - Step-by-Step Execution

Process Flow - Scaling services with replicas

Start with 1 service instance

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Decide number of replicas

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Run docker service scale command

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Docker creates additional replicas

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All replicas running

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Load balancer distributes requests

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Service scales up or down as needed

This flow shows how Docker scales a service by creating multiple replicas and balancing requests among them.

Execution Sample

Docker

docker service create --name myweb nginx

docker service scale myweb=3

Create a web service with one replica, then scale it to 3 replicas.

Process Table

Step	Command	Action	Replicas Running	Result
1	docker service create --name myweb nginx	Create service with default 1 replica	1	Service 'myweb' created with 1 replica
2	docker service scale myweb=3	Scale service to 3 replicas	3	Service 'myweb' now has 3 replicas running
3	docker service scale myweb=1	Scale service down to 1 replica	1	Service 'myweb' scaled down to 1 replica

💡 Scaling commands complete; service replicas adjusted as requested

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
replicas	0	1	3	1

Key Moments - 2 Insights

Why does the service start with 1 replica by default?

By default, Docker creates a service with 1 replica as shown in Step 1 of the execution_table. This ensures the service is running before scaling.

What happens if you scale to fewer replicas than currently running?

As shown in Step 3, scaling down reduces the number of replicas by stopping extra instances, so the service runs only the requested number.

Visual Quiz - 3 Questions

Look at the execution_table, how many replicas are running after Step 2?

A 1

B 2

C 3

D 0

💡 Hint

Check the 'Replicas Running' column for Step 2 in the execution_table

At which step does the service scale down to 1 replica?

A Step 3

B Step 2

C Step 1

D None

💡 Hint

Look at the 'Command' and 'Action' columns in the execution_table for scaling down

If you run 'docker service scale myweb=5' after Step 3, what will happen to replicas?

A Replicas stay at 1

B Replicas increase to 5

C Replicas decrease to 0

D Service is deleted

💡 Hint

Scaling up increases replicas as shown in Step 2; scaling to 5 will create 5 replicas

Concept Snapshot

Scaling services with replicas in Docker:
- Use 'docker service create' to start a service (default 1 replica)
- Use 'docker service scale servicename=number' to change replicas
- Docker adds or removes containers to match the replica count
- Load balancer spreads requests across replicas
- Scaling up adds instances; scaling down removes them

Full Transcript

This lesson shows how to scale Docker services by changing the number of replicas. First, a service is created with one replica. Then, the 'docker service scale' command changes the number of replicas. Docker adds or removes containers to match the requested count. This helps handle more or fewer requests by running multiple instances of the service. The load balancer sends traffic to all replicas evenly. Scaling up means more replicas; scaling down means fewer. The execution table traces these steps with commands and replica counts.