1. Introduction

In this tutorial, we will review the basics of scaling and rolling updates in Docker Swarm. Docker Swarm is a container orchestration tool built into Docker, allowing for the management of Docker nodes and services.

Goals

• Understand the concepts of service scaling and rolling updates
• Learn how to perform these tasks in Docker Swarm

Prerequisites

• Basic knowledge of Docker
• Docker installed on your computer

2. Step-by-Step Guide

Service Scaling

Service scaling is the process of increasing or decreasing the number of replicas (instances) of a service. In Docker Swarm, you can scale services using the docker service scale command.

For example, if you want to scale a service named my_service to 3 replicas, you would use the following command:

docker service scale my_service=3

Rolling Updates

Rolling updates allow you to update the services in your swarm without downtime. Docker Swarm will incrementally update the service across the nodes ensuring that the service is still available during the update.

The update command can be used to change several service configurations, including the image version. For example:

docker service update --image my_service:1.2.1 my_service

This command will update the my_service service to use the image version 1.2.1.

3. Code Examples

Scaling a Service

Let's create a service named nginx_service using the nginx image:

docker service create --name nginx_service nginx

Now, scale this service to 3 replicas:

docker service scale nginx_service=3

You can confirm the service has been scaled by using the docker service ps command:

docker service ps nginx_service

Performing a Rolling Update

Assuming we have a service my_service that is currently using the 1.2.0 version of its image, we can update it to 1.2.1 using the following command:

docker service update --image my_service:1.2.1 my_service

You can confirm the update has been made by using the docker service inspect command:

docker service inspect --pretty my_service

4. Summary

In this tutorial, we have learned how to scale services and perform rolling updates in Docker Swarm. These are powerful features that allow you to manage your applications with high availability and zero downtime.

For further learning, you can delve into other Docker Swarm features such as service discovery, secrets management, and overlay networks. Docker's official documentation is a great resource for this.

5. Practice Exercises

1. Create a new service using the httpd image and scale it to 5 replicas.
2. Update the service created above to use the httpd:2.4 image version.
3. Try to scale down a service and observe what happens.

Remember, practice makes perfect! Keep practicing these tasks until you're comfortable with them. Docker's command-line interface is very intuitive, which makes it a great tool for learning container orchestration.