Managing Docker Swarm

Introduction

Before you can start managing a Docker Swarm cluster, you will need to create one. You can do this by running the following command on a single node in your cluster:

This command will initialize your node as the manager of the swarm and give you the command to add more nodes to the cluster. Once your cluster is created, you can start deploying services to it.

Monitoring Cluster Using a Visualizer

One of the most important things to do when managing a Docker Swarm cluster is to monitor it. One way to do this is by using a visualizer. A visualizer is a service that runs on your cluster and provides a visual representation of the services and nodes in your cluster.

You can deploy a visualizer service to your cluster by running the following command:

This command will create a service called "viz" that runs on the manager nodes of the cluster and listens on port 8080. You can then access the visualizer by navigating to http://<manager-node-ip:8080 in your web browser.

Analyzing Services with Docker Inspect

Another important aspect of managing a Docker Swarm cluster is analyzing the services that are running on it. One way to do this is by using the docker inspect command. This command allows you to view detailed information about a service, such as its configuration, the nodes it is running on, and the tasks that it is running.

For example, to inspect a service called "web", you can run the following command:

Understanding the Stacks and Stack Files

Docker Swarm allows you to deploy your services as a stack, which is a group of services that are deployed together. Stacks are defined in a stack file, which is a YAML file that describes the services in the stack and their configuration.

A simple stack file might look like this:

This file defines two services, "web" and "db", that are deployed together as a stack.

How to Manipulate the Running Stack of Services?

Docker Swarm allows you to create and manage a stack of services that make up your application. You can create a new stack by running the following command:

This command will create a new stack called "myapp" using the configuration defined in the "stack.yml" file. You can view the status of your stack by running the following command:

To update your stack, you can run the following command:

This command will update the stack with the new configuration defined in the new-stack.yml file.

Modifying the Network Ports

When deploying a service in a Docker Swarm, you can specify the network ports that the service should use. You can do this by adding a "ports" section to the service definition in your compose file, like so:

This service definition will publish port 80 on the host to port 80 in the container.

Mounting Volumes

Docker Swarm allows you to mount volumes to your services so that your data can persist even if the container is recreated. You can do this by adding a "volumes" section to the service definition in your compose file, like so:

This service definition will mount the host directory "/var/lib/postgresql/data" to the container directory "/var/lib/postgresql/data".

Replicated vs Global Services

When deploying a service in a Docker Swarm, there are two options to choose from; replicated or global services. A replicated service runs multiple instances of the service on different nodes in the swarm ensuring high availability and automatic failover. Global services, however, run a single instance of the service on every node in the swarm, allowing for easy accessibility of the service from any node.