The Basics of Docker Swarm

The software development process has come a long way from the traditional waterfall model to the latest Agile and DevOps methodologies. DevOps is already being adopted to streamline the software development process, as most organizations need to reduce the time to market for any software that they build.

Coming across Docker is inevitable if you are exploring DevOps and the tools used for this approach. This article introduces you to Docker and provides an in-depth understanding of Docker Swarm. 

Here are the topics covered in this Docker Swarm article:

  • What is Docker and Docker container?
  • What is Docker Swarm?
  • Features of Docker Swarm
  • How does a Docker Swarm work?
  • Demo: how to create a service in Docker Swarm
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What is Docker and Docker Container?

Docker is a tool used to automate the deployment of an application as a lightweight container so that the application can work efficiently in different environments. Docker container is a lightweight software package that consists of the dependencies (code, frameworks, libraries, etc.) required to run an application. 

We can use Docker Swarm to make Docker work across multiple nodes, allowing them to share containers with each other. It's an environment where you can have various Docker images running on the same host operating system. 

What is Docker Swarm?

A Docker Swarm is a service that allows you to create and manage a cluster of Docker nodes and schedule containers. Each node of a Docker Swarm is a Docker daemon, and all Docker daemons interact using the Docker API. Each container within the Swarm can be deployed and accessed by nodes of the same cluster. 

docker-os

There are five critical elements within a doctor environment:

  1. Docker container 
  2. Docker daemon
  3. Docker images 
  4. Docker client 
  5. Docker registry 

Consider an environment having Docker containers as shown:

docker-containers

If one of the containers fails, we can use the Swarm to correct that failure.

docker-swarm

Docker Swarm can reschedule containers on node failures. Swarm node has a backup folder which we can use to restore the data onto a new Swarm. 

docker-daemon

Features of Docker Swarm

Some of the most essential features of Docker Swarm are:

  • Decentralized access: Swarm makes it very easy for teams to access and manage the environment 
  • High security: Any communication between the manager and client nodes within the Swarm is highly secure 
  • Autoload balancing: There is autoload balancing within your environment, and you can script that into how you write out and structure the Swarm environment 
  • High scalability: Load balancing converts the Swarm environment into a highly scalable infrastructure
  • Roll-back a task: Swarm allows you to roll back environments to previous safe environments

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How Does Docker Swarm Work?

In Swarm, containers are launched using services. A service is a group of containers of the same image that enables the scaling of applications. Before you can deploy a service in Docker Swarm, you must have at least one node deployed. 

There are two types of nodes in Docker Swarm:

  1. Manager node. Maintains cluster management tasks 
  2. Worker node. Receives and executes tasks from the manager node

Consider a situation where a manager node sends out commands to different worker nodes. 

manager-node.

The manager node knows the status of the worker nodes in a cluster, and the worker nodes accept tasks sent from the manager node. Every worker node has an agent that reports on the state of the node's tasks to the manager. This way, the manager node can maintain the desired state of the cluster. 

The worker nodes communicate with the manager node using API over HTTP. In Docker Swarm, services can be deployed and accessed by any node of the same cluster. While creating a service, you'll have to specify which container image you're going to use. You can set up commands and services to be either global or replicated: a global service will run on every Swarm node, and on a replicated service, the manager node distributes tasks to worker nodes. 

Now a question may arise: don't task and service refer to the same thing? The answer is no. 

A service is a description of a task or the state, whereas the actual task is the work that needs to be done. Docker enables a user to create services that can start tasks. When you assign a task to a node, it can't be assigned to another node. It is possible to have multiple manager nodes within a Docker Swarm environment, but there will be only one primary manager node that gets elected by other manager nodes. 

Therefore, the working of the Swarm can be summarized as follows:

managernode-workernode      

A service is created based on the command-line interface. The API that we connect in our Swarm environment allows us to do orchestration by creating tasks for each service. The task allocation will enable us to allocate work to tasks via their IP address. The dispatcher and scheduler assign and instruct worker nodes to run a task. The worker node connects to the manager node and checks for new tasks. The final stage is to execute the tasks that have been assigned from the manager node to the worker node.

Want to test your knowledge on the various concepts of Docker? Try answering these Docker Certified Associate Exam Dumps and find out now.

Demo: How to Create a Service in Docker Swarm

Watch this video to understand how to create a service in Docker Swarm.

How can Simplilearn Enhance Your Knowledge of Docker?

Broad understanding of container concepts like Docker is one of the most critical skills that a DevOps engineer should have. You can add this credential to your skill set by enrolling in Simplilearn's Docker Certified Associate (DCA) Certification Training Course. Get hands-on experience with Docker Compose and Docker hub, create flexible, customizable environments and networks, and much more with this comprehensive training course using Simpilearn's unique Blended Learning approach.   

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