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Introduction to Docker Swarm in Container Orchestration

August 17, 2021

Docker Swarm refers to a container orchestration tool that allows users to manage multiple containers. It is used by developers to create, deploy, and manage a cluster of Docker nodes. This tool is resourceful because it allows easy deployment.

This article provides the basics of Docker Swarm and explains its key features. It also highlights some of its uses and explains how it works.

An overview of Docker swarm

Before understanding what Docker Swarm is, it is important to learn the terms Docker and Swarm. A Swarm is a term used to describe many Docker hosts or systems. Docker container is a software that consists of libraries, codes, and frameworks for running applications.

Docker Swarm is a tool used for managing container orchestration. It runs on Docker applications and enables users to create a group of Docker nodes and deploy them.

It can also be termed as a Docker tool used for scheduling and network clustering. Docker Swarm enables you to manage containers when their size increases. It uses an overlay network to discover services and a built-in load balancer to scale the services up or down.

In Docker Swarm, the network and volume of docker services can be configured automatically without necessarily configuring them manually. Docker updates the configurations and stops the configuration of service tasks that are out of date. It also creates new service tasks to match the required configuration.

Features of Docker Swarm

The following are the main features of Docker Swarm:

  • Security: The nodes in Docker Swarm allow the enforcement of encryption and mutual authentication to enhance high security in communications between nodes.
  • Scaling: Docker Swarm consists of automatic addition or removal of tasks that allow users to scale up or down as per their needs.
  • Decentralized design: It consists of a decentralized design that allows you to create a swarm from one disk image.
  • Integration: The cluster management of Docker Swarm has been integrated with Docker Engine. This allows users to manage swarms without requiring another orchestrations software.
  • Rolling updates: During rollout, service updates on nodes can be made incrementally. In case of a problem, you can roll back to a previous safe service.
  • Declarative service: Docker Swarm consists of a declarative service model that allows users to define the required state of services in the swarm environment.
  • Service discovery: The manager nodes assign a DNS to each service. This means you can use an embedded DNS server to query a container that runs within the swarm.

How developers can use Docker Swarm

Docker Swarms are important to developers, especially in the Docker ecosystem. They enable them to create and deploy Docker node clusters.

Developers can also use Docker Swarm to perform the following tasks:

  • To scale up or down the size of containers.
  • To enhance the coordination of two or more containers.
  • To allocate tasks to various container clusters.
  • To roll out service updates on nodes.
  • To manage multiple container clusters, including their lifecycle, and functionality.
  • To reschedule containers in case of node failure.

How Docker Swarm works

Docker Swarm uses services, tasks, and nodes to manage containers. Let’s gain an overview of these aspects before diving into how they are used in Docker Swarm.

A service is a list of tasks that need to be executed on nodes. Services allow app images to be deployed to the Docker Swarm. They enable users to launch containers. Examples of services include HTTP Servers, DB Servers, and Web Servers.

A task refers to work that needs to be done. A Swarm node is a singular Docker Engine or an instance of it. It consists of two types of nodes: worker nodes and manager nodes.

The main role of a manager node is to allocate tasks to the worker nodes. They also perform managerial tasks required for the effective operation of a swarm. A worker node plays the role of executing the allocated tasks. It requires at least one manager node to function well.

One or more nodes should be deployed before a service can be deployed. A service is created by setting up commands. The manager node consists of an API that connects to the swarm environment. The API enhances communication between the worker node and the manager node through the HTTP protocol.

The manager node enhances container orchestration through the creation of tasks for every service. These tasks are allocated and dispatched to a worker node. The worker node checks for tasks, receives tasks and executes them. The status of the tasks is reported to the manager node using the API. If a task fails, the Docker Swarm is not restarted. Instead, the orchestration replaces it with a new task that matches the state of the service.

Docker Swarm service modes

When you create a service, you may have to specify the container image that will be used. A container image is a package that consists of executable files. It consists of the libraries, code, binaries, configuration files, and runtime required for running applications.

There are two main service modes in Docker Swarm: global service and replicated service.

  • Global service: This is a service that runs on every node. Global services monitor containers that run on swarm nodes.
  • Replicated service: In this service mode, the tasks are distributed by the manager node to the worker nodes. The worker nodes then execute them.

Advantages and disadvantages of Docker Swarm

Advantages

  • It allows easier deployment of containers.
  • It is easy to install.
  • It is easy to learn. Using Docker Swarm doesn’t require much experience.
  • It can be integrated with Docker compose and Docker CLI.

Disadvantages

  • Manual intervention may be required when scaling services.
  • It has a relatively smaller community than other orchestration tools. A good example is Kubernetes.
  • The fault tolerance in Docker Swarm is limited.

Conclusion

This article provided an overview of Docker Swarm in container orchestration.

To summarize:

  1. We gained a better understanding of Docker Swarm.
  2. We have looked at the features of Docker Swarm.
  3. We have learned how developers can use Docker Swarm when managing containers.
  4. We have gained an overview of services, tasks, and nodes.
  5. We learned how Docker Swarm uses services and nodes to achieve container orchestration.
  6. We gained an overview of the two types of service modes in Docker Swarm.
  7. We have learned the advantages and disadvantages of Docker Swarm.

Happy learning!


Peer Review Contributions by: Onesmus Mbaabu