Containers on their own are not a new technology and have in fact been around for many years. What truly sets Docker apart is the tooling and ease of use they have brought to the community. Modern development practices promote the use of Continuous Integration and Continuous Deployment. These techniques, when done right, can have a profound impact on your software product quality.
ThoughtWorks defines Continuous Integration as a development practice that requires developers to integrate code into a shared repository several times a day. By having a continuous process of building and deploying code, organizations are able to instill quality control and testing as part of the everyday work cycle. The result is that updates and bug fixes happen much faster and the overall quality improves.
However, there has always been a challenge in creating development environments that match that of testing and production. Often inconsistencies in these environments make it difficult to gain the full advantage of continuous delivery.
Using Docker, developers are now able to have truly portable deployments. Containers that are deployed on a developer's laptop are easily deployed on an in-house staging server. They are then easily transferred to the production server running in the cloud. This is because Docker builds containers up with build files that specify parent layers. One advantage of this is that it becomes very easy to ensure OS, package, and application versions are the same across development, staging, and production environments.
Because all the dependencies are packaged into the layer, the same host server can have multiple containers running a variety of OS or package versions. Further, we can have various languages and frameworks on the same host server without the typical dependency clashes we would get in a virtual machine (VM) with a single operating system.
The well-defined isolation and layer filesystem also make containers ideal for running systems with a very small footprint and domain-specific purposes. A streamlined deployment and release process means we can deploy quickly and often. As such, many companies have reduced their deployment time from weeks or months to days and hours in some cases. This development life cycle lends itself extremely well to small, targeted teams working on small chunks of a larger application.
