Containerisation

What it means

Containerisation means packaging an application together with everything it needs to run, its libraries, settings and dependencies, into an isolated container. The container behaves the same on a developer's laptop, a test server and in production in the cloud, which removes the familiar it works on my machine problem.

Unlike a virtual machine, a container does not include a whole operating system but shares the host's kernel. It is therefore lighter, starts in seconds, and many more can run on one server. The best-known tool is Docker, which popularised the container image format.

Containerisation often goes hand in hand with a microservices architecture, where an application is split into small independent parts. Each runs in its own container and can be deployed and scaled independently, and orchestrators such as Kubernetes manage large numbers of containers.

How it works in practice

For a development team, containers simplify deployment. The application image is built automatically in the CI/CD pipeline, tested, and the very same image is deployed to production. Differences between environments disappear and rolling back to the previous version takes minutes.

For a company that commissions software, containerisation helps when changing supplier or hosting. An application packaged in standard containers is easier to move to another cloud provider than a system installed by hand on a particular server.

Containers fit well with a DevOps culture, where development and operations work together. Infrastructure is described in files that are versioned alongside the code, so it is clear who changed what, and the environment can be recreated at any time.

What to watch out for

Containers are not secure by default. Images often contain outdated libraries with known vulnerabilities, and credentials must not be baked into the image. Regular image scanning, updates and secrets management outside the code are needed.

For a small application, containerisation with orchestration can be needlessly complex and expensive to maintain. A simple website or internal tool is often better served by a managed platform or a serverless solution, where the provider looks after servers and containers.