Of the containers described so far, operating system containers are fundamentally different from the current container trend led by Docker, Kubernetes, and LXD. Instead of focusing on recreating an entire operating system with services such as syslog and cron, application containers focus on running a single process within a container – just the application.
Proprietary solutions replace all the usual OS-level services. These solutions provide a unified way to manage the applications within a container. For example, instead of using syslog to handle logs, the standard output of the process with PID 1 is considered as application logs. Instead of using a mechanism such as init.d or systemd, the application container's lifecycle is handled by the runtime application.
Since Docker is at the moment the dominant solution for application containers, we will mostly use it as an example throughout this book. To make the picture complete, we will present...
