Docker, Java, Signals and Pid 1

Running a Java application in a container seems to be very easy. The devil is in the details and I want to shed some light on the PID 1 problem when you run Java applications in containers. Theoretical, processes in containers should not have state so you just don’t care, but reality is different.

Signals are used to message running process to behave in certain ways. A common case is to terminate a process nicely sending a SIGTERM to the process identified by the process id using the tool ps. People can implement logic shutting down an application gracefully. When you issue the command kill <pid> you send a SIGTERM to a process. We have other signals you can use, especially when application provide terminals. Hitting CTRL + C sends SIGINT to the terminal process. Some signals are quite out of time, the signal to hangup SIGHUP comes from terminal applications. It is often used nowadays telling a running process to re-read configurations.