I would like to use journald within a container without systemd. Is this possible? If so, how to do this? What are the issues to take care?
Journald is an inseparable component of Systemd bundle. From systemd for Administrators, XVII:
If you are wondering what the journal is, here's an explanation in a few words to get you up to speed: the journal is a component of systemd, that captures Syslog messages, Kernel log messages, initial RAM disk and early boot messages as well as messages written to STDOUT/STDERR of all services, indexes them and makes this available to the user. It can be used in parallel, or in place of a traditional syslog daemon, such as rsyslog or syslog-ng.
How systemd handles containers is described in The New Control Group Interfaces, e.g.
systemd through its "unit" concept already implements a dependency network between objects where propagation can take place and contains a powerful execution queue. Also, a major part of the objects resources need to be controlled for are already systemd objects, most prominently the services systemd manages.
Why is this not managed by a component independent of systemd?
Well, as mentioned above, a dependency network between objects, usable for propagation, combined with a powerful execution engine is basically what systemd is. Since cgroups management requires precisely this it is an obvious choice to simply implement this in systemd itself.
Implementing a similar propagation/dependency network with execution scheduler outside of systemd in an independent "cgroup" daemon would basically mean reimplementing systemd a second time. Also, accessing such an external service from PID 1 for managing other services would result in cyclic dependencies between PID 1 which would need this functionality to manage the cgroup service which would only be available however after that service finished starting up. Such cyclic dependencies can certainly be worked around, but make such a design complex.
This kind of structure has been criticized of being against the UNIX philosophy:
"Systemd flies in the face of the Unix philosophy: 'do one thing and do it well,' representing a complex collection of dozens of tightly coupled binaries1. Its responsibilities grossly exceed that of an init system, as it goes on to handle power management, device management, mount points, cron, disk encryption, socket API/inetd, syslog, network configuration, login/session management, readahead, GPT partition discovery, container registration, hostname/locale/time management, and other things. Keep it simple, stupid.”