Introducing libeconf - Bringing systemd-like configuration layering to everything else

systemD has a very distribution-friendly way of providing it's configuration, with distributions providing defaults in /usr and users being able to override things either selectively or entirely with their own files in /etc. This is especially nice for distributions wishing to be in some way stateless, support a read-only root filesystem, or provide some kind of factory-reset. libeconf is a newly written C library to ease the adoption of similar configuration layering in other programs across the Linux ecosystem.

This talk will give a brief introduction to libeconf, how to use it in your existing programs and demonstrate some examples that have already adopted libeconf (eg. PAM, util-linux, rebootmgr, etc).

The session will also share some future plans and welcome suggestions for future contributions, especially for additional features, language bindings, etc.

The target audience is primarily developers of 'low level' distribution plumbing (eg. core daemons & services, package managers, etc) that are most likely to benefit from libeconf, but might be of interest to anyone developing any service for linux distributions.

Transcript

1. Introduction to libeconf
   Bringing Peace to the Configuration War
   Linux Distribution Engineer
   Richard Brown
   rbrown@opensuse.org
   sysrich on Freenode.net
   @sysrich
   

   View Slide

2. Who’s responsible for configuration?
   

   View Slide

3. Who’s responsible for configuration?
   

   View Slide

4. Distributions
   ●
   Distributions are responsible for providing
   a good out of the box experience
   ●
   This includes distributing “sensible”
   defaults
   ●
   New upstream developments often
   require new settings
   

   View Slide

5. Users
   ●
   Users need to be able to configure their
   system as they see fit
   ●
   This may include “unsensible” choices
   ●
   They might not know of new
   features/changes needing new settings
   

   View Slide

6. Problems
   ●
   Distribution config installed in
   /etc/$foo.conf
   ●
   User edits /etc/$foo.conf
   ●
   New $foo package wishes to change
   defaults/introduce new parameters
   ●
   …….
   

   View Slide

7. View Slide

8. Imperfect Solutions
   ●
   Package PostScripts editing existing /etc config
   – Users: “The Distro broke my config!”
   ●
   Backing up user config and installing new in /etc
   (eg .rpmsave)
   – Users: “The Distro broke my config!”
   ●
   Avoiding user config in /etc and storing new
   alongside (eg .rpmnew)
   – Distros/Upstreams: “Sorry your setup is not supported”
   

   View Slide

9. Atomic Distros (eg. openSUSE MicroOS)
   ●
   Growing availability of
   Atomic/Transactional Distributions
   ●
   Many allow users changing /etc for
   runtime configuration changes
   ●
   Need some way of atomically delivering
   new configuration with their atomically
   installed packages
   

   View Slide

10. systemD
    ●
    Nice structured ‘Unit’ files
    ●
    Dedicated location for distro config
    – /usr/lib/systemd/system
    ●
    Dedicated location for user config
    – /etc/systemd/system
    

    View Slide

11. systemD drop-ins
    ●
    Selected parameters can be overridden by
    ‘drop-in’ files
    – /usr/lib/systemd/system/$foo.service.d
    – /etc/systemd/system/$foo.service.d
    ●
    Perfect for distros with multiple
    ‘consumers’ of a service & and for users
    wanting to make minimal changes
    

    View Slide

12. libeconf
    ●
    Flexible & configurable C library for
    implementing systemd-like configuration
    layering
    ●
    Supports both “/usr vs /etc” configs and
    “/usr/*/$foo*.d/ vs /etc/*/$foo*.d/” drop-
    ins
    

    View Slide

13. Current Limitations
    ●
    Not magic; software must add libeconf support
    ●
    Only suitable for “.ini style” config values with
    key/value/group structure
    [Group]
    Key=Value
    Key2=Value2
    

    View Slide

14. API Call Examples
    ●
    econf_readFile – read config file and stores
    it’s contents into a key_file
    ●
    econf_readDirs – read config files in directory
    ●
    econf_getGroups – get [Groups] from key_file
    ●
    econf_getKeys – get Keys from Group
    ●
    econf_get{$FOO}Value – get Value (of type
    {$FOO}) from Key
    

    View Slide

15. API Call Examples
    ●
    econf_set{$FOO}Value – set Value of type
    {$FOO} in a Key
    ●
    econf_mergeFiles – merge contents of two
    key_file
    ●
    econf_writeFile – write config file from
    key_file (not yet stable)
    

    View Slide

16. libeconf & openSUSE - /usr/etc
    ●
    /etc is considered ‘user data’ and treated
    with the same reverence & avoidance as
    /home, /srv and similar.
    ●
    libeconf managed config files moving to
    /usr/etc
    ●
    These locations are configurable by
    bother distros and software developers
    

    View Slide

17. Examples
    ●
    pam
    – Distro config in /usr/etc/pam.d
    – User config in /etc/pam.d
    – Both parsed with libeconf at runtime
    – /etc values always win
    

    View Slide

18. Examples
    ●
    shadow
    – Distro config in /usr/etc/pam.d &
    /usr/login.defs
    – User config in /etc/pam.d & /etc/login.defs
    – Both parsed with libeconf at runtime
    – /etc values always win
    

    View Slide

19. Examples
    ●
    util-linux (su, login, remote, runuser)
    – Distro config in /usr/etc/pam.d
    – User config in /etc/pam.d
    – Both parsed with libeconf at runtime
    – /etc values always win
    

    View Slide

20. Examples
    ●
    Rebootmgr
    – Distro config in /usr/etc/rebootmgr.conf
    – User config in /etc/rebootmgr.conf
    – Both merged libeconf at runtime, written to
    /etc/rebootmgr.conf
    

    View Slide

21. Plans
    ●
    Augeas
    – Highly reliable parser with support for more
    complex merging, diverse filetypes & layouts
    – Not Certain: Larger dependencies, more code
    complexity
    ●
    econftool
    – Generic ‘helper’ tool for understanding and
    editing layered configuration
    

    View Slide

22. Other Ideas
    ●
    Different file parser
    ●
    Language Bindings
    – Python?
    – Go?
    – Rust?
    

    View Slide

23. How to Contribute
    ●
    https://github.com/openSUSE/libeconf
    ●
    Contributions Welcome
    ●
    Changes Welcome
    ●
    New Features Welcome
    

    View Slide

24. View Slide