[systemd-devel] journald API backward compatibility

[Lennart Poettering]

On Fri, 18.11.16 10:26, Jakub Filak (jfilak at redhat.com) wrote:

> Hello,
> 
> I have a question regarding journald and backward compatibility. Are there
> any known issues with accessing journals that were created by different
> versions of journald?
> 
> I want to use the latest libsystemd in my project but I might end up reading
> journals created by an ancient systemd (e.g. systemd-200).

The general structure has been stable, and current code should be fine
with reading any kind of journal file. There is one exception however:
some builds enabled experimental LZ4 compression support very early
(but not on Fedora/RHEL), at a time where the LZ4 bit format wasn't
stable. Journal files created with that cannot be read with the
current codebase, because the LZ4 bitsream changed. We clearly
indicated that the LZ4 back then was experimental, but I doubt that is
much consolation. We declared LZ4 support stable (and enabled it in
Fedora/RHEL) as soon as the LZ4 upstream folks decided to stick with
it, too.

Hence: yes, the journal file format has been stable since basically
its inception, but the LZ4 bitstream we made use of through the LZ4
libraries has not been. 

Lennart

-- 
Lennart Poettering, Red Hat