Juju Leader Election and Application-Specific Leadership

[Dmitrii Shcherbakov]

Hi everybody,

I have a question with regards to Juju leader unit election and
application-specific leadership.

https://jujucharms.com/docs/2.1/reference-charm-hooks#leader-elected
"leader-elected is run at least once to signify that Juju decided this
unit is the leader. Authors can use this hook to take action if their
protocols for leadership, consensus, raft, or quorum require one unit
to assert leadership. If the election process is done internally to
the service, other code should be used to signal the leader to Juju.
For more information read the charm leadership document."

This doc says
"If the election process is done internally to the service, other code
should be used to signal the leader to Juju.".

However, I don't see any hook tools to assert leadership to Juju from
a charm based upon application-specific leadership information
http://paste.ubuntu.com/24319908/

So, as far as I understand, there is no manual way to designate a
leader and the doc is wrong.

Does anyone know if it is supposed to be that way and if this has not
been implemented for a reason?

It might be tricky to implement as if you have a network partition you
may actually have multiple application-level leaders in separated
clusters. If Juju has access to both clusters (say, via a management
network) but applications themselves cannot reach each other, it may
get conflicting requests for leadership from that "other code that
should be used to signal the leader to Juju".

The "require one unit to assert leadership" part does make sense - if
an application supports a manual leader specification then you can
rely on Juju to pick a leader unit which will have code to set a
leader manually via application's own tools on leader-elected event.

In other words, I can see how it works for

juju leader unit configured -> application-specific leader configured

scenario, but not for

juju leader unit configured <- 'custom code' <- application-specific
leader configured

Doing a pickaxe search for a part of the above doc reveals that this
has originally been pushed into the juju docs repo in 2015.

➜  docs git:(master) git log -S '`leader-elected` run at least once to'

commit 2868eebe2abf2019a39b1dba56487ef22adc648d
...
Date:   Tue Aug 30 08:13:56 2016 -0500

    added missing hooks

commit 7df18e4f11350077c88e40d560a6f34911cd34f4
...
Date:   Thu Nov 12 16:16:48 2015 -0600

    Adding the reference for charm hooks and links to it.

commit 7d94261f4ae3541bb1af93707414102c81515b43
...
Date:   Thu Nov 12 16:16:48 2015 -0600

    Adding the reference for charm hooks and links to it.

----

Does anyone know the current official stance on this?

Thanks in advance!

Best Regards,
Dmitrii Shcherbakov

Field Software Engineer
IRC (freenode): Dmitrii-Sh