More on logging changes in Juju 1.26.0

[Menno Smits]

In a recent thread I highlighted the upcoming change in Juju's logging
where logs from machine and unit agents will be streamed over Juju's API
instead of via rsyslogd. Two things came out of that discussion:

1. a desire for juju debug-logs to support a "non-tailing" mode (as per bug
1390585 <https://bugs.launchpad.net/juju-core/+bug/1390585>)
2. a desire for a tool to dump the logs out of the database in the case of
Juju being unavailable for some reason.

I'm happy to report that #1 is being worked by right now as part of wider
changes to remove the "jes" and "db-log" feature flags, and #2 is done as
of today. Under 1.26, machine agents will install a /usr/bin/juju-dumplogs
command which can be run on any state server to generate complete dumps of
all stored logs (one file per environment).

There are some things to be aware of for when existing environments are
upgraded to 1.26.0 or later:

   - The existing all-machines.log will stop receiving updates very soon
   after the state server upgrades to the new Juju version.
   - all-machines.log will be left behind, but will be compressed.
   - Some text will be appended to the bottom of all-machines.log
   explaining that the file is no longer used with some pointers to relevant
   documentation.
   - There will be a small window where the state servers have upgraded to
   the new Juju version but some agents haven't. Those agents will continue
   trying to log to rsyslogd but it will no longer be running. Logs sent
   during that period will not be recorded in all-machines.log or in the new
   logs database. They will still appear in the machine or unit's local log
   file however.
   - We do not plan to pre-seed the logs database with existing logs from
   all-machines.log.

As always, questions and comments welcome.

- Menno
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