Structured logging (was Early review of std.logger)

[Robert Schadek]

On 10/15/2013 05:20 PM, Johannes Pfau wrote:
> I think one increasingly important point for std.log is 'structured
> logging'.
>
> Structured logging is basically not simply logging textual messages, but
> also logging additional KEY/VALUE pairs of data. The idea is that logs
> should not only be readable by humans but also easy to parse and
> analyze. Structured logging often also includes UUIDs to simplify
> finding similar errors/problems in a log file.
>
> For example: Instead of logging
> logf("Couldn't login as %s on server %s : Reason: %s", user, server,
>       errorCode);
>
> we'd do:
> log(LOGIN_FAILED_UUID, "Couldn't log in", ["User": user, "Server":
>     server, "ErrorCode": errorCode]); 
>
> The log can then be queried for all events with 'LOGIN_FAILED_UUID'
> uuid, Server="..." ErrorCode=42, etc.
>
> As a nice benefit structured logging can also log the function, module
> and line of the source file that logged the message, Exceptions can be
> written to the log in a nice way, etc.
>
> SystemDs journal [1] is a structured replacement for syslog and is
> already being used on some linux distributions (ArchLinux, Fedora).
> It's likely that the journal will replace syslog on linux so at least
> for server-side software structured logging support is becoming
> important. The GNOME guys are also working on a log viewer for
> systemd's journal [2]. 
>
> std.log certainly doesn't need to provide a backend for the journal
> right now. But some questions regarding structured logging need to
> taken into account:
>
> * Will structured logging be integrated with the normal logging at all?
> * Will the public API somehow collide with the proposed std.log API?
> * Should the API for log backends already support structured logging or
>   is it possible to add this later on?
>
> Most important:
> * What to do about structured logging calls when logging to a simple
>   text logger? Ignore the additional fields or serialize them? What
>   about the UUID?
>
> Even if we don't have a backend implementation it'd be nice to have the
> public API available. Otherwise new projects cannot use structured
> logging with std.log right now and the user code needs to be
> rewritten once std.log does support structured logging.
>
> [1]
> http://0pointer.de/blog/projects/journalctl.html
>
> [2]
> https://mail.gnome.org/archives/gnome-announce-list/2013-September/msg00097.html
Currently there is no structured logging implemented in std.logger.

That been said, you can add it. The method Logger.logf is a variadic
template. You can simple create your own Logger Class and overwrite that
method and implemented your structured logging approach there. The only
pitfall is that you need a ref of type WhateverYourLoggerClassIsCalled
and call logf on that or you fall back to the default logf
implementation provided by the Logger class. Another way would be to
parse the message passed to logMessage and go from there.

Long story short. Global default Logger means structured logging
requires parsing. All other logger require correct reference type of logger.