Re: from multiple sources to multiple destinations using s6-log

From: Laurent Bercot <>
Date: Wed, 18 Feb 2015 11:16:30 +0100

On 18/02/2015 09:12, Gorka Lertxundi wrote:
> Imagine I have a single process which generates multiple kind of logs, for example, nginx, or even better mysql, which could generate three types of logs: general, error and slow-query. Piping all of them to the same destination in order to be eatable by s6-log, will converge typologies into one log processing chain.

  Hi Gorka,
  I'm not sure what your objective is:

  1. Being able to pipe several log sources into the same pipe to a
unique s6-log process, which would then select lines on a token given
in the input, and log into a separate logdir for each original source;


  2. Being able to have several supervised s6-log processes all reading
from the same service, one per log flux.

  I don't recommend going after 1. Generally, if you have several log
sources, you don't want to merge them into a single destination - that's
the syslogd design and is inefficient. The sources are different for a
reason, and it's always easier to concatenate several sets of logs for
analysis than it is to parse information in a single set to retrieve the
origin of a log line. But from this diagram:

> general -> /dev/mypipe1 (prepend “g:”) --\ /-- if g s6-log /generalpath
> error -> /dev/mypipe2 (prepend “e:”) ---|— /dev/stdout -|--- if e s6-log /errorpath
> slow-quey -> /dev/mypipe3 (prepend “sq:”) --/ \-- if sq s6-log /slow-query

it looks like you're going after 2, and feel constrained by the need to
pipe all the logs into the service's stdout for them to be picked up
by a logger.

  So my answer to 2 is: don't feel constrained. The "one logger per
service" model isn't an absolute one, it's just a special treatment
of the common case by s6-svscan to make it easier; you can have as
many log fluxes as you need, there just won't be any specific help
from s6-svscan.

  - Create a supervised service (without its own logger!) that runs a
s6-log process reading from a fifo for every flux you have: i.e.
a "mysqld general-path log" service, a "mysqld error-path log" service,
and a "mysqld slow-query log" service.
  - In your mysqld's run script, redirect the outputs of your daemon
to the appropriate fifo.
  - There, you're done. But if you want the same guarantees of not
losing any logs when you restart a s6-log process as s6-svscan gives
you with a service's "native" logger, you should perform two more
    * Make sure you have a s6-fdholder-daemon service running.
    * At some point in your initialization scripts, open each of your
fifos for reading and store the descriptor into the fd-holding daemon.

  Now your fifos will never close - unless you kill the fdholder without
transferring its contents first, so don't do that before shutdown time.

  Honestly, now that s6 can perform fd-holding, the specific logger
handling performed by s6-svscan becomes largely irrelevant. I may even
deprecate it in a very distant future. (Very distant! Please don't
freak out.)


Received on Wed Feb 18 2015 - 10:16:30 UTC

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