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Docker captures stderr; you can configure Postgres to simply use stderr logging, and then capture it in the host system. Another option is syslog, which Postgr

Docker captures stderr; you can configure Postgres to simply use stderr logging, and then capture it in the host system. Another option is syslog, which Postgres also supports, with which you would log to the host's syslog daemon. (You can mount the host's /dev/log if you don't want to deal with setting up the networking.)

sandGorgon 12y ago
Capturing it in the host system would imply logrotate on the host - which I am not sure about because logrotate needs to issue sighup to log-creator. Currently, I am bind mounting /dev/log which means syslog is logging to the host system - but again, the same issues.
- meatmanek 12y ago
  If you're sending logs out to the host's syslog, then the log creator is your syslog daemon. Also, processes in containers exist as processes on the host (they have different PIDs inside and outside of the container due to PID namespacing); so logrotate should be able to send signals to containerized processes.
  - sandGorgon 12y ago
    hey thanks for that comment - could you talk about it in a slight more detail. I have rsyslog on the host and I'm sending logs to host syslog through bind mounting of /dev/log . Is this what you meant by sending out logs to host ? I'm having a lot of trouble figuring out how to do it any other way. Could you also confirm that rsyslog on host will be able to send appropriate signals to the container's syslog ... which forwards it to the containerized process ? I'm unable to find documentation in rsyslog (or syslog-ng) that talks about this behavior so I'm not sure.
    - noselasd 12y ago
      If you bind mount /dev/log of the host into your container, you only run rsyslogd on the host. There's no need to send any SIGHUP to anything in the container.
    - lobster_johnson 12y ago
      No signaling needed other than rsyslog itself. Sighup is needed when processes (such as Postgres) write directly to log files. These files are open, and when you want to rotate the file, you have to tell the process to close the files so that it will start writing to a new file. If you tell Postgres to log to rsyslog (or any other syslog daemon), the log data will be sent to rsyslog via UDP, TCP or a Unix socket. Postgres itself will not have any files open, so there is no need to sighup it. You will have to sighup rsyslog in the host system, though.

sandGorgon 12y ago

Capturing it in the host system would imply logrotate on the host - which I am not sure about because logrotate needs to issue sighup to log-creator. Currently, I am bind mounting /dev/log which means syslog is logging to the host system - but again, the same issues.

meatmanek 12y ago
If you're sending logs out to the host's syslog, then the log creator is your syslog daemon. Also, processes in containers exist as processes on the host (they have different PIDs inside and outside of the container due to PID namespacing); so logrotate should be able to send signals to containerized processes.
- sandGorgon 12y ago
  hey thanks for that comment - could you talk about it in a slight more detail. I have rsyslog on the host and I'm sending logs to host syslog through bind mounting of /dev/log . Is this what you meant by sending out logs to host ? I'm having a lot of trouble figuring out how to do it any other way. Could you also confirm that rsyslog on host will be able to send appropriate signals to the container's syslog ... which forwards it to the containerized process ? I'm unable to find documentation in rsyslog (or syslog-ng) that talks about this behavior so I'm not sure.
  - noselasd 12y ago
    If you bind mount /dev/log of the host into your container, you only run rsyslogd on the host. There's no need to send any SIGHUP to anything in the container.
  - lobster_johnson 12y ago
    No signaling needed other than rsyslog itself. Sighup is needed when processes (such as Postgres) write directly to log files. These files are open, and when you want to rotate the file, you have to tell the process to close the files so that it will start writing to a new file. If you tell Postgres to log to rsyslog (or any other syslog daemon), the log data will be sent to rsyslog via UDP, TCP or a Unix socket. Postgres itself will not have any files open, so there is no need to sighup it. You will have to sighup rsyslog in the host system, though.