A few considerations to note about logging.

  1. Logging Python multiprocessing threads is not easy.
  2. Rotating Python TimedRotatingFileHandler logs in not easy.
  3. logrotate on Python multiprocessing threads is not easy.
  4. Logging something that eats through as much data and does as many things as Skyline does is not easy.

Skyline logging can be viewed as having 2 contexts:

  1. Logging the Skyline application operations - Skyline app log
  2. Logging the details of anomalous time series - syslog

Any long time users of Skyline will have undoubtedly run into a logging pain with Skyline at some point. Whether that be in terms of logs being overwritten or no errors being logged but a Skyline module broken or hung. Although logging is very mature, especially in an ecosystem as mature as Python and one may be led to believe it should be easy, however in reality it is non-trivial.

A modified logging methodology

The logging implementation in Skyline is quite complicated due to the abovementioned reasons. It is important to note that some optimisations have had to be added to the logging process which could be described as unintuitive AND simply as un-log like.

Therefore it is important to explain why it is necessary to use a modified logging methodology and so that the user understands conceptually what is being logged and what is not being logged.

In terms of Analyzer if errors are encountered in any algorithms, we sample the errors. This achieves a balance between reporting errors usefully and not using lots of I/O and disk space if something goes wrong. Skyline has the potential to do a runaway log, but not reporting errors is not useful when you are trying to pinpoint what is wrong.

However, it is right that algorithms should just True or False and not impact on the performance or operation of Skyline in anyway.

This achieves a balance.

Skyline app logs

Each Skyline app has its own log. These logs are important from a Skyline perspective in terms of monitoring the Skyline processes. See [Monitoring Skyline]((monitoring-skyline.html))

The logs are also very important in terms of being able to verify all the steps and results in the analysis pipeline.

It is recommended to NOT stream the Skyline app logs through any logging pipeline e.g. rsyslog, logstash, elasticsearch, etc. The syslog alert trigger was added for this purpose. If you wish to parse and rate anomalies do so via syslog (see below). Or look at the skyline Graphite metrics or set some alert tuples on some of the skyline. Graphite namespace metrics.

Log rotation is handled by the Python TimedRotatingFileHandler and by default keeps the 5 last logs:

handler = logging.handlers.TimedRotatingFileHandler(
    settings.LOG_PATH + '/' + skyline_app + '.log',

The Skyline app logs and the rotation is relatively cheap on disk space.

` 141M    /var/log/skyline 248M    /var/log/skyline 139M    /var/log/skyline 330M    /var/log/skyline `

Skyline app log preservation

It should be noted that the bin/ bash scripts are used to ensure logs are preserved and not overwritten by any Python multiprocessing processes or the lack of mode=’a’ in the Python TimedRotatingFileHandler. It is for this reason that the Skyline app logs should not be streamed through a logging pipeline as logstash, et al as this in a logging pipeline with say rsyslog can result in the log being pushed multiple times due to the following scenario:

  • Skyline app bin is called to start
  • Skyline app bin makes a last log from the Skyline app log
  • Skyline Python app is started, creates log with mode=w
  • Skyline Python app pauses and the app bin script concatenates the last log and new log file
  • Skyline bin script exits and Skyline Python app continues writing to the log

In terms of rsyslog pipelining this would result in the log being fully submitted again on every restart.

It is possible that a fragment of the source of that the original Skyline logging implementation was made from and it is possible that all this is due to a method change from the source, it tested OK, but has not been implemented as logging is working and needs to work. So it is horribly fixed, but works and testing something else new under all possible conditions is difficult. So it reamins this way for the forseeable future.


With this in mind a syslog alert trigger was added to Skyline apps to handle the logging the details of anomalous time series to syslog, groking the syslog for Skyline logs and rates etc is the way to go or just look as the skyline Graphite namespace metrics.

A special mention of the settings.py variable


This essentially now a hard requirement for Panorama and Luminosity now. If this special alerter is disabled a large chunk of Skyline’s new functionality is turned off.