Improving Logging to Aid in Debugging

Operators have been asking for mnemonic identifiers for log messages since the start of the Folsom cycle in 2012. This proposal tries to provide the features of those identifiers, without having to update every log message and string being passed through the logging modules.

Problem description

Many commercial (closed-source) enterprise applications, and even some open source projects, include unique identifiers for every user and log message. The IDs are used as reference points in documentation and bug reports, either instead of or in addition to the text of the log message. Message IDs also mean that online searches can be language independent, to some degree, although translating the results of a search may still require someone to be able to read English if there are no results in their primary language.

As an open source project with a widely distributed contributor base and multi-service architecture, adding message IDs to OpenStack presents some unique challenges not faced in other environments. A central repository of unique IDs will be difficult to maintain. Adding IDs to every log message in anticipation of documentation to be written later will produce a lot of patches with little value, triggering reviews, test jobs, and rebases of other patches. Requiring IDs for each log call, or for only some log levels, will require incompatible changes to our logging library APIs. This proposal tries to address all of those issues in a way that will allow us to deliver the benefit of those message IDs without the expense of introducing unique IDs to all logging calls.

The benefits of unique IDs for log messages have been given as

  1. A unique ID for an error can act as a mnemonic, when discussing the error with another operator or when a user reports an error through support channels.

  2. A unique ID for a message helps locate that message in the source for the program, making it easier to troubleshoot and debug by being able to jump directly to the source code.

Proposed change

Although unique message IDs are useful, maintaining the list would grow tedious. The new rules reviewers would need to learn in order to manage a segmented ID value correctly would lead to frustration and nit picking on reviews. Adding the IDs in the first place would take manual work, and keeping them correctly organized over time would increase our maintenance burden. The two primary features leading to the request for message IDs are both available in other forms with minimal source code changes, so this proposal describes how to achieve the goals in the easiest way possible.

Message Locations

The location of a message (filename and line number) can be exposed today in any version of OpenStack by adding the relevant fields to the logging format configuration option. No source code changes are needed at all. There are 3 different ways to expose the location, depending on how much detail is desired.

  1. The form in use today is the %(name)s field, which introduces the “logger name” into the log line. The standard practice in OpenStack is to use the Python module name, accessible via the __name__ variable, to get a logger object for each file. Therefore in most cases, the logger name and module name will match. For example, in nova/api/ the line:

       LOG = logging.getLogger(__name__)
    causes the logger to be called ``nova.api.auth``.
  2. The %(filename)s field inserts the last part of the full file name into the log line. For example, nova/api/ produces OpenStack contains quite a few modules with the same base file name, so this version is not likely to be very useful for debugging.

  3. The %(pathname)s field inserts the full name of the file into the log line. For example nova/api/ produces something like /usr/local/lib/python2.7/site-packages/nova/api/ (the actual value depends on how the software is packaged and installed). These paths can be quite long, but for a site where that level of detail is desired it is available.

The file name is only one part of the necessary information for finding a log message. The other is the line number. The %(lineno)s field will insert the line number of the location of the logging call that introduces a line to the log. For example, the line:

LOG.debug("Neither X_USER_ID nor X_USER found in request")

appears on line 102 of nova/api/, so %(lineno)s would insert 102 into the log line.

As stated above, the %(name)s field already appears in the default format string set by oslo.log. The %(lineno)s field can be added there easily. This information can be added to the oslo.log documentation to make it easier for users to find. Although this may lead to operators running with non-default configurations, it is less likely to break the logging pipelines in existing deployments. Over time, as we see how useful the line numbers are, we can revisit this approach.

Error Identifiers

Most errors in Python programs are associated with an instance of a class derived from Exception. These classes have message text, frequently with the ability to insert dynamic values (such as the name of a specific thing that has a problem). Python’s logging library recognizes exception instances, and when a log message is emitted while there is an active exception the exception can be included in the output, producing a traceback.

Exceptions are logged by calling the exception() method of the logger, or by passing exception information to another method such as error() or warning(). In all cases, the logging formatter is responsible for taking the arguments given and producing the appropriate log output line.

Given the example logging call:

LOG.exception('the exception log msg')

Today when an exception is logged, the output produced looks like:

2017-04-24 15:15:54.823 1108 ERROR oslo.tester:24 [-] the exception log msg
2017-04-24 15:15:54.823 1108 ERROR oslo.tester Traceback (most recent call last):
2017-04-24 15:15:54.823 1108 ERROR oslo.tester   File "./", line 21, in <module>
2017-04-24 15:15:54.823 1108 ERROR oslo.tester     raise RuntimeError('the error text')
2017-04-24 15:15:54.823 1108 ERROR oslo.tester RuntimeError: the error text
2017-04-24 15:15:54.823 1108 ERROR oslo.tester

Note that the exception name (RuntimeError) and error text appear several lines removed from the first error line, which contains the string passed to the exception() method.

The log formatter used by oslo.log can be modified to insert the summary of the exception into the first log line, along with the local message. The results would look like:

2017-04-24 15:15:54.823 1108 ERROR oslo.tester:24 [-] the exception log msg: RuntimeError: the error text
2017-04-24 15:15:54.823 1108 ERROR oslo.tester Traceback (most recent call last):
2017-04-24 15:15:54.823 1108 ERROR oslo.tester   File "./", line 21, in <module>
2017-04-24 15:15:54.823 1108 ERROR oslo.tester     raise RuntimeError('the error text')
2017-04-24 15:15:54.823 1108 ERROR oslo.tester RuntimeError: the error text
2017-04-24 15:15:54.823 1108 ERROR oslo.tester

We can actually do this no matter the log level, so that if a log message is emitted in the context of handling an exception, that error is included in the output even if the level for the log message is INFO or DEBUG.

See for an example change. The other log formatters provided by oslo.log will also need to be updated.

This change works with any Python library, not just OpenStack modules, so errors produced by upstream dependencies are treated the same way.

We can simultaneously allow operators to add the error summary to their default format, so if they want to place it earlier in the line they can. If the default error summary value is “-”, the field will be present, making log line parsing easier. For example, the logging_context_format_string value can be set to

%(asctime)s.%(msecs)03d %(process)d %(levelname)s %(name)s:%(lineno)s [%(request_id)s %(user_identity)s] [%(error_summary)s] %(instance)s%(message)s

to produce a line like the following (when there is an error)

2017-04-24 15:15:54.823 1108 INFO oslo.tester:24 [-] [RuntimeError: the error text] a regular log message goes here

or like the following (when there is not an error)

2017-04-24 15:15:54.823 1108 INFO oslo.tester:24 [-] [-] a regular log message goes here

see for an example implementation of this additional flexibility.


  1. The first public specification related to this topic was posted in glance-specs under the title “Glance Error Codes”:

    The Glance spec was eventually moved to the cross-project specs repo with the title “OpenStack wide Error Codes for Log Messages”:

  2. We could, once and for all, declare that this is not a feature we are going to add to OpenStack.

  3. We could adopt the “situation ID” proposal in the cross-project spec proposed as, or one of the variations described in the alternatives section of that document.

  4. We could add the directives to the default logging format string to add the line numbers automatically. This would likely break the log management pipelines in existing deployments, since the line format would change.




Primary assignee:

oslo.log work: dhellmann

documenting exceptions in more detail: TBD

Work Items

  • Add documentation about how to include line numbers in the log messages.

  • Make the oslo.log context formatter add exception summary when logging an exception (

  • Make the other oslo.log formatters add the exception summary, where appropriate. (JSON, journald, etc.)

  • Make the other oslo.log formatters and handlers include the line number, where appropriate. (JSON, journald, etc.)





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