Tech Blog: JSON logging in Python

Both uWSGI and especially nginx logs are indeed more important than application logs for figuring out how the microservice-oriented infrastructure breathes in general. However, when problems occur, collecting informative application logs is a must. In this part of the JSON logging series, we discuss our solution to JSON logging in Python.

JSON logging everywhere

If you haven’t already, be sure to check out our previous blog post where we discussed how we configured uWSGI to use JSON logging. There was one important detail we practically skimmed over. We assumed application logs would already be JSON formatted. This is, however, not the case for Python, and now we will discuss the exact way our team tackled that specific issue.

Custom-built JSON formatter

Existing solutions

Before starting, we decided that we wanted a solution filling out these requirements

1. an easy way to log LogRecord attributes in JSON log using arbitrary keys
2. log additional static values, ie. app_name, etc.
3. log values provided as argument extra in logging methods

The first thing we did was, of course, a search for other open-source solutions for JSON logging. There are existing solutions out there, however, none of them seemed a good fit for our use case. A good example is python-json-logging which offers a wide variety of features we do not intend to use, and more importantly, fails at our first and second requirement.

Our solution

Finally, we built a custom JSONFormatter allowing us simple configuration by passing the argument fields to replace Python’s LogRecord keys with our own, for example:

{
    "message": "message",
    "time": "created",
    "log_level": "levelname",
    "process": "process",
    "process_name": "processName",
    "thread": "thread",
    "thread_name": "threadName",
    "traceback": "exc_text"
}

where we evaluate values as LogRecord attributes and log them under their keys. Similarly, we can also pass always_extra key during formatter initialization to log additional static values. Just like python-json-logging does as well, we also log values provided as argument extra in logging methods.

Since we want to use the same logging for all of our Python microservices, we also need a simple way to set up our log formatting. This is where the function useful.logs.setup with sane defaults comes in. This makes JSON logging a breeze.

import logging
import useful.logs

useful.logs.setup()
_log = logging.getLogger(__name__)


if __name__ == '__main__':
    _log.info("testing", extra={"app": "AppName", "metric": 1.2})

Edge cases

It is good to have JSON logging in cases you explicitly call logging methods. There are, however, situations where you simply do not catch all possible exceptions and Python writes the exception traceback to stderr. Python provides a mechanism for solving this issue. We do it by overriding a function sys.excepthook which will serve as an alternative “logging” function called in case of uncaught exceptions. Here we can override writing to stderr and instead use our standard JSON logging.

import logging
import sys

def except_logging(exc_type, exc_value, exc_traceback):
    logging.error("Uncaught exception",
                  exc_info=(exc_type, exc_value, exc_traceback))

sys.excepthook = except_logging

This is better, but what happens when it is impossible to catch the exception? Yes, such cases do exist. One example would be an exception during garbage collection. Again, the Python sys module gives us the solution! Simply set the hook sys.unraisablehook in a similar manner as sys.excepthook.

import logging
import sys

def unraisable_logging(args):
    exc_type, exc_value, exc_traceback, err_msg, _ = args
    default_msg = "Unraisable exception"

    logging.error(err_msg or default_msg,
                  exc_info=(exc_type, exc_value, exc_traceback))

sys.unraisablehook = unraisable_logging

Sadly, we are not done yet. We still have to handle exceptions raised by threading.Thread.run() and as of Python version 3.8.0 we can override the hook threading.excepthook the same way we did sys.excepthook.

import logging
import threading

def threading_except_logging(args):
    exc_type, exc_value, exc_traceback, _ = args
    logging.error("Uncaught threading exception",
                  exc_info=(exc_type, exc_value, exc_traceback))

threading.excepthook = threading_except_logging

The final issue is with the Python (as of writing, the latest stable version is 3.8.1) multiprocessing module, which implements and calls its own exception handling routine that writes to stderr instead of calling the sys.excepthook. As the answer on the StackOverflow suggests, you can “fix” this issue by overriding method multiprocessing.Process.run which will manually catch all exceptions and call sys.excepthook. We do not use multiprocessing in any of our microservices and have thus decided not to implement any solution to this problem since we haven’t found a clean way to do it.

Besides multiprocessing, all other edge cases mentioned above are handled inside useful.logs.setup.

Request tracing id

When building REST APIs in a microservice environment, it is very important to have a way of tracing requests across the infrastructure. A simple way to do it is to save some sort of identification value in the request header set by the first service in the infrastructure (nginx in our case) and log it in every service afterward. This requires you to have access to the request at the moment of logging, which can be a bit impractical, especially when using a framework where current request isn’t stored in some sort of global variable like Flask does. Since we use Falcon that has no such implementation and we are running on uwsgi with threads, we implemented our own useful.logs.context based on threading.local that can be used to store request tracing id and is accessible inside log formatter. All that is left is a simple manner of implementing custom Falcon middleware to automatically read the request header and store it into useful.logs.context. If you need support for greenlets, werkzeug provides an awesome alternative implementation supporting both threads and greenlets along with the way to “clean” the context after each request since WSGI does not guarantee that threads won’t be reused for multiple requests. If you are using DataDog you can use Python module ddtrace to solve this issue.

We still have to explore how to configure nginx for JSON logging and request tracing along with log collecting infrastructure, so stay tuned!

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