Cosmolog

Cosmolog provides structured log formatting for your application. It treats logs as event streams and will record each log event as a json object with a schema.

CosmologEvent Schema

A cosmolog event looks like the following:

{
    "version": 0,
    "stream_name": "foo_nginx",
    "origin": "foo-api1.com",
    "timestamp": "2016-09-02T16:34:12.019105Z",
    "format": "service {name} started",
    "level": 400,
    "payload": {"name": "foo"}
}

version     (int)    The version of the logging schema.
stream_name (string) The name that identifies the log stream.
origin      (string) FQDN of the host on which the stream originated.
timestamp   (string) UTC ISO8601 formatted datetime string.
format      (string) Optional Python Format String to format the payload to
                     make it human-readable. Please use format strings with
                     replacement fields that are delimited with "{}".
level       (int)    The log level.
payload     (dict)   A flat dictionary of key-value pairs where keys are
                     strings and values can be any scalar type.

Installation

pip install cosmolog

Quick Start

from cosmolog import setup_logging
from cosmolog import Cosmologger

setup_logging()

l = Cosmologger('foo')
l.info('Hello World')
l.info('Hello {person}', person='Dave')
l.info(value1=0.98, value2=4.0)

Human

human is a command line tool that formats machine readable logs for humans. It reads stdin and writes formatted lines to stdout.

$ echo '{"origin": "enterprise.starfleet.com", "stream_name": "telemetry", "format": "Measurement complete: gravity={gravity}", "timestamp": "2016-10-19T04:13:15.049920Z", "level": 400, "version": 0, "payload": {"gravity": 1.8}}' | human
Oct 19 04:13:15 enterprise.starfleet.com telemetry: [INFO] Measurement complete: gravity=1.8

Advanced Usage: setup_logging

setup_logging provides basic Python logging configuration with CosmologgerFormatter class as the default logging formatter. The default logging configuration looks like the following:

LOGGING_CONFIG = {
    'version': 1,
    'disable_existing_loggers': False,
    'formatters': {
        'cosmolog': {
            '()': CosmologgerFormatter,
            'origin': origin,
            'version': 0,
        },
    },
    'handlers': {
        'default': {
            'class': 'logging.StreamHandler',
            'formatter': 'cosmolog'
        },
    },
    'root': {
        'handlers': ['default'],
        'level': level,
    }
}

By default, origin is set to socket.getfqdn(). To set it yourself:

setup_logging(origin='my-fully-qualified-domain.com')

You can configure Cosmologger with a custom configuration dictionary, as you would with regular Python logging. Let's say you want to stream your log entries to a file in the CosmologEvent schema, as well as stderr in a human- readable format:

my_custom_config = {
    'version': 1,
    'disable_existing_loggers': False,
    'formatters': {
        'cosmolog': {
            '()': CosmologgerFormatter,
            'origin': origin,
            'version': 0,
        },
        'cosmolog-human': {
            '()': CosmologgerHumanFormatter,
            'origin': origin,
            'version': 0,
        },
    },
    'handlers': {
        'file_handler': {
            'class': 'logging.FileHandler',
            'formatter': 'cosmolog',
            'filename': '/path/to/my/file.log'
        },
        'stderr': {
            'class': 'logging.StreamHandler',
            'formatter': 'cosmolog-human',
	'color': True
        },
    },
    'root': {
        'handlers': ['file_handler', 'stderr'],
        'level': 'DEBUG',
    }
}

setup_logging(custom_config=my_custom_config)

By default, your log level will be set to INFO. You can set it to a different level:

setup_logging(level='DEBUG')

The table below shows the level names, and the numberic values they will be mapped to in the log output:

Level	Numeric value
FATAL	100
ERROR	200
WARN	300
INFO	400
DEBUG	500
TRACE	600

Advanced usage: Exceptions

Normally, all keyword arguments are faithfully passed into the CosmologEvent payload field, except in the case of logging exceptions. Here, the conventions of the builtin logging library are followed. Calling Cosmologger.exception or any of the other logging methods with the keyword argument exc_info=1 will add traceback information to the payload's exc_text field. If there is no {exc_text} specified in the format string, then the format field will be appended with '\n{exc_text}'.

Development

pip install -e .[test]
tox