ASAB logging is built on top of a standard Python logging module. It means that it logs to stderr when running on a console and ASAB also provides file and syslog output (both RFC5424 and RFC3164) for background mode of operations.

Log timestamps are captured with sub-second precision (depending on the system capabilities) and displayed including microsecond part.

Logging Levels

ASAB uses Python logging levels with the addition of LOG_NOTICE level. LOG_NOTICE level is similar to logging.INFO level but it is visible in even in non-verbose mode.

Level Numeric value Syslog Severity level
CRITICAL 50 Critical / crit / 2
ERROR 40 Error / err / 3
WARNING 30 Warning / warning / 4
LOG_NOTICE 25 Notice / notice / 5
INFO 20 Informational / info / 6
DEBUG 10 Debug / debug / 7

Verbose mode

The command-line argument -v enables verbose logging. It means that log entries with levels DEBUG and INFO will be visible. It also enables asyncio debug logging.

The actual verbose mode is avaiable at asab.Config["logging"]["verbose"] boolean option.

L.debug("This message will be visible only in verbose mode.")

Structured data

ASAB supports a structured data to be added to a log entry. It follows the RFC 5424, section STRUCTURED-DATA. Structured data are a dictionary, that has to be seriazable to JSON.

L.warning("Hello world!", struct_data={'key1':'value1', 'key2':2})

Example of the output to the console:

25-Mar-2018 23:33:58.044595 WARNING myapp.mymodule [sd key1="value1" key2="2"] Hello world!

Logging to file

The command-line argument -l on command-line enables logging to file. Also non-empty path option in the section [[logging:file]] of configuration file enables logging to file as well.

Example of the configuration file section:

format="%%(asctime)s %%(levelname)s %%(name)s %%(struct_data)s%%(message)s",
datefmt="%%d-%%b-%%Y %%H:%%M:%%S.%%f"

Log rotation

ASAB supports a log rotation. The log rotation is triggered by a UNIX signal SIGHUP, which can be used e.g. to integrate with logrotate utility. It is implemented using logging.handlers.RotatingFileHandler from a Python standard library. Also, a time-based log rotation can be configured using rotate_every option.

backup_count specifies a number of old files to be kept prior their removal. The system will save old log files by appending the extensions ‘.1’, ‘.2’ etc., to the filename.

rotate_every specifies an time interval of a log rotation. Default value is empty string, which means that the time-based log rotation is disabled. The interval is specified by an integer value and an unit, e.g. 1d (for 1 day) or 30M (30 minutes). Known units are H for hours, M for minutes, d for days and s for seconds.

Logging to syslog

The command-line argument -s enables logging to syslog.

A configuration section [[logging:syslog]] can be used to specify details about desired syslog logging.

Example of the configuration file section:


enabled is equivalent to command-line switch -s and it enables syslog logging target.

format speficies which logging format will be used. Possible values are:

  • 5 for (new) syslog format (RFC 5424 ) ,
  • 3 for old BSD syslog format (RFC 3164 ), typically used by /dev/log and
  • m for Mac OSX syslog flavour that is based on BSD syslog format but it is not fully compatible.

The default value is 3 on Linux and m on Mac OSX.

address specifies the location of the Syslog server. It could be a UNIX path such as /dev/log or URL. Possible URL values:

  • tcp://syslog.server.lan:1554/ for Syslog over TCP
  • udp://syslog.server.lan:1554/ for Syslog over UDP
  • unix-connect:///path/to/syslog.socket for Syslog over UNIX socket (stream)
  • unix-sendto:///path/to/syslog.socket for Syslog over UNIX socket (datagram), equivalent to /path/to/syslog.socket, used by a /dev/log.

The default value is a /dev/log on Linux or /var/run/syslog on Mac OSX.


class asab.log.AsyncIOHandler(loop, family, sock_type, address, facility=17)[source]

Bases: logging.Handler

A logging handler similar to a standard logging.handlers.SocketHandler that utilizes asyncio. It implements a queue for decoupling logging from a networking. The networking is fully event-driven via asyncio mechanisms.


This is the entry point for log entries.

class asab.log.Logging(app)[source]

Bases: object

class asab.log.MacOSXSyslogFormatter(fmt=None, datefmt=None, style='%', sd_id='sd')[source]

Bases: asab.log.StructuredDataFormatter

It implements Syslog formatting for Mac OSX syslog (aka format m).

class asab.log.StructuredDataFormatter(facility=16, fmt=None, datefmt=None, style='%', sd_id='sd')[source]

Bases: logging.Formatter

The logging formatter that renders log messages that includes structured data.

empty_sd = ''

Format the specified record as text.

formatTime(record, datefmt=None)[source]

Return the creation time of the specified LogRecord as formatted text.


Return the string with structured data.

class asab.log.SyslogRFC3164Formatter(fmt=None, datefmt=None, style='%', sd_id='sd')[source]

Bases: asab.log.StructuredDataFormatter

It implements Syslog formatting for Mac OSX syslog (aka format 3).

class asab.log.SyslogRFC5424Formatter(fmt=None, datefmt=None, style='%', sd_id='sd')[source]

Bases: asab.log.StructuredDataFormatter

It implements Syslog formatting for Mac OSX syslog (aka format 5).

empty_sd = ' '