python: writing server-style Python sources

The Python source allows you to write your own source in Python. You can import external Python modules to receive or fetch the messages. Since many services have a Python library, the Python source makes integrating AxoSyslog very easy and quick.

You can write two different type of sources in Python:

  • Server-style sources that receives messages. Write server-style sources if you want to use an event-loop based, nonblocking server framework in Python, or if you want to implement a custom loop.

  • Fetcher-style sources that actively fetch messages. In general, write fetcher-style sources (for example, when using simple blocking APIs), unless you explicitly need a server-style source.

This section describes server-style sources. For details on fetcher-style sources, see python-fetcher: writing fetcher-style Python sources.

The following points apply to using Python blocks in AxoSyslog in general:

  • Python parsers and template functions are available in AxoSyslog version 3.10 and later.

    Python destinations and sources are available in AxoSyslog version 3.18 and later.

  • Supported Python versions: 2.7 and 3.4+ (if you are using pre-built binaries, check the dependencies of the package to find out which Python version it was compiled with).

  • The Python block must be a top-level block in the AxoSyslog configuration file.

  • If you store the Python code in a separate Python file and only include it in the AxoSyslog configuration file, make sure that the PYTHONPATH environment variable includes the path to the Python file, and export the PYTHON_PATH environment variable. For example, if you start AxoSyslog manually from a terminal and you store your Python files in the /opt/syslog-ng/etc directory, use the following command: export PYTHONPATH=/opt/syslog-ng/etc.

    In production, when AxoSyslog starts on boot, you must configure your startup script to include the Python path. The exact method depends on your operating system. For recent Red Hat Enterprise Linux, Fedora, and CentOS distributions that use systemd, the systemctl command sources the /etc/sysconfig/syslog-ng file before starting AxoSyslog. (On openSUSE and SLES, /etc/sysconfig/syslog file.) Append the following line to the end of this file: PYTHONPATH="<path-to-your-python-file>", for example, PYTHONPATH="/opt/syslog-ng/etc".

  • The Python object is initiated every time when AxoSyslog is started or reloaded.

  • The Python block can contain multiple Python functions.

  • Using Python code in AxoSyslog can significantly decrease the performance of AxoSyslog, especially if the Python code is slow. In general, the features of AxoSyslog are implemented in C, and are faster than implementations of the same or similar features in Python.

  • Validate and lint the Python code before using it. The AxoSyslog application does not do any of this.

  • Python error messages are available in the internal() source of AxoSyslog.

  • You can access the name-value pairs of AxoSyslog directly through a message object or a dictionary.

  • To help debugging and troubleshooting your Python code, you can send log messages to the internal() source of AxoSyslog. For details, see Logging from your Python code.


Python sources consist of two parts. The first is a AxoSyslog source object that you define in your AxoSyslog configuration and use in the log path. This object references a Python class, which is the second part of the Python source. The Python class receives or fetches the log messages, and can do virtually anything that you can code in Python. You can either embed the Python class into your AxoSyslog configuration file, or store it in an external Python file.

   source <name_of_the_python_source>{
                "option1" "value1",
                "option2" "value2"
    python {
    from syslogng import LogSource
    from syslogng import LogMessage
    class <name_of_the_python_class_executed_by_the_source>(LogSource):
        def init(self, options): # optional
            self.exit = False
            return True
        def deinit(self): # optional
        def run(self): # mandatory
            while not self.exit:
                # Must create a message
                msg = LogMessage("this is a log message")
        def request_exit(self): # mandatory
            self.exit = True

Methods of the python() source

Server-style Python sources must be inherited from the syslogng.LogSource class, and must implement at least the run and request_exit methods. Multiple inheritance is allowed, but only for pure Python super classes.

You can implement your own event loop, or integrate the event loop of an external framework or library, for example, KafkaConsumer, Flask, Twisted engine, and so on.

To post messages, call LogSource::post_message() method in the run method.

For the list of available optional parameters, see python() and python-fetcher() source options.

init(self, options) method (optional)

The AxoSyslog application initializes Python objects every time when it is started or reloaded. The init method is executed as part of the initialization. You can perform any initialization steps that are necessary for your source to work.

When this method returns with False, AxoSyslog does not start. It can be used to check options and return False when they prevent the successful start of the source.

options: This optional argument contains the contents of the options() parameter of the AxoSyslog configuration object as a Python dictionary.

run(self) method (mandatory)

Use the run method to implement an event loop, or start a server framework or library. Create LogMessage instances in this method, and pass them to the log paths by calling LogSource::post_message().

Currently, run stops permanently if an unhandled exception happens.

For details on parsing and posting messages, see Python LogMessage API.

request_exit(self) method (mandatory)

The AxoSyslog application calls this method when AxoSyslog is shut down or restarted. The request_exit method must shut down the event loop or framework, so the run method can return gracefully. If you use blocking operations within the run() method, use request_exit() to interrupt those operations and set an exit flag, otherwise AxoSyslog is not able to stop. Note that AxoSyslog calls the request_exit method from a thread different from the source thread.


Closes the current source-side batch. Source-side batching helps AxoSyslog to effectively process a larger chunk of messages, instead of processing messages each message. For example, when feeding a destination queue and instead of taking a lock on the queue for every message (causing contention), we only take it once per batch.

The native drivers built into AxoSyslog typically close batches once every mainloop iteration, allowing a single iteration to process multiple messages. For instance, when receiving multiple messages in a single TCP datagram, all of those messages can be processed as a part of the same batch.

In Python-based log sources, a batch will automatically be closed after every message posted via post_message(), except if self.auto_close_batches is set to False during initialization. In case self.auto_close_batches is set to False, the driver has to call close_batch() explicitly, preferably at a natural boundary between incoming batches of messages. A good example is when we retrieve several messages via the same HTTP REST call, then the right time to close the batch would be after the last message in the response is posted.

The deinit(self) method (optional)

This method is executed when AxoSyslog is stopped or reloaded. This method does not return a value.

set_transport_name(self, name)

Set the transport name used to retrieve messages This function can be called to customize the ${TRANSPORT} name-value pair.