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syslog-ng Premium Edition 7.0.18 - Release Notes

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 syslog-ng PE 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 syslog-ng PE in general:

  • Python parsers and template functions are available in syslog-ng PE version 7.0.2 and later.

    Python destinations and sources are available in syslog-ng PE version 7.0.11 and later.

  • Supported Python versions: 2.7.

  • The Python block must be a top-level block in the syslog-ng PE configuration file.

  • If you store the Python code in a separate Python file and only include it in the syslog-ng PE configuration file, make sure that the PYTHON_PATH environment variable includes the path to the Python file, and export the PYTHON_PATH environment variable. For example, if you start syslog-ng PE 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 syslog-ng PE 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 syslog-ng PE. (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 syslog-ng PE is started or reloaded.


    If you reload syslog-ng PE, existing Python objects are destroyed, therefore the context and state information of Python blocks is lost. Log rotation and updating the configuration of syslog-ng PE typically involves a reload.

  • The Python block can contain multiple Python functions.

  • Using Python code in syslog-ng PE can significantly decrease the performance of syslog-ng PE, especially if the Python code is slow. In general, the features of syslog-ng PE 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 syslog-ng PE application does not do any of this.

  • Python error messages are available in the internal() source of syslog-ng PE.

  • You can access the name-value pairs of syslog-ng PE 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 syslog-ng PE. For details, see Logging from your Python code.

  • Support disclaimer

    Using Python in syslog-ng PE is recommended only if you are familiar with both Python and syslog-ng PE. Product support applies only to syslog-ng PE: that is, until the entry point of the Python code and passing the specified arguments to the Python code. One Identity is not responsible for the quality, resource requirements, or any bugs in the Python code, nor any syslog-ng PE crashes, message losses, or any other damage caused by the improper use of this feature, unless explicitly stated in a contract with One Identity.


Starting with 7.0.19, syslog-ng PE assigns a persist name to Python sources and destinations. The persist name is generated from the class name. If you want to use the same Python class multiple times in your syslog-ng PE configuration, add a unique persist-name() to each source or destination, otherwise syslog-ng PE will not start. For example:

log {
    source { python(class(PyNetworkSource) options("port" "8080") persist-name("<unique-string>); };
    source { python(class(PyNetworkSource) options("port" "8081")); };

Alternatively, you can include the following line in the Python package: @staticmethod generate_persist_name. For example:

from syslogng import LogSource
  class PyNetworSource(LogSource):
    def generate_persist_name(options):
        return options["port"]
    def run(self):
    def request_exit(self):

Python sources consist of two parts. The first is a syslog-ng PE source object that you define in your syslog-ng PE 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 syslog-ng PE 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.

init(self, options) method (optional)

The syslog-ng PE 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.


If you reload syslog-ng PE, existing Python objects are destroyed, therefore the context and state information of Python blocks is lost. Log rotation and updating the configuration of syslog-ng PE typically involves a reload.

When this method returns with False, syslog-ng PE 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 syslog-ng PE 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 syslog-ng PE application calls this method when syslog-ng PE 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 syslog-ng PE is not able to stop. Note that syslog-ng PE calls the request_exit method from a thread different from the source thread.

The deinit(self) method (optional)

This method is executed when syslog-ng PE is stopped or reloaded. This method does not return a value.


If you reload syslog-ng PE, existing Python objects are destroyed, therefore the context and state information of Python blocks is lost. Log rotation and updating the configuration of syslog-ng PE typically involves a reload.

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

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