python: writing custom Python destinations

The Python destination allows you to write your own destination in Python. You can import external Python modules to process the messages, and send them to other services or servers. Since many services have a Python library, the Python destination makes integrating syslog-ng OSE very easy and quick.

The following points apply to using Python blocks in syslog-ng OSE in general:

Python parsers and template functions are available in syslog-ng OSE version 3.10 and later.

Python destinations and sources are available in syslog-ng OSE 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 syslog-ng OSE configuration file.
If you store the Python code in a separate Python file and only include it in the syslog-ng OSE 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 OSE 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 OSE 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 OSE. (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 OSE is started or reloaded.

CAUTION: If you reload syslog-ng OSE, 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 OSE typically involves a reload.

The Python block can contain multiple Python functions.
Using Python code in syslog-ng OSE can significantly decrease the performance of syslog-ng OSE, especially if the Python code is slow. In general, the features of syslog-ng OSE 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 OSE application does not do any of this.
Python error messages are available in the internal() source of syslog-ng OSE.
You can access the name-value pairs of syslog-ng OSE 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 OSE. For details, see Logging from your Python code.

NOTE: Starting with 3.26, syslog-ng OSE 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 OSE configuration, add a unique persist-name() to each source or destination, otherwise syslog-ng OSE 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 PyNetworkSource(LogSource):
   @staticmethod
   def generate_persist_name(options):
       return options["port"]
   def run(self):
       pass
   def request_exit(self):
       pass

Declaration

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

destination <name_of_the_python_destination>{
    python(
        class("<name_of_the_python_class_executed_by_the_destination>")
    );
};

python {
class <name_of_the_python_class_executed_by_the_destination>(object):

    def open(self):
        """Open a connection to the target service

        Should return False if opening fails"""
        return True

    def close(self):
        """Close the connection to the target service"""
        pass

    def is_opened(self):
        """Check if the connection to the target is able to receive messages"""
        return True

    def init(self, options):
        """This method is called at initialization time

        Should return false if initialization fails"""
        return True

    def deinit(self):
        """This method is called at deinitialization time"""
        pass

    def send(self, msg):
        """Send a message to the target service

        It should return True to indicate success. False will suspend the
        destination for a period specified by the time-reopen() option."""
        return True

    def flush(self):
        """Flush the queued messages"""
        pass
};

NOTE: From version 3.27, syslog-ng OSE supports the arrow syntax for
declaring custom Java and Python options. You can alternatively declare
them using a similar syntax:
options(  
   "host" => "localhost"  
   "port" => "1883"  
   "otheroption" => "value"  
)                    

Methods of the python() destination

init(self, options) method (optional)

The syslog-ng OSE 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, syslog-ng OSE 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 OSE configuration object as a Python dictionary.

is_opened(self) method (optional)

Checks if the connection to the target is able to receive messages, and should return True if it is. For details, see Error handling in the python() destination.

open(self) method (optional)

The open(self) method opens the resources required for the destination, for example, it initiates a connection to the target service. It is called after init() when syslog-ng OSE is started or reloaded. If send() returns with an error, syslog-ng OSE calls close() and open() before trying to send again.

If open() fails, it should return the False value. In this case, syslog-ng OSE retries it every time-reopen() seconds. By default, this is 1 second for Python sources and destinations, the value of time-reopen() is not inherited from the global option. For details, see Error handling in the python() destination.

send(self, message) method (mandatory)

The send method sends a message to the target service. It should return True to indicate success, or self.QUEUED when using batch mode. For other possible return values, see the description of the flush() method. Note that for batch mode, the flush() method must be implemented as well.

This is the only mandatory method of the destination.

If a message cannot be delivered after the number of times set in retries() (by default: 3), syslog-ng OSE drops the message and continues with the next message. For details, see Error handling in the python() destination.

The method can return True, False, or one of the following constants:

self.DROP: The message is dropped immediately.
self.ERROR: Corresponds to boolean False. The message is put back to the queue, and sending the message is attempted (up to the number of the retries() option). The destination is suspended for time-reopen() seconds.
self.SUCCESS: Corresponds to boolean True. The message was sent successfully.
self.QUEUED: The send() method should return this value when using batch mode, if it has successfully added the message to the batch. Message acknowledgment of batches is controlled by the flush() method.
self.NOT_CONNECTED: The message is put back to the queue, and the destination is suspended. The open() method will be called, and the sending the messages will be continued with the same message/batch.
self.RETRY: The message is put back to the queue, and sending the message is attempted (up to the number of the retries() option). If sending the message has failed retries() times, self.NOT_CONNECTED is returned.

flush(self) method (optional)

Send the messages in a batch. You can use this method to implement batch-mode message sending instead of sending messages one-by-one. When using batch mode, the send() method adds the messages to a batch (for example, a list), and the flush() method sends the messages as configured in the batch-bytes(), batch-lines(), or batch-timeout() options.

The method can return True, False, or one of the following constants:

self.DROP: The messages cannot be sent and the entire batch is dropped immediately.
self.ERROR: Corresponds to boolean False. The message is put back to the queue, and sending the message is attempted (up to the number of the retries() option). The destination is suspended for time-reopen() seconds.
self.SUCCESS: Corresponds to boolean True. The message was sent successfully.
self.NOT_CONNECTED: The message is put back to the queue, and the destination is suspended. The open() method will be called, and the sending the messages will be continued with the same message/batch.
self.RETRY: The message is put back to the queue, and sending the message is attempted (up to the number of the retries() option). If sending the message has failed retries() times, self.NOT_CONNECTED is returned.

close(self) method (optional)

Close the connection to the target service. Usually it is called right before deinit() when stopping or reloading syslog-ng OSE. It is also called when send() fails.

The deinit(self) method (optional)

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

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

Error handling in the python() destination

The Python destination handles errors as follows.

Currently syslog-ng OSE ignores every error from the open method until the first log message arrives to the Python destination. If the fist message has arrived and there was an error in the open method, syslog-ng OSE starts calling the open method every time-reopen() second, until opening the destination succeeds.
If the open method returns without error, syslog-ng OSE calls the send method to send the first message.
If the send method returns with an error, syslog-ng OSE calls the is_opened method.
- If the is_opened method returns an error, syslog-ng OSE starts calling the open method every time-reopen() second, until opening the destination succeeds.
- Otherwise, syslog-ng OSE calls the send method again.
If the send method has returned with an error retries() times and the is_opened method has not returned any errors, syslog-ng OSE drops the message and attempts to process the next message.

Example: Write logs into a file

The purpose of this example is only to demonstrate the basics of the Python destination, if you really want to write log messages into text files, use the file destination instead.

The following sample code writes the body of log messages into the /tmp/example.txt file. Only the send() method is implemented, meaning that syslog-ng OSE opens and closes the file for every message.

destination d_python_to_file {
    python(
        class("TextDestination")
    );
};
log {
    source(src);
    destination(d_python_to_file);
};
python {
class TextDestination(object):
    def send(self, msg):
        self.outfile = open("/tmp/example.txt", "a")
        self.outfile.write("MESSAGE = %s\n" % msg["MESSAGE"])
        self.outfile.flush()
        self.outfile.close();
        return True
};

The following code is similar to the previous example, but it opens and closes the file using the open() and close() methods.

destination d_python_to_file {
    python(
        class("TextDestination")
    );
};
log {
    source(src);
    destination(d_python_to_file);
};
python {
class TextDestination(object):
    def open(self):
        try:
            self.outfile = open("/tmp/example.txt", "a")
            return True
        except:
            return False

    def send(self, msg):
        self.outfile.write("MESSAGE = %s\n" % msg["MESSAGE"])
        self.outfile.flush()
        return True

    def close(self):
        try:
            self.outfile.flush()
            self.outfile.close();
            return True
        except:
            return False
};

For a more detailed example about sending log messages to an MQTT (Message Queuing Telemetry Transport) server, see the Writing Python destination in syslog-ng: how to send log messages to MQTT blog post.

Example: Print logs in batch mode

The following is a simple destination that uses the flush() method to print the messages in batch mode.

class MyDestination(object):
    def init(self, options):
        self.bulk = list()
        return True

    def send(self, msg):
            self.bulk.append(msg["MSG"].decode())
            return self.QUEUED

    def flush(self):
        print("flushing: " + ",".join(self.bulk))
        self.bulk = list()
        return self.SUCCESS

For the list of available optional parameters, see python() destination options.