python: Write 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 AxoSyslog very easy and quick.

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.

  Warning

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

• 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.

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

   from syslogng import LogSource
      class PyNetworSource(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 AxoSyslog destination 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 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 AxoSyslog 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
    };

Methods of the python() destination

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.

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 AxoSyslog is started or reloaded. If send() returns with an error, AxoSyslog calls close() and open() before trying to send again.

If open() fails, it should return the False value. In this case, AxoSyslog 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), AxoSyslog 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 AxoSyslog. It is also called when send() fails.

The deinit(self) method (optional)

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

Error handling in the python() destination

The Python destination handles errors as follows.

1. Currently AxoSyslog 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, AxoSyslog starts calling the open method every time-reopen() second, until opening the destination succeeds.

2. If the open method returns without error, AxoSyslog calls the send method to send the first message.

3. If the send method returns with an error, AxoSyslog calls the is_opened method.

   • If the is_opened method returns an error, AxoSyslog starts calling the open method every time-reopen() second, until opening the destination succeeds.

   • Otherwise, AxoSyslog calls the send method again.

4. If the send method has returned with an error retries() times and the is_opened method has not returned any errors, AxoSyslog 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 AxoSyslog 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.