how to reconnect when connection lost?

[xdewx]

    h = logstash.LogstashHandler(host, port, **kwargs)
    logger.addHandler(h)

If restart logstash, the connection will lost. how to ensure logger to reconnect logstash as soon as possible in each log?

[RomkaSk]

Please see documentations for class SocketHandler in python/logging/handlers.py:529 namely pay attention to the methods emit() and handleError().

def emit(self, record):
    """
    Emit a record.

    Pickles the record and writes it to the socket in binary format.
    If there is an error with the socket, silently drop the packet.
    If there was a problem with the socket, re-establishes the
    socket.
    """
    ...


def handleError(self, record):
    """
    Handle an error during logging.

    An error has occurred during logging. Most likely cause -
    connection lost. Close the socket so that we can retry on the next event.
    """
    ...

If you don't want to lose packets, you can create a custom handler that extends the logstash.LogstashHandler/TCPLogstashHandler and overrides the emit() method. Within the custom handler, you can implement the reconnection logic to handle connection failures.
Something like:

class ReconnectingLogstashHandler(logstash.LogstashHandler):
    def __init__(self, host, port, **kwargs):
        super().__init__(host, port, **kwargs)

    def emit(self, record):
	try:
	    if not self.sock:
	        self.createSocket()
	    super().emit(record)
	except (ConnectionError, OSError):
	    # Connection lost, handle reconnection
	    self.sock = None
	    self.createSocket()  # Create a new socket
	    # Wait n-seconds before re-emitting the log record
	    time.sleep(1) 
	    self.emit(record)