log4j: How does a Socket Appender work?

Question

I'm not sure how Socket Appender works. I know that logging events are sent to particular port. Then we can print logs on a console or put into a file.

My question is more about the way logs are sent. Is there e.g. one queue? Is it synchronous or asynchronous? Can using it slow down my program?

I've found some info here, but it isn't clear for me.

Answer 1

From the SocketAppender documentation

Logging events are automatically buffered by the native TCP implementation. This means that if the link to server is slow but still faster than the rate of (log) event production by the client, the client will not be affected by the slow network connection. However, if the network connection is slower then the rate of event production, then the client can only progress at the network rate. In particular, if the network link to the the server is down, the client will be blocked.

On the other hand, if the network link is up, but the server is down, the client will not be blocked when making log requests but the log events will be lost due to server unavailability.

Since the appender uses the TCP protocol, I would say the log events are "sort of synchronous".

Basically, the appender uses TCP to send the first log event to the server. However, if the network latency is so high that the message has still not been sent by the time a second event is generated, then the second log event will have to wait (and thus block), until the first event is consumed. So yes, it would slow down your application, if the app generates log events faster than the network can pass them on.

As mentioned by @Akhil and @Nikita, JMSAppender or AsyncAppender would be better options if you don't want the performance of your application to be impacted by the network latency.

Answer 2

Socket Appender sends the logs as a serialized Obect to a SocketNode or log server. In the appender the Connector Thread with a configured reconnectionDelay will check for the connection integrity and will dump all the logs if the connection is not initialized or disconnected.Hence no blocking on the application flow. If you need better JMS features in sending log info across JVM try JMSAppender.

1. Log4j JMS appender can be used to send your log messages to JMS broker.The events are serialized and transmitted as JMS message type ObjectMessage.

You can get a sample program HERE.