Examples of org.apache.log4j.net.SocketHubAppender

• org.apache.log4j.net.SocketHubAppender
  Sends {@link LoggingEvent} objects to a set of remote log servers,usually a {@link SocketNode SocketNodes}.

  Acts just like {@link SocketAppender} except that instead ofconnecting to a given remote log server, SocketHubAppender accepts connections from the remote log servers as clients. It can accept more than one connection. When a log event is received, the event is sent to the set of currently connected remote log servers. Implemented this way it does not require any update to the configuration file to send data to another remote log server. The remote log server simply connects to the host and port the SocketHubAppender is running on.

  The SocketHubAppender does not store events such that the remote side will events that arrived after the establishment of its connection. Once connected, events arrive in order as guaranteed by the TCP protocol.

  This implementation borrows heavily from the {@link SocketAppender}.

  The SocketHubAppender has the following characteristics:

  • If sent to a {@link SocketNode}, logging is non-intrusive as far as the log event is concerned. In other words, the event will be logged with the same time stamp, {@link org.apache.log4j.NDC}, location info as if it were logged locally.

  • SocketHubAppender does not use a layout. It ships a serialized {@link LoggingEvent} object to the remote side.

  • SocketHubAppender relies on the TCP protocol. Consequently, if the remote side is reachable, then log events will eventually arrive at remote client.

  • If no remote clients are attached, the logging requests are simply dropped.

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

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

    The single remote client case extends to multiple clients connections. The rate of logging will be determined by the slowest link.

    The BufferSize param provides a cyclic buffer of recently appended events. As new clients attach to the SocketHubAppender, the clients also receive the buffered events.

  • If the JVM hosting the SocketHubAppender exits before the SocketHubAppender is closed either explicitly or subsequent to garbage collection, then there might be untransmitted data in the pipe which might be lost. This is a common problem on Windows based systems.

    To avoid lost data, it is usually sufficient to {@link #close}the SocketHubAppender either explicitly or by calling the {@link org.apache.log4j.LogManager#shutdown} method beforeexiting the application.

  @author Mark Womack

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