SocketHubAppender

Class Overview

Sends LoggingEvent objects to a set of remote log servers, usually a SocketNodes.

Acts just like SocketAppender except that instead of connecting 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 SocketAppender.

The SocketHubAppender has the following characteristics:

• If sent to a 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, NDC, location info as if it were logged locally.

• SocketHubAppender does not use a layout. It ships a serialized 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.

• 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 close() the SocketHubAppender either explicitly or by calling the shutdown() method before exiting the application.

Summary

[Expand] Inherited Fields

From class org.apache.log4j.AppenderSkeleton protected boolean closed Is this appender closed? protected ErrorHandler errorHandler It is assumed and enforced that errorHandler is never null. protected Filter headFilter The first filter in the filter chain. protected Layout layout The layout variable does not need to be set if the appender implementation has its own layout. protected String name Appenders are named. protected Filter tailFilter The last filter in the filter chain. protected Priority threshold There is no level threshold filtering by default.

[Expand] Inherited Methods
From class org.apache.log4j.AppenderSkeleton void activateOptions() Derived appenders should override this method if option structure requires it. void addFilter(Filter newFilter) Add a filter to end of the filter list. abstract void append(LoggingEvent event) Subclasses of AppenderSkeleton should implement this method to perform actual logging. void clearFilters() Clear the filters chain. synchronized void doAppend(LoggingEvent event) This method performs threshold checks and invokes filters before delegating actual logging to the subclasses specific append(LoggingEvent) method. void finalize() Finalize this appender by calling the derived class' close method. ErrorHandler getErrorHandler() Return the currently set ErrorHandler for this Appender. Filter getFilter() Returns the head Filter. final Filter getFirstFilter() Return the first filter in the filter chain for this Appender. Layout getLayout() Returns the layout of this appender. final String getName() Returns the name of this appender. Priority getThreshold() Returns this appenders threshold level. boolean isAsSevereAsThreshold(Priority priority) Check whether the message level is below the appender's threshold. synchronized void setErrorHandler(ErrorHandler eh) Set the ErrorHandler for this Appender. void setLayout(Layout layout) Set the layout for this appender. void setName(String name) Set the name of this Appender. void setThreshold(Priority threshold) Set the threshold level.
From class java.lang.Object Object clone() boolean equals(Object arg0) void finalize() final Class<?> getClass() int hashCode() final void notify() final void notifyAll() String toString() final void wait() final void wait(long arg0, int arg1) final void wait(long arg0)
From interface org.apache.log4j.Appender abstract void addFilter(Filter newFilter) Add a filter to the end of the filter list. abstract void clearFilters() Clear the list of filters by removing all the filters in it. abstract void close() Release any resources allocated within the appender such as file handles, network connections, etc. abstract void doAppend(LoggingEvent event) Log in Appender specific way. abstract ErrorHandler getErrorHandler() Returns the ErrorHandler for this appender. abstract Filter getFilter() Returns the head Filter. abstract Layout getLayout() Returns this appenders layout. abstract String getName() Get the name of this appender. abstract boolean requiresLayout() Configurators call this method to determine if the appender requires a layout. abstract void setErrorHandler(ErrorHandler errorHandler) Set the ErrorHandler for this appender. abstract void setLayout(Layout layout) Set the Layout for this appender. abstract void setName(String name) Set the name of this appender.
From interface org.apache.log4j.spi.OptionHandler abstract void activateOptions() Activate the options that were previously set with calls to option setters.