Provide a way to filter log events, where the decision on whether to discard the message or actually log them cannot be made until after the application has already logged the message.
In many systems, particularly event processing applications, log files contain a lot of repetitive log messages. Suppose an application needs to do some calculation to decide whether or not to react to some event, and a lot of detail is logged during this calculation. Imagine that 99% of the time, the application decides to take no action. Once the application arrived at that conclusion it would be nice if we could go back and undo all the detailed logging and print a summary instead. When the application does decide to take some action, however, we do want the detailed log messages.
A Unit of Work for logging would allow us to group a set of log messages and either discard them or log them together. (Inspired by Martin Fowler's Unit of Work pattern.)
This should result in log files where a lot of the "uninteresting" logging is filtered out, significantly reducing the amount of data logged.
Some applications do this in an ad hoc manner, for example by passing a Collection to its components, where these components can add log message strings to this Collection. When the discard/retain decision is made, the application then either clears the Collection or logs the contents of the Collection. This works, but having to pass the Collection down the component tree is clunky and the result often omits details like logger name, timestamp and other details that come for free with normal logging. Log4j can provide a reusable and less intrusive way to accomplish this.
There would need to be some API for the application to mark the start of the unit of work, and some API to signal whether the log messages that are part of that unit of work need to be discarded or logged (retained).
Not all logging that occurs after a unit of work was started is part of that unit of work. The application may want some messages to be logged regardless of whether the unit of work was discarded or not. There needs to be a flexible way (or multiple ways) to include or exclude logging statements from the unit of work.
The application may also designate multiple units of work, which may be sequential, nested or partially overlapping. Each unit of work may define its own rules for which log messages are considered included in or excluded from the unit of work.