At some point, the application might decide to push a piece of data onto the NDC stack, which other parts of the application might want to reuse. For example, at the beginning of a web request in a server, the application might decide to push the IP address of the client onto the stack to provide it for other loggers down the road without having to pass the data from function to function.
The Log::Log4perl::Layout::PatternLayout class even provides the handy %x placeholder which is replaced by the blank-separated list of elements currently on the stack.
This module maintains a simple stack which you can push data on to, query what's on top, pop it off again or delete the entire stack.
Its purpose is to provide a thread-specific context which all Log::Log4perl loggers can refer to without the application having to pass around the context data between its functions.
Since in 5.8.0 perl's threads don't share data only upon request, global data is by definition thread-specific.
This is typically done when a context is entered.
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