Exponential back-off logging

[knoepfel]

I help develop a computing framework that particle-physics experiments use to process their data. The framework contains a logging facility that I'd love to replace with spdlog. However, one of the features we provide is an exponential back-off of messages logged to a particular sink.

For example, a user could write something like:

some_logger::set_backoff_factor(2);
for (std::size_t i = 1; i != 100; ++i) {
  some_logger::info("Processing {}", i);
}

and the output would be something like:

Processing 1
Processing 2
Processing 4
Processing 8
Processing 16
Processing 32
Processing 64

Does spdlog have such a back-off facility (perhaps through a custom logger)? Or would this have to be a new feature?

[tt4g]

Backtrace of logger is available

// Debug messages can be stored in a ring buffer instead of being logged immediately.
// This is useful in order to display debug logs only when really needed (e.g. when error happens).
// When needed, call dump_backtrace() to see them.

spdlog::enable_backtrace(32); // Store the latest 32 messages in a buffer. Older messages will be dropped.
// or my_logger->enable_backtrace(32)..
for(int i = 0; i < 100; i++)
{
  spdlog::debug("Backtrace message {}", i); // not logged yet..
}
// e.g. if some error happened:
spdlog::dump_backtrace(); // log them now! show the last 32 messages

// or my_logger->dump_backtrace(32)..

https://github.com/gabime/spdlog/tree/v1.11.0#backtrace-support

There is no feature to back off the logs of a specific sink, so it must be implemented custom sink.

See the Wiki for a custom sink implementation: https://github.com/gabime/spdlog/wiki/4.-Sinks#implementing-your-own-sink
See spdlog::logger::backtrace() for how to implement back off.

spdlog/include/spdlog/logger-inl.h

Lines 107 to 122 in ad0e89c

	// create new backtrace sink and move to it all our child sinks
	SPDLOG_INLINE void logger::enable_backtrace(size_t n_messages)
	{
	tracer_.enable(n_messages);
	}
	// restore orig sinks and level and delete the backtrace sink
	SPDLOG_INLINE void logger::disable_backtrace()
	{
	tracer_.disable();
	}
	SPDLOG_INLINE void logger::dump_backtrace()
	{
	dump_backtrace_();
	}

spdlog/include/spdlog/logger-inl.h

Lines 210 to 219 in ad0e89c

	SPDLOG_INLINE void logger::dump_backtrace_()
	{
	using details::log_msg;
	if (tracer_.enabled())
	{
	sink_it_(log_msg{name(), level::info, "****************** Backtrace Start ******************"});
	tracer_.foreach_pop([this](const log_msg &msg) { this->sink_it_(msg); });
	sink_it_(log_msg{name(), level::info, "****************** Backtrace End ********************"});
	}
	}

[tt4g]

spdlog::sinks::ringbuffer_sink is available, although it may be slightly different from the required back-off.

https://github.com/gabime/spdlog/blob/v1.11.0/include/spdlog/sinks/ringbuffer_sink.h