Different logging targets

[tastyherring]

I would like to have different logging targets with a single application so I can vary the log levels for these different things.

Something like
MYAPP_LEVEL=info,target1=trace,target2=info

Rust's logging trait does this with a logging target, in the meta data recorded at each callsite?
https://docs.rs/log/latest/log/struct.Metadata.html#method.target

What is the approach to achieving this with SPDLOG? Do I need to define different loggers for each 'target' or is there any example of this this might be done.

[tt4g]

You can pass different log levels for each logger by environment variables or argv.
See README: https://github.com/gabime/spdlog/blob/v1.15.2/README.md#load-log-levels-from-the-env-variable-or-argv

Load log levels from the env variable or argv

#include "spdlog/cfg/env.h"
int main (int argc, char *argv[])
{
    spdlog::cfg::load_env_levels();
    // or specify the env variable name:
    // MYAPP_LEVEL=info,mylogger=trace && ./example
    // spdlog::cfg::load_env_levels("MYAPP_LEVEL");
    // or from the command line:
    // ./example SPDLOG_LEVEL=info,mylogger=trace
    // #include "spdlog/cfg/argv.h" // for loading levels from argv
    // spdlog::cfg::load_argv_levels(argc, argv);
}

So then you can:

$ export SPDLOG_LEVEL=info,mylogger=trace
$ ./example

The logger will have its log level set when it is registered in the spdlog registry.

spdlog/include/spdlog/details/registry-inl.h

Lines 65 to 68 in 48bcf39

	// set new level according to previously configured level or default level
	auto it = log_levels_.find(new_logger->name());
	auto new_level = it != log_levels_.end() ? it->second : global_log_level_;
	new_logger->set_level(new_level);

Tip

You must create the loggers yourself.

See also tests: https://github.com/gabime/spdlog/blob/v1.15.2/tests/test_cfg.cpp

[tastyherring]

I want my library code to be able to direct it logs at a specific "logging targets", without having to define a full blown logger. I can't have my library code creating loggers, because it doesn't know what will be appropriate for the application that eventually consumes the lib. I think your suggesting my library code needs to chose if logs go to syslog or go to file, but this my vary depending on how the lib is used.

Can this decision be deferred? So I only need to make this choice at the application level, and have the application decide what should be done with the different logging targets something like "target1=trace,target2=info".

[tt4g]

spdlog does not support automatic logger creation from configuration.

because it doesn't know what will be appropriate for the application that eventually consumes the lib. I think your suggesting my library code needs to chose if logs go to syslog or go to file, but this my vary depending on how the lib is used.

This idea applies directly to spdlog.
Depending on how the library or application uses spdlog, it should design how to create loggers.

For example, if you have designed the spdlog logger to be created from a configuration file, then that feature should be implemented in your source code (you can send a pull request if you think the feature is needed by many users).

FYI. There is a library https://github.com/guangie88/spdlog_setup that creates spdlog loggers from TOML configuration files.

[tt4g]

Another way, is to add an API to the library that accepts the loggers to be used: https://github.com/gabime/spdlog/wiki/How-to-use-spdlog-in-DLLs