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Comment by jdmichal | Hacker News Reader

I have always used them as follows. Deciding between DEBUG and INFO is the most difficult. INFO, WARN, and ERROR are simple decisions.

DEBUG: Information which is relevant for tracing activity and data through the program.

INFO: Important workflow decisions made within the normal operation of the program. Allows you to tell what path a particular operation flowed down, but not with tracing every activity.

WARN: Non-fatal errors which do not prevent the operation from completing. Example: Failing to close a file after having read from it.

ERROR: Fatal errors which prevent the operation from completing. Example: Failing to open the file or reading from it.

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What about FATAL errors which prevent the entire program from continuing?

i think OP is implying that error level and fatal level should be regarded as the same

Error and fatal are the same thing. If an operation fails and cannot be recovered, that's an error. There is no meaningful difference between an operation failing and the entire program failing. Or, from another perspective, keeping the program running is just another operation that can have fatal or non-fatal errors.

That depends on the nature of the program. We use error and critical for server software. If there is a protocol violation on a particular connection, log an error and drop the connection, but otherwise continues. If the `accept` system call fails that's critical and the program shuts down. Depending on what we're doing there may be other critical operations, but some software doesn't have any.

Picture an Erlang web-application server.

A request that causes a 500 error response to the client, should also produce an Error log output on the backend. The request crashed. But, because this is an actor-based system, that crash didn't affect anything other than the request. It was Fatal to that interaction, but not to the system.

A request that causes the entire server process to spontaneously halt, should generate a Fatal log message. That is never supposed to be something that happens. The system is built, on a number of levels, to ensure that it doesn't happen. But it'd be really great, if it does happen, to have enough information to know why.

Ideally, a Fatal log message is accompanied by a crash dump.

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