Let's assume that you've programmed a client application that always should be connected to the server.  Disconnection from server prevents  the application from functioning but we cannot say that it is a FATAL situation because server will get up and run soon possibly.  We'll have to say it is in a FATAL situation if client cannot proceed the reconnect process by some reason. 
So I think ERROR and FATAL level are both required.  WARN, ERROR and FATAL both represents exceptional situation, but three has some different meaning:
* WARN - exceptional situation, but safely ignorable because the application can recover itself.
* ERROR - exceptional situation that requires user's attention because of temporary malfunction of the application.  The application can recover from this situation automatically if other environment changes.
* FATAL - exceptional situation that we really didn't expected or that makes the application stop permanently.
2005/7/5, Ceki Gülcü <listid@qos.ch>:

Trustin Lee wrote:

> SLF4J doesn't have "FATAL" level AFAIK.  Would this be OK?

This question is answered in the SLF4J FAQ [1] quoted below:

  Why doesn't the org.slf4j.Logger interface have methods for the FATAL

  From the  stand point  of a logging  system, the distinction  between a
  fatal error and an error  is usually not very useful. Most programmers
  exit the  application when  a fatal error  is encountered.  However, a
  logging library cannot (and should not) decide on its own to terminate
  an application. The initiative to exit the application must be left to
  the developer.

  Thus, the most the FATAL level can do is to highlight a given error as
  the cause for application to  crash. However, errors are by definition
  exceptional events  that merit attention. If a  given situation causes
  errors  to be  logged, the  causes should  be attended  to as  soon as
  possible. However, if the "error" is actually a normal situation which
  cannot  be prevented  but merits  being aware  of, then  it  should be
  marked as WARN, not ERROR.

  Assuming  the ERROR level  designates exceptional  situations meriting
  close attention,  we are inclined to  believe that the  FATAL level is

Does this answer your question?

[1] http://slf4j.org/faq.html#2.5


Ceki Gülcü

  The complete log4j manual: http://www.qos.ch/log4j/

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