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A maximally useful log message contains a number of fixed items, usually in a fixed-format header of some sort, and text for the human reader to use to understand the implications of the problem.

From memory, the fixed information includes enough to allow for mechanical sorting by nastiness and occasionally mechanical processing:

- date/time
- origin, meaning machine- or domain-name
- source, in some detail,, including the executable name and process id as a minimum, if applicable, and optionally the file, function and line, it is good to make this one token, for ease of parsing and resilience when one line has "sendmail:parse.c:parse_it:332:1948" and another has only "mconnect:1293"
- pre-classification, meaning the application type, error type and severity. DFAs can switch on this, and should.

The old ARPA format was error type source and severity as three decimal digits, which you still see when smtp says "250 ok". The 2 was permanent success, the 5 meant "the app", in this case smtp, and 0 was the severity. I prefer ascii, not numbers (;-))
- then the text for the human, saying the meaning of the error, the same way you're supposed to write the **meaning** of code in comments, not just say what the code does.

Syslog does about half of this, metze's did most of it.

One very handy pair was the ARPA command and return-code standard.

A command was four letters or less at the beginning of a line (record, packet), often monocase, so it could be treated as a 4-byte integer and switched on.

For example, smtp starts ups with
helo localhost
250 froggy Hello localhost [127.0.0.1], pleased to meet you
The "HELO" is the command, and the next line the response.

the first character is an ascii digit, where
1 means "informational message", and is rare
2 means permanent success
3 means partial success, as in a series of steps.
4 means temporary failure, such as "no space", and
5 means permanent failure

The second digit is 5 for "this app" and 9 for "the OS"

The third digit is the severity, so
599 I must close down, my CPU is on fire
is a very sever and permanent error (:-))

The fourth character is an ascii blank if the reply is complete on this line, a "-" if it continues to additional lines. For example, smtp has a help command:
help
214-2.0.0 This is sendmail version 8.12.8+Sun
214-2.0.0 Topics:
214-2.0.0 HELO EHLO MAIL RCPT DATA
214-2.0.0 RSET NOOP QUIT HELP VRFY
214-2.0.0 EXPN VERB ETRN DSN
214-2.0.0 For more info use "HELP ".
214-2.0.0 To report bugs in the implementation contact Sun Microsystems
214-2.0.0 Technical Support.
214-2.0.0 For local information send email to Postmaster at your site.
214 2.0.0 End of HELP info

The three digits and the "-" for continuation allows one to write as simple or as complex a DFA as you like, by doing trivial masking on fixed-length strings.