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From Cliff Skolnick <cl...@organic.com>
Subject Re: SSI handlers
Date Tue, 11 Jul 1995 21:47:35 GMT

Both issues are handler issues, but they are distinct.  I like pulling them
apart.  In fact maybe two API's for handlers instead of one.  One to
get objects, one to manipulate them.

On Mon, 10 Jul 1995 19:49:47 EDT, rst@ai.mit.edu (Robert S. Thau) wrote:
}    From: Cliff Skolnick <cliff@organic.com>
} First, topic #2.
} 
}    I'm not sure we will need threading for HTTP-NG.  I don't see why one
}    thread can't handle multiple requests at once to be honest.  All you
}    really need is some sort of async I/O.  This can be simulated in an OS
}    neutral way, unlike threads.  Of course many of the same concerns and issu
  es
}    would be the same in the code.
} 
} To begin with, my problem with your <TCL> tag has less to do
} with multithreading per se than it has to do with simply having
} multiple requests served by one process, regardless of how that's
} accomplished.  Consider a "document" with the following embedded code:
} 
}    <!--#TCL-->
}    while {1} {open /dev/null r}
}    <!--#/TCL-->
} 
} --- I hope you'll pardon me for using my own suggested syntax.
} 
} Whatever process winds up serving this document will quickly run out
} of file descriptors (too quickly to be caught by a timeout, for
} instance) --- at which point all other requests being served by the
} same process would be totally screwed.  As I've already pointed out to
} you, detecting and recovering from these sorts of situations, in the
} general case, is very difficult.  This has nothing to do with
} multithreading per se --- it has to do with having one process serving
} multiple requests, no matter how that is accomplished.

Fine on the syntax, I like the idea of all non-standard tags sticking
out.  Forget I even mentioned <TCL> :).  I understand the issue here and
I don't think we can protect people from doing stupid things.  I'd like
to see a very restrictive language instead of Tcl, but I don't want to
write one.  So I will use Tcl and keep my fingers crossed.  If there is
something better I will jump on it.  I'm not arguing to hang people, just
give 'em the rope.  I want this ability in the API, but I don't want to see
it distributed with the server.  Is that OK?

} (You can say that no one would write that sort of thing deliberately,
} and at Organic, that may even be true.  But sooner or later, it's
} going to happen by accident --- or worse, you'll have a slow leak
} which doesn't get detected until the "document" gets loose on your
} primary server and brings *it* down.  If you're doing one request per
} process, you have exit() as a last-ditch way to escape from these
} situations, but if multiple requests are being served by the same
} process, and for HTTP-NG they basically have to be, there is no way
} out).

Again, this will be use at your own risk.  In fact since you can add
handlers are we protecting again someone adding a bad one with a leak?
I think not.  Nothing is making them use your alloc lines.

} Now, on to topic #1 --- skipping point-by-point replies, the heart of
} it is:
} 
}    I don't expect the Apache group to do this, but I do want to see APIs that
}    support a database vendor or an interested third party (like Organic)
}    doing this.  This is the future.
} 
} Here is a list of ways in which Shambhala is currently wedded to a
} filesystem as a back-end:
} 
}    1) Translation handlers have to be translating into some sort of a
}       namespace; currently, the filesystem is it.

This is actually fine for a database.  Most information can be aranged
hierarchically, which maps to file names.  Nothing needed to change here.
} 
}    2) The server core scans the translated pathnames, looking for
}       .htaccess files to read per-directory permissions out of.  A
}       database-back-ended server would presumably want a similar
}       mechanism, but coming up with a suitably general interface
}       is extremely difficult.

This may still work, the server could "fetch" and object "{somename}/.htaccess"
which may be returned by the database.  I'd rather have a "is this object
accessed controlled" call, which would be a backing store specifc funtion
combined with an access method.  I am sure the database vendor would like to
store the user/password database themselves and not have it look like a flat
file.  Perhaps the module itself should be given the translated object
(filename) and return if it needs to be called for further access control.

} 
}    3) The response handlers all invariably use fopen() to get at the
}       filesystem object whose name popped out of the translation
}       handler. 
} 
} I've thought fairly long and hard about how to come up with an API
} which generalizes all these things, and I can't.  I've then given up on
} 3), decided that anything which was in the DB back-end would need its
} own response handler, and I have a few ideas about how that could work
} (it helps to start distinguishing internal object type from the type
} that will be served to the client, so that you can dispatch on the
} former and do content negotiation on the latter), but it's still quite
} messy (particularly #2 --- what interface do you provide to the
} command stuff)?

I don't know the answer to this, but I could see two things:

1 module that knows how to
	start access to an object (open)
	read data from an object (read)
	stop access (close)

mod_filesystem.c and mod_db.c would have these routines.  I think we
can safely assume sequencial access to an object so this should be OK.

handlers like mod_include.c would get handed an object where data was ready
to be read.  The framework would do the start access and stop access
to the object.  The framework could also provide glue that would allow
the output of handler A to be fed into handler B.

} It took me a couple of *months* to come up with clean APIs for what
} Shambhala does now --- I was effectively AWOL for quite a bit longer
} than people seem to have noticed.  I expect it would take at least
} an equivalent amount of time to come up with a good clean design for
} this and make it work, and I'm not sure I have the time for that right
} now.  Sigh...

Hey, this stuff is #ifdef FUTURE.  Maybe even a 2.0 thing.  I think you've
done most excellent work and have thought about these implimentation issues
more than me.  I just have a very strong feeling that this is where
the web is going, and I want to be there.  I was hoping this would be easier,
but you've given many good arguments as to why these are a pain.

Our priority should be to get this code out there and have people look at
and make suggestions.  Let's release with only changes that you (rst) see
as high priority.  I'll even shut up about this until we release.  Hmm, maybe
I should shutup after release as a motivation for the release. :)

Cliff

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