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HTTP Design Issues, lessons from WebDAV's Property and Depth head er experiences

From: Yaron Goland <yarong@microsoft.com>
Date: Thu, 24 Dec 1998 00:00:57 -0800
Message-ID: <3FF8121C9B6DD111812100805F31FC0D08792BF4@RED-MSG-59>
To: "'Slein, Judith A'" <JSlein@crt.xerox.com>, "Masinter, Larry <masinter@parc.xerox.com>" <masinter@parc.xerox.com>, Jim Davis <jdavis@parc.xerox.com>, w3c-dist-auth@w3.org
Cc: "Henrik Frystyk Nielsen (E-mail)" <frystyk@w3.org>, "Roy T. Fielding (E-mail)" <fielding@ics.uci.edu>, "Jim Whitehead (E-mail)" <ejw@ics.uci.edu>, "Henry Sanders (Exchange)" <henrysa@exchange.microsoft.com>
<Skipping to the End>
This one is long and complex so I will just skip to the end. Instead of
using one method for one function, WebDAV essentially re-invented the HTTP
request/response format in XML so it could glue a bunch of requests
together. The reasons it did this are:

1) Pipelining - It is and isn't supported in HTTP/1.1. Currently HTTP/1.1
bans the use of non-idempotent methods across pipelining. This is fine for
PROPFIND but a real problem if we are to use PROPPATCH in pipelined mode.
Even so, the reality is that WebDAV will have to run primarily across
HTTP/1.0 connections for a real long time due the relatively sparse support
for HTTP/1.1 proxies. So we wanted to pack a bunch of related messages into
a single request. Hopefully, when HTTP/1.1 proxy support is sufficiently
wide spread, this motivation will go away.

2) Byte Bloat - In many cases multiple method calls on the wire will be
almost identical. For example, imagine if PROPFIND did the right thing and
only allowed you to request a single property. Now imagine you wanted to
retrieve 50 properties. You would end up with 50 requests that were
completely identical except for one string. There was a sticky header
proposal but WebDAV makes extensive use of the body so we would also need a
sticky body/diff proposal as well.

3) Relatedness - Every time a request comes in a server has to roll out a
new context to handle that message. However if you are going to get a bunch
of related requests, say 50 PROPPATCHs, each wanting to change a single
property, you would rather just keep re-using the same context. But because
of the shear volume of messages coming in, a server wants to be told if a
context should be re-used. WebDAV achieves this by allowing methods to
implicitly contain multiple requests, like PROPFIND letting you set 50
properties at once. We need a way in HTTP to do the same without having to
re-invent HTTP, as WebDAV has had to do. We need to tell the server "the
following requests are all related".

4) Unmatched Responses - There are times when a method needs to apply across
an unknown number of resources. For example, performing a COPY with a depth
header. You don't know which resources existed in the hierarchy when you
executed the request. However you still need to get back responses for any
resources that may have had a problem with the COPY, even though you never
even knew they existed. WebDAV deals with this by using the multistatus
response which just wraps up all the information. Of course, multistatus is
really just the HTTP response format in XML. What would be better is if HTTP
supported the ability to send responses w/out associated requests. In order
to keep bytes down you would also then need to be able to apply the "Byte
Bloat" solution to the responses. Bonus points are that the folks in
notification land also need this feature.

So if HTTP/1.1 was enhanced or HTTP/1.2 was introduced with support for
features 1-4 then WebDAV could get out of the protocol
bloating/extensibility killing business of re-inventing HTTP and instead
actually use HTTP secure in the knowledge that the performance it so
desperately needs will be there.

	<Side Note>
I can hear Henrik in my head. Unfortunately I hear him in Danish so I can't
understand a word he is saying. But knowing Henrik I suspect it is something
like "Repent ye sinners, HTTP NG is the one true way." True or not, I for
one really don't want to throw away an entire invested base. Especially
given that I think the previous features, with a little creative use of the
BATCH method, could be built over the existing infrastructure.
	</Side Note>
</Skipping to the End>

<Why I Bother>
And now, for the two case studies which lead to the previous conclusions. If
you actually make it to the end, you clearly have way the hell too much time
on your hands. I'm writing these out mostly so:
	A) I don't forget
	B) I can beat people over the head with them when they try to make
the same damn stupid fool mistakes
</Why I Bother>

<"This is another fine protocol you've gotten me into!">
In a previous episode
(http://lists.w3.org/Archives/Public/w3c-dist-auth/1998OctDec/0074.html) I
provided some history behind WebDAV's property architecture. However once
the basic architecture was agreed upon there was still much work to be done.

The first issue was the ability to set the value of multiple properties in a
single atomic act. It was assumed by the working group that multiple
dependent properties would exist on a resource. That is, that the value of
one property would be related to the value of another. Thus if a client
wishes to change both values and only one actually got changed the resource
would be in an inconsistent state. Hence it was critical that a mechanism be
provided to allow clients to set multiple property values atomically. In
retrospect I wonder how critical this requirement really was. Especially
given that none of the existing implementations are really atomic, at least
not in the transacted sense meant in the spec. But, at the time, we were
positive that this requirement was critical and so it ended up in the spec.

In trying to fulfill the requirement my primary concern was to keep the
methods simple. I have always liked slogans and I vaguely remember inventing
"one method, one property". This certainly would result in a simpler
protocol. But if each property was sent in a different method, how would we
meet the atomicity requirement? One possibility was to use a protocol like
TIP to implement real transactioning. But this was clearly too heavy a
requirement for base DAV servers. The next proposal was to use an "atomic"
method. The idea was that one would send the atomic method followed by the
property change methods followed by another atomic method. However there
were two problems:

	1) HTTP servers were generally designed to handle each request in
total isolation. Each request, even over a pipelined connection, was handled
by a different execution context. Thus it was very difficult for the servers
to associate requests with each other. Even if the servers could associate
the requests, this still left the relatively high cost of having to create a
brand new context for each and every method. The message from the server
community was crystal clear - the less requests across the wire, the better.
This was especially true given that the server community was not ready to
completely redesign their servers to do better context handling.

	2) Even though WebDAV is officially an HTTP/1.1 protocol, in reality
everyone knew it was going to spend the first few years of its life running
over HTTP/1.0. This meant there was no pipelining support. Thus every
property to be set, in addition to the "atomic" methods, would have to be
sent on their own, independent, round trips. The performance ramifications
were too horrific to consider.

"one method, one property" was still not quite dead. A proposal had been
kicking around for a BATCH method. The idea was that one would take a group
of methods, glue them together, shove them inside of the BATCH method and
send them to the server. The atomic nature of the requests would be implicit
to the BATCH. There was even a proposal to throw in an "atomic" switch so
that it would be possible to not ask for atomic behavior. The assumption
being that outside of certain predefined contexts such as a group of
PROPPATCHs, servers wouldn't be able to support atomicity.

In retrospect I wish I had given this choice more attention, it would have
resulted in a significantly simpler protocol. Alas, I helped to lead the
effort to kill BATCH. The argument was that BATCH essentially tunnels
through HTTP. It robs HTTP of its caching and firewalling features. These
arguments are true, but the real choice was amongst greater and lesser
evils. Which was worse, to try and fix some very serious performance issues
in HTTP in a backwards compatible manner or to make WebDAV about a factor of
2 more complex?

With both separate methods and BATCH considered unacceptable the "one
method, one property" philosophy was dead. The only remaining solution was
to create a composite method inside of PROPPATCH. I don't remember if this
solution coincided with the introduction of multistatus for
COPY/MOVE/PROPFIND/PROPPATCH results but I wouldn't be surprised. They both
suffer from the same illness, they essentially re-invent BATCH but for only
a single method and in a completely non-reusable way. That is, if we had
actually worked the bugs out of BATCH we could have used it as essentially a
"transport" and run our methods over it. The composite forms used in
PROPFIND/PROPPATCH and multi-status, on the other hand, end up having to
re-invent most of the HTTP features and to keep on re-inventing them over
and over again for each method.

From there it was an obvious step to adding the same composite support for
PROPFIND. Everyone was ecstatic about both methods. The property store guys
could implement the properties easily, the server guys could process the
messages and responses efficiently and the client guys could do all their
business in one round trip. What could possibly be better? I would find the
irony funny if I wasn't responsible for so many of the bad decisions. 
</"This is another fine protocol you've gotten me into!">

<Out of our Depth>
The "one method, one property" slogan got some more play as the "one method,
one action" slogan a.k.a. the Depth Wars. The idea was a simple one. People
like to COPY/MOVE/DELETE using the URL hierarchy. That way you can say
"Delete this resource and all its children" without even having to know who
the children are. The proposed solution to allow this very cool
functionality was the depth header. Just slap it on a request and it will
apply to that resource and all the children of that resource to the
specified level. Very neat.

But there was a lone voice which objected. The voice belonged to Henry
Sanders. Even though most of the people working on WebDAV had no idea who
Henry was, he still got a lot of respect. As it happens Henry is one of the
founding members of the HTTP WG and basically is "the" DEV for the HTTP
stack in IIS. Before writing IIS's HTTP stack he also wrote Microsoft's
TCP/IP implementation. So if you do know who Henry is, you listen to what he

Henry felt that the depth header unnecessarily complicated the protocol. He
felt that people were trying to build their UI in the protocol, one of the
cardinal WebDAV no-nos. But most importantly, he felt that the depth header
was essentially a form of denial of service attack. His argument went as

	1) File systems have absolutely no support for depth operations.
When you execute that recursive copy in Windows or MAC or UNIX, the copy
program is actually talking to the OS and doing a file by file copy. That
was what Henry meant by UI. Recursive copy is a fiction created by UI.

	2) Each individual copy/move operation takes a long time. To put
this time into context, serving a single non-recursive copy/move request can
take longer than a full round trip between the client and server. Now
imagine how long it would take to do all the requests for the whole tree in
one go.

	3) Servers can only contain a certain number of message contexts at
a time. By requiring the server to keep a context open and running for the
enormous length of time needed to perform a recursive operation we were
denying service to everyone else. Put another way, a server can only handle
N simultaneous requests. A recursive operation takes so damn long that for
all intents and purposes a full context is assigned to a single user.
Meaning that all the other users are forced to share the remaining contexts.
If, on the other hand, the client had to make a separate request for each
copy/move (meaning no depth header support) then the client's request would
be "spread out" over a period of time and everyone else would get a fair
chance at the server's resources.

	Therefore - The depth header should not be supported. Instead the
client should perform a look up of the name of all the children of the
current resource. Then the client should issue a series of COPY requests for
all those children. Pipelining would keep down the round trip costs and it
is worth the extra context costs on the server because separating the
messages guaranteed equal access to the server's resources by all clients.

The counter voices belonged mostly to people who didn't use the file system
as their stores or had extremely advanced file systems. These stores could
do things like copy/move on write/read. The server would leave itself a note
saying that "resource a/b was copied to a/c." So long as no one made any
changes to a/b the server would service any requests for a/c's value from
a/b. Stores could do this for entire hierarchies using a single note to
themselves. Thus these stores could handle depth requests extremely
efficiently. However if the requests were broken up into pieces, thus
loosing the depth header semantics, then these powerful features were
essentially useless.

The debate went on for over six months. It went on this long mostly because
it was intertwined with the COPY/MOVE debate
There were meetings, there were discussions, there were e-mails, it just
kept on going around. Both sides were right, how did you decide? In the end
Henry gave in, you will have to ask him why. Without Henry holding up the
"Depth is a bad idea" side, the Depth header won by default.

Which brought up a problem. HTTP's request/response format is based on the
idea that a request goes to a single resource and the response is scoped to
that resource. But in this case the request went to a single resource but
applied to any number of resources. So we ended up re-inventing HTTP's
response format inside of the XML multistatus response. It never occurred to
anyone to have unmatched HTTP responses with sticky header/body. The
were well over by this point so once the Depth header got the o.k.
multistatus was a done deal.
</Out of our Depth>
Received on Thursday, 24 December 1998 03:01:08 UTC

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