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TAG input to EXI WG on Efficient XML Interchange and EXI Measurements

From: Henry S. Thompson <ht@inf.ed.ac.uk>
Date: Thu, 29 Nov 2007 19:34:59 +0000
To: public-exi@w3.org
Cc: www-tag@w3.org
Message-ID: <f5b4pf4j15o.fsf@hildegard.inf.ed.ac.uk>

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Following on from the many useful discussions at group and individual
level which took place during the recent TPAC week, the TAG has
arrived at the following requests and recommendations for the
Efficient XML Interchange Working Group:

1) Making the case for EXI

On the face of it two propositions need to be confirmed in order for
the proposed EXI Format to go ahead to REC:

  1a) The proposed EXI format is the best available choice for the job;
  1b) The performance of the proposed EXI format makes it worth going

The _Efficient XML Interchange Measurements Note_ has a huge amount of
data in it, but it is _very_ difficult to digest.  On the face of it,
it is difficult to determine the extent to which it confirms either
(1a) or (1b).  A detailed set of comments on this document is provided
at the end of this message.

What is needed is an summary analysis of the measurement results which
specifically addresses (1a) and (1b) above.  With respect to (1a),
this would presumably have something like the following form:

 EXI is better than GZIP because, as shown in tables xxx and yyy, it
 produces comparable compression with zzz% less time to compress and
 ZZZ% less time to decompress and parse for documents in classes P, Q
 and R;

 EXI is better than XMill because. . .

 and so on.

With respect to (1b), as the TAG suggested in its feedback [1] on the
outputs of the XML Binary Characterization WG, "concrete targets
should be set for the size and/or speed gains that would be needed to
justify the disruption introduced by a new format".  It's too late to
set those targets in advance -- what is required now is that a small
number of key use cases be identified, the expected benefits of EXI
for those use cases be quantified, and concrete evidence cited that
those benefits are a) not liable to be seriously eroded by the onward
rush of Moore's law and friends and b) sufficient to make adoption
likely, bearing in mind that EXI is unlikely to be widely available
with a W3C-approved REC behind it for several years.

The use-case we have heard most commonly nominated to fill this role
is delivery of data-intensive web-services to mobile devices, where
computationally intensive processes place heavy demands on power
supply, and battery technology is _not_ advancing at anything like
Moore's law rates.  Documenting and quantifying this story, if true,
would be a very good start.  Statements of requirements from potential
deployers would be particularly useful, that is, getting a mobile
device manufacturer to say "We will deploy service X on devices of
class Y iff we can get Z amount of XML data off the wire and into
memory in less than M milliseconds using an N MHz processor.  The best
we project with current technologies is MM msec using an NN MHz
processor", accompanied, of course, by a demonstration that EXI could
actually meet that requirement.

2) Positioning EXI going forward

The value of XML interoperability is enormous.  If EXI goes forward,
it must do so in a way which does not compromise this.  The WG needs
to take every opportunity to get the message across that EXI is
primarily aimed at a set of core use-cases, where one-off binary
formats and/or lower-performance generic compression techniques are
already in use.  _No_ aspect of the public presentation of EXI should
suggest that generic XML tools will or ought to evolve to read or
produce the EXI format.

If the EXI WG agrees with this perspective on positioning, that TAG
will be happy to assist in any way it can to promote it.  If the EXI
WG does _not_ agree, further discussion is needed urgently to try to
find a position which both parties can agree to.

- -------------

Detailed comments on the Measurement documents

The 2005 TAG message called for testing to involve "best possible
text-based XML 1.x implementations", and the XBC's call for
implementations [2] specifically called for only XML parsers, not for
the surrounding application or middleware stacks, JDKs or Java Virtual
Machines.  The benchmarks have not been against the best possible
text-based XML 1.x implementations.
The measurements document acknowledges the issue of stack integration
in "Stack integration considers the full XML processing system, not
just the parser. By selectively combining the components of the
processing stack through abstract APIs, the system can directly
produce application data from the bytes that were read. Two prominent
examples of this technique are [Screamer [3] and [EngelenGSOAP
[4]]. Both of these can also be called schema-derived as they compile
a schema into code. However, neither simply generates a generic
parser, but rather a full stack for converting between application
data and serialized XML. This gives a significant improvement compared
to just applying the pre-compilation to the parsing layer."  But
neither of these prominent examples appears in the test data.
Further, there were no "real-world end to end" use cases tested, such
as a Web service application, a mobile application, etc.  Thus we do
not know the overall effect of any particular technology on overall
application performance.

The measurements document states "To begin with, the XBC
Characterization Measurement Methodologies Note [5] defines thresholds
for whether a candidate format achieves sufficient compactness " [5].
But in the XBC Characterization Measurement Methodologies Note [5]
itself we find no threshholds: "Because XML documents exist with a
wide variety of sizes, structures, schemas, and regularity, it is not
possible to define a single size threshold or percentage compactness
that an XML format must achieve to be considered sufficiently compact
for a general purpose W3C standard."
We attempted to determine the differences between Efficient XML and
Gzip but found the methodology confusing.  The measurements document
specifies that "In the Document [...] and Both [...]  classes,
candidates are compared against gzipped XML, while in the Neither
[...] and Schema [...]  cases, the comparison was to plain XML".
Examining Document and Both compactness graphs, Gzip appears to offer
improvements over XML that track the other implementations, with the
noteworthy point that Efficient XML's improvements over Gzip are
significant in a significant part of the Both chart but similar in the
Document.  Examining Processing Efficiency graphs, it appears as
though XML is clearly superior in Java Encoding in Document and Both.
GZip appears further inferior but yet all solutions vary wildly around
XML in Decoding Document and Both.  A worrying statement is "An
interesting point to note in the decoding results is the shapes of the
graphs for each individual candidate. Namely, these appear similar to
each other, containing similar peaks and troughs. Even more
interestingly, this is also the case with Xals, indicating that there
is some feature of the JAXP parser that is implemented suboptimally
and triggered by a subset of the test documents."  The measurements
document states "For instance, preliminary measurements in the EXI
framework indicate that the default parser shipped with Java improved
noticeably from version 5.0 to version 6, showing 2-3-fold improvement
for some cases.", and the measurements used JDK 1.5.0_05-b05 for Java
based parsing and JDK 1.6.0_02-ea-b01 for native.  Perhaps an improved
JDK, Java Virtual Machine, or virtualized JVM would further improve
results.  These leads us to wonder whether a combination GZip with
improved technologies such as Parsers, JDKs, VMs, or even Stack
Integration technology (that is Schema aware and hence covered under
Both and Schema) would suffice for the community.
Examining the data sets used, there are a number of military
applications (ASMTF, AVCL, JTLM) and yet comparatively few generic
"Web service" applications.  The Google test suite lists Web services
for small devices and Web services routing; the Invoice test suite
lists Intra/InterBusiness Communication which immediately limits its
scope to "A large business communicates via XML with a number of
remote businesses, some of which can be small business partners. These
remote or small businesses often have access only to slow transmission
lines and have limited hardware and technical expertise."; and there
is a WSDL test suite.  This seems to avoid the "common" Web service
case of the many Web APIs provided by hosted solutions like Google,
Yahoo, Amazon, eBay, Salesforce, Facebook, MSN, etc.  Examining the
test data shows that the Google test cases used 5 different test cases
(0,7,15, 24,30) which includes 1 soap fault (case #24).  There are 2
AVCL, 5 Invoice, 8 Location Sightings, 6 JTLM, 5 ASMTF, 2 WSDL test
cases as well.  There appears to be broad based coverage of each,
though the rationale for the various weightings aren't documented.
For example, why 4 Google "success cases" and 2 WSDL cases?  Surely
there are more than 2 times as many SOAP messages than WSDL messages
being sent around the internet.

Henry S. Thompson
on behalf of the TAG

[1] http://lists.w3.org/Archives/Public/public-xml-binary/2005May/0000.html
[2] http://lists.w3.org/Archives/Public/public-exi/2006Mar/0004.html
[3] http://www.w3.org/TR/2007/WD-exi-measurements-20070725/#ref-screamer
[4] http://www.w3.org/TR/2007/WD-exi-measurements-20070725/#ref-Engelen-gsoap
[5] http://www.w3.org/TR/xbc-measurement/
- -- 
 Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
                     Half-time member of W3C Team
    2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
            Fax: (44) 131 650-4587, e-mail: ht@inf.ed.ac.uk
                   URL: http://www.ltg.ed.ac.uk/~ht/
[mail really from me _always_ has this .sig -- mail without it is forged spam]
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Received on Thursday, 29 November 2007 19:35:32 UTC

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