The advantages of Web services reliable message delivery for e-Commerce
and e-Business has been well documented by IBM, Microsoft and others.
An excellent whitepaper on this topic can be downloaded from msdn.microsoft.com/library/default.asp?url=/library/en-us/dnglobspec/ht ml/ws-rm-exec-summary.asp
However, there has been almost nothing published on how Grid data,
control and management messages will be reliably delivered to one or
more endpoints [over a Grid network]. A GRIDtoday article by
this author noted that WS Reliability was being used in the Japanese
government sponsored Business Grid Project, for event notifications.
Please refer to www.gridtoday.com/04/1115/104252.html
No other announcements have been forthcoming on use of WS reliable
message delivery for Grids. We do know that Grid networks are different
from conventional Intranets and Extranets, but what's so special about
Grids? How do they differ from previous distributed systems? Franco
Travostino of Nortel, chair of the GGF Grid High Performance Network
Research Group (GHPN RG), provided some answers at last October's
- Grid user knows of a resource pool; the pool (but not its
constituents) knows about the user. However, there is not a prior
knowledge of specific resources by the user.
- Capability to "gang-schedule" a subset of a resource pool (CPU, storage, sensors, etc.).
- Grids may straddle across administrative boundaries, trust
boundaries and large distances (Editor's Note: early deployment of
enterprise Grids is usually contained within a single administration
and trust cloud. This is because Grid security has not been
standardized yet and is dependent on WS Security roadmap of IBM-MSFT).
- Dynamic matchmaking in time space between "virtual organizations" and resource pools.
The complete article can be read at news.taborcommunications.com/msgget.jsp?mid=298965&xsl=story.xsl
Are Web Services Relevant for Grids?
Some Grid developers think that Web services are not needed at all and
that TCP/IP will be sufficient for reliable message delivery, perhaps
augmented by compression, acceleration or some type of proprietary
"throughput boosting" network solution. In sharp contrast, the GGF OGSA
WG long ago defined a Grid service as a Web service (implying SOAP/XML
encoded messages) with certain additional properties. The GGF
recommends use of IPv6 for its expanded address space, but has not yet
taken a position on the Transport protocol. While most assume it will
be TCP, there are issues and concerns about that protocol in a Grid
networking environment, due to low throughput when different packet
sizes are mixed over a single network interface. Indeed, the GGF High
Performance Network Research Group is exploring alternative Transport
layers. The GGF OGSA WG should be defining all Web services
specifications that are part of the Grid infrastructure, but progress
on this front has been very slow. Consider that all Web Services
messages are SOAP/XML encoded, which significantly expands the original
(un-encoded) message size and further degrades end to end throughput.
Also note that certain application data (e.g., real time video/graphics
or floating point numbers) can not be well-represented as SOAP/XML
streams. So, the jury is still deliberating how and where Web services
will be used for Grids.
Hence, we are left with many unanswered questions as to what role Web
services and reliable messaging play in a Grid environment:
- Should Grid application data messages use SOAP/XML encoding and
be represented as a Web service? If so, should some or all of those
messages use WS reliable messaging?
- What WS format should application data messages take (e.g., as
user data in the SOAP message body or as a minimal SOAP with
[un-encoded] attachments message)?
- What about Grid control, management and security messages? [Here,
most GGF participants assume the use of Web services and emerging Web
services standards (e.g., WS Security, WS Distributed Management, WS Resource Framework and WS Notification).
All of these WS protocols will need to be enveloped in some type of
reliable messaging SOAP header and "composed" with one of two competing
WS Reliable Messaging specifications -- see below].
- Consider that there are two very mature reliable message delivery
specifications for Web services (WS Reliability and WS Reliable
Messaging) and two OASIS TCs working to progress the standardization of
each one of these separately (WSRM TC has standardized WS Reliability,
while the new WSRX TC will standardize WS Reliable Messaging and WS
Policy Assertions). Which version/set of specs should be supported when
there is a need for WS reliable message delivery in a Grid environment?
Can both of these specifications co-exist or will WS-Reliability be
usurped and made obsolete by the WSRX specs?
What Type of WS Reliable Messaging (assuming Web services provide the basic Grid infrastructure)?
Let's look a bit deeper at this last conundrum. First, let's consider the WS Reliable Messaging
spec. First published in March 2003 (coincidentally, this is when the
WSRM TC was created by a different set of vendors), the WS Reliable
Messaging spec is co-authored by IBM, Microsoft, BEA and TIBCO. It now
has very wide industry support as evidenced by the three interop events
where implementations of this spec were tested. The interop events also
involved "composability" testing of WS Reliable Messaging with
WS-Addressing (now submitted to W3C), WS-Security (an OASIS standard)
and, most recently, WS-Secure Conversation and WS-Trust (these latter
two specs have not yet been submitted to a standards body).
To read about the third of these interop events (April 2005), which
tested implementations of WS Reliable Messaging composed with WS Secure
Conversation, please refer to news.taborcommunications.com/msgget.jsp?mid=363854&xsl=story.xsl
Note that WS Reliable Messaging
depends on WS Policy
, which has not yet been submitted to a standards body and has not yet been tested for interoperability.
In February 2005, the WS Reliable Messaging spec was revised. The spec,
along with an abstract, XML Schema and associated WSDL, may be
downloaded free from www-128.ibm.com/developerworks/Webservices/library/specification/ws-rm/
or from a vendor neutral site (no abstract) at schemas.xmlsoap.org/ws/2005/02/rm
On May 3, a call for participation for the new WSRX TC was announced by OASIS. It may be referenced from xml.coverpages.org/ni2005-05-04-a.html
Forty-one individuals identified as "WSRX TC co-proposers" have already
agreed to support the work of the new TC, representing at least 28
corporate institutions. These include: ACORD, Actional, Adobe, Arjuna,
BEA Systems, Blue Titan, Choreology, Entrust, Ericsson, *Hitachi, IBM,
IONA, Microsoft, *NEC, Nortel, Novell, OAGi, *Oracle, Reactivity, SAP,
*SeeBeyond, *Sonic Software, *Sun Microsystems, Systinet, TIBCO, United
Kingdom e-Government Unit, The University of North Carolina at Chapel
Hill and WebMethods.
* These companies were also proponents of the WSRM TC and the WS-Reliability standard.
From the "coverpages" link above:
The WS-RX TC will continue development of the (BEA Systems, IBM, Microsoft and TIBCO Software) Web Services Reliable Messaging
specification (WS-Reliable Messaging) submitted to the TC. The defined
mechanism by which Web services express support for reliable messaging
and related useful parameters "will be based upon the Web Services Reliable Messaging Policy Assertion (*WS-RM Policy Assertion)
specification," also to be submitted to the TC. The WS-RX work will
be designed to compose
with the WSS (Security) TC specifications and will utilize the
WS-Addressing (W3C version) functions where appropriate, and avoiding
the creation of overlapping functions.
* assumed to be a subset of WS-Policy spec: schemas.xmlsoap.org/ws/2005/02/rm/policy
Many approaches have been taken to reliable message transfer. The WS-RX
TC Call for Participation includes a discussion of related work, which
recognizes the OASIS Web Services Reliable Messaging TC and the OASIS
ebXML Business Process TC (which developed the ebMS spec that uses WS
Reliability and WS Security). The WSRM TC, although it has a goal
similar to that of WS-RX, "has a fundamentally different view with
regard to scope of the specification, required functions, policy and
Web services architecture composability."
Some members of the WSRM TC might disagree with the quote above.
They would suggest that the only key difference here is that policy is
embedded in WS-Reliability protocol (or the associated WSDL file) and
does not require a separate WS Policy specification. In particular,
WS-Reliability defines an abstract contract that can be bound to
different representations. The QoS required for reliable message
delivery (exactly once, duplicate elimination, or sequential/ordered
delivery) may be specified via the WS-Reliability protocol, when sent
to the recipient (or consumer) of the WS reliable message. This would
allow the policy or agreement to be deployed on the client side only,
while the Web Service endpoint would advertise reliable message QoS
capabilities (e.g. through the associated WSDL file). The QoS can also
be specified on a per message basis.
One key difference in spec functionality is that WS Reliability includes a Polling capability
while WS Reliable Messaging does not. Polling enables reliable
messaging protocol to reach WS endpoints behind a firewall (that would
otherwise block Request/Response message exchanges).
[Note: Chris Ferris, a very knowledgeable and experienced Web services
architect from IBM, takes the view that while polling is necessary, it
is a generic function that is orthogonal to reliable message delivery. We wonder if any user behind a firewall would be willing to send and receive unreliable messages
With respect to composability, the views seem to be consistent and not
at all different. NEC and Fujitsu had previously proposed an interop
test of WS Reliability composed with WS Security
. This has the
same basic goal as the interop testing of WS Reliable Messaging with WS
Secure Conversation (that spec is not part of the WSRX TC charter).
Again, kindly refer to news.taborcommunications.com/msgget.jsp?mid=363854&xsl=story.xsl
Quick Take on WS Reliability
WS Reliability defines three ways for the receiver to send back an
Acknowledgment message or a Fault message to the sender. These are
referred to as the "RM Reply patterns," which are defined as follows:
- Response RM-Reply Pattern
We say that a Response RM-Reply pattern is in use if the
outbound Reliable Message is sent in the underlying protocol request,
and the resultant Acknowledgment message (or Fault message) is
contained in the underlying protocol response message which corresponds
to the original request. In essence, the Acknowledgement is
"piggybacked" onto the business response message.
- Callback RM-Reply Pattern
We say that a Callback RM-Reply pattern is in use if the
Acknowledgment message (or Fault message) is contained in an underlying
protocol request of a second request/response exchange (or a
second one-way message), operating in the opposite direction to the
message containing the outbound Reliable Message.
- Polling RM-Reply Pattern (not included in WS Reliable Messaging)
We say that the Polling RM-Reply pattern is being used if a
second underlying protocol request is generated, in the same direction
as the one containing the outbound Reliable Message, to act as a
"request for acknowledgment." The Acknowledgment message (or Fault
message) is contained in the underlying protocol response to this
request. This polling pattern can be used in instances where it is
inappropriate for the sender of reliable messages to receive underlying
protocol requests (e.g., the sender behind a firewall).
These three reply patterns provide "the users" with flexibility to send
reliable request/response or one-way SOAP messages (Callback and
Polling patterns). Callback is important for one-way request message
patterns and for batching of acknowledgements and fault messages. We
have already mentioned the importance of Polling to reach a WS endpoint
behind a firewall. Otherwise, the two reliable message delivery specs
have very similar functionality (WS Reliable Messaging has a few
What About Implementations of these Specs?
WS Reliability has an open source implementation, developed by the
three Japanese companies (Fujitsu, Hitachi, NEC) involved in the
Business Grid Project. Referred to as "Reliable Messaging for Grid
Services or RM4GS," it was supported by the Japanese Ministry of
Economy, Trade, and Industry (MITI). Please refer to www.adtmag.com/article.asp?id=10291
** There is no current plan in the Japanese business Grid project to
replace WS-Reliability with WS-Reliable Messaging or any other WSRX TC
In addition to the RM4GS, several vendors have developed their own
implementations of WS-Reliability. Two of these are believed to be
Fujitsu Software USA and Oracle Corp.
WS Reliable Messaging also has an open source version. It is being
developed by an Apache group based in Sri Lanka. To be sure, WS
Reliable Messaging has been implemented by many more vendors than
WS-Reliability, as evidenced by double digit participation in the
interop events. There are also Internet endpoints available from IBM,
Microsoft, BEA, Systinet and others for more informal interop testing
with emerging implementations.
But which one of the two specifications will be more relevant for
Grids? Even more important, what role will WS reliable message delivery
play in the Grid infrastructure/ecosystem? We have no answers here and
look to GGF for guidance.
The author thanks Jacques Durand of Fujitsu and Chris Ferris of IBM for
their value contributions and clarifications made in the review and
editing of this article.
About Alan J. Weissberger
Alan J. Weissberger is actively seeking clients in need of his
expertise in the telecommunications field. If you would like to speak
personally with Alan about how he could help your company, feel free to
contact him via e-mail at email@example.com
. To learn more about his extensive qualifications, read his annotated biography below.
As the founder and Technical Director of Data Communications Technology
(DCT), a technical consulting firm started in March 1983, Alan J.
Weissberger specializes in telecommunications standards and their
implementation. His clients have included network providers (AT&T,
NTT, Pacific Bell, US West, Entel and CTC in Chile, Telkom South
Africa, Moroccan PTT, others), equipment and semiconductor
manufacturers, and large end users. In 1995 and 1996 Alan was the
principal architect for the European Commission's multi-service,
multi-country ATM network -- the largest private network in Europe
(that network has now evolved into Gig Ethernet over CWDM). In 2000-01,
he was Ciena's lead ITU-T delegate, contributing to the standardization
of the optical control plane in SG13 and SG15. Alan now represents NEC
Corp in several OASIS TCs dealing with Web Services, while also
attending the Global Grid Forum and the Optical Internetworking Forum
To read his entire biography, please visit www.gridtoday.com/04/1011/bio.html