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© IBM Corporation OASIS Symposium: Reliable Infrastructures for XML Critical Comparison of WS-RM and WS-R April 27, 2004 Christopher Ferris Senior Technical Staff Member IBM Emerging Technology
© IBM Corporation Agenda Common features What's different? Conclusions Resources
© IBM Corporation Common features Both are SOAP-based protocol extensions Both provide same delivery assurance semantics –At-Least-Once –At-Most-Once –Exactly-Once –Ordered delivery Each message has a group identifier and unique sequence number –WS-R has MessageId –WS-RM has Sequence –Both are roughly equivalent Both support "piggy-backing" for bi-directional reliability
© IBM Corporation Common features (continued) Each has some form of acknowledgement –Each provides for a range of messages to be acknowledged Each can survive loss of acknowledgement messages –Although it isn't abundantly clear that there is any guarantee of survival on part of WS-R –WS-R appears to have no requirement that all messages in a group be acknowledged in each SequenceReplies: WS-R - "This element MUST contain the values of the original MessageIds of the messages delivered for a group, and for each Fault Code being reported, the MessageIds of messages which encountered the particular Fault Code." WS-RM - "Every acknowledgement issued by the RM Destination MUST include within an acknowledgement range or ranges the sequence number of every message successfully received by the RM Destination and MUST exclude sequence numbers of any messages not yet received."
© IBM Corporation What's different Complexity levels Processing model Nack Composability –Addressing, Policy Implementation details Efficiency of resource reclamation
© IBM Corporation Complexity WS-R is 78 pages as compared with WS-RM's 40 WS-R has 62 occurances of the word "if", WS-RM only 22 –Much of the conditional language in the WS-R spec relates to implementation detail which should be out of scope of a protocol specification WS-R has two versions of its schema; one for SOAP1.1 and another for SOAP1.2 –Why? This means that there are effectively two versions of the protocol just to accommodate the soap:mustUnderstand attribute? WS-R seems to have invented its own fault mechanism rather than building on SOAP Fault It is unclear that WS-R offers any guarantee of correctness
© IBM Corporation Example WS-R Request Header T03:01:03-03:50 Response
© IBM Corporation Example WS-RM Sequence Header 10
© IBM Corporation Example WS-R Response Header
© IBM Corporation Example WS-RM SequenceAcknowledgement
© IBM Corporation Processing model WS-R acknowledges messages only after they have been delivered to the application –IMO, this completely misses the whole point –What if the application is off-line or busy processing other messages? Will the sending RMP keep resending just because the receiving RMP has not delivered messages to the application? WS-RM acknowledges receipt of a message –Preserves separation of concerns; an application level acknowledgement needs to be an application-level message e.g. equivalent of an EDI 855 PO Acknowledgement –RM destination takes responsibility for delivery to application and acknowledges receipt immediately
© IBM Corporation WS-RM Processing Model
© IBM Corporation Nack WS-RM added a Nack with the express purpose to allow an RM Destination to request that the RM Source resend a message WS-R provides for faults which could possibly be leveraged to this effect –Not clear why a source needs to know why, only adds to complexity
© IBM Corporation Composability WS-R Message Reply Patterns –Sprinkles in addressing aspect that really should be orthogonal –Not at all clear that WS-R can effectively be composed with WS-Addressing WS-RM designed for composability with other WS-* specs –While WS-RM depends upon some form of addressing, the protocol itself is completely independent. Furthermore, WS- Addressing is not dependent upon WS-RM –Can be composed with WS-Policy*, WS-Security*, …
© IBM Corporation Implementation details WS-R imposes requirements on implementation. e.g. "NOTES: Given the above definition of ExpiryTime, in case Duplicate Elimination is required, when a received message is processed, it is sufficient to only check for its duplicates among MessageIds of past messages that have not expired yet at the time of the duplicate check." "Section 5 Operational Aspects and Semantics" –Implementation detail/advice has no place in a protocol spec WS-RM defines the protocol exclusively in terms of what appears "on the wire" –Provides more flexibility in potential use cases for WS-RM
© IBM Corporation Efficiency of resource reclamation WS-R depends upon synchronization of clocks for resource reclamation WS-RM adds capability for destination to allocate Sequence identifiers –CreateSequence, TerminateSequence operations defined –If RM Destination receives Sequence/Identifier that it does not recognize, then it can be assured that it can safely disregard the message –RM Destination can reclaim resources associated with a Sequence as soon as it receives TerminateSequence message –RM Destination can reclaim resources after Sequence expires even if it never receives the TerminateSequence message
© IBM Corporation Conclusions Similar if not identical requirements Similar feature set Different approaches Time will tell which specification the market prefers
© IBM Corporation Resources WS-Reliable Messaging WS-Reliability Reliability pdf
© IBM Corporation Thank you!
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