WS-CoL: another domain-independent policy constraint language

[Anne Anderson <Anne.Anderson@sun.com>]

Colleagues,

This group may be interested in a recently published paper, "WS-Policy 
for Service Monitoring" [1], which proposes another domain-independent 
policy assertion language, WS-CoL (Web Services Constraint Language), 
for use with WS-Policy.  It is specifically designed to be used in 
monitoring WS-BPEL processes.  The authors imply that it is not 
proprietary, but there is no pointer to a language specification or 
other background documentation.

I think this is interesting work, and bears further study.  I will 
invite the authors to participate in dipal-discuss.

A few notes and points of comparison:

- The authors do not address the problems of policy and assertion 
intersection, assuming that the policy they are monitoring is the set of 
effective assertions for the process instance being monitored.  This is 
the major drawback to this effort for our purposes.

- The language and its implementation are integrated with WS-BPEL and 
with WS-Policy, and the monitoring code is automatically generated.  I 
think it is worth studying how the integration with WS-BPEL is done.

- The language binds the variable data retrieval mechanism fairly 
tightly to the language syntax by using links to WSDL specifications for 
a service interface that will provide the required data.  In XACML, this 
binding is done indirectly in a separate component called the Context 
Handler.  The XACML Context Handler could be written to associate WSDL 
links and operations with various XACML "Attributes", and I think this 
is an interesting approach.

- The language so far supports only integer, string, and Boolean data 
types.  They use "XlinkIt" [2] and "CLiX" [3] to implement the language.

- The language does not seem to handle problems like multiple nodes 
satisfying a single XPath expression.  It appears to assume there will 
always be exactly one value retrieved per data reference.  CLiX [3], 
however, may include more complex expressions than those shown as 
examples in the paper.

- In an example, they translate a WS-SecurityPolicy Assertion into 
WS-CoL.  Based on our experience with WS-PolicyConstraints, I think it 
will not be possible to automatically generate monitors for all 
WS-SecurityPolicy Assertions as not all can be verified in the message 
itself.  They also do not deal with the problem of multiple Assertions 
that might affect the construction of the same message; it appears that 
their approach would end up generating two separate forms of the same 
message.

References:

[1] "WS-Policy for Service Monitoring", by L. Baresi, S. Guinea, and P. 
Plebani of the Dipartimento di Elettronica ed Informazione,Politecnico 
di Milano, 6th VLDB Workshop on Technologies for E-Services; September 2 
- 3, 2005.  http://www.elet.polimi.it/upload/baresi/papers/TES2005.pdf

[2]"XlinkIt: A Consistency Checking and Smart Link Generation Service", 
ACM Transactions on Software Engineering and Methodology, pp. 151-185, 
May 202.

[3] "CLiX: Constraint Language in XML".  http://www.clixml.org/clix/1.0

Regards,
Anne
-- 
Anne H. Anderson               Anne.Anderson@sun.com
Sun Microsystems Labs          1-781-442-0928
Burlington, MA USA