1. Inline Vs. External Policy Attachment SCA Policy Framework
Sanjay Patil
Anish Karmarkar

2. Inline Vs. External Policy Attachment
Comparison Criteria
Inline Policy Attachment (IPA)
External Policy Attachment (EPA)
Separation of roles:
Developer
(creates composite)
Deployer
(attaches policy sets)
Mixes the two roles.
Simpler model when SCDL documents are to be manually edited for attaching policy sets
Maintains separation of the roles.
Composite developer does not have to deal with policy sets.
Enables independent management of the policy attachment data (separate from that of SCDL)
Domain-wide policy attachment
Not supported
Easier to describe domain-wide policy attachment rules
Fine grained policy attachment
(per message/operation)
Complicates the SCDL Syntax (by duplicating the relevant message/ operation constructs in the SCDL for the purposes of inline policy attachment)
Expressions can be formulated for externally attaching policy sets at the message/operation level without requiring any changes to the SCDL metamodel

3. EPA is sufficient, prevalent and simpler
EPA supports all the use cases addressed by IPA and more
Usage of EPA seems to be more prevalent
Supporting both the mechanisms (EPA and IPA) makes the specs complex
Definition of each mechanism
Processing rules when EPA and IPA are used together

4. Other Considerations …
What does it mean to disallow IPA but still allow component implementation code to specify defaults for policy sets?
How to handle the scenario where a composite developer wants to experiment with attaching different policy sets to a SCDL without involving the external policy attachment mechanism?

5. Proposal
Option I: Support only EPA, remove support for IPA, and disallow component implementation code to specify policy defaults
Cleaner architecture
Simpler specifications and conformance rules
Minimal/no loss of functionality
Option II: IPA as an optional feature, not to be used together with EPA (Note: an attribute on composite element indicates whether IPA or EPA is to be used, with EPA as the default)
Allows use of IPA while reducing complexity (no need to define overriding rules, etc)

6. Bindings
Arguments for the Inline Vs. External Policy Attachment discussion also apply for bindings attachment
Treatment of bindings attachment can be based upon the resolution for Policy Attachment