Support for mutually-exclusive intents
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Key: TUSCANY-2239
URL: https://issues.apache.org/jira/browse/TUSCANY-2239
Project: Tuscany
Issue Type: New Feature
Components: Java SCA Core Runtime
Reporter: Greg Dritschler
The SCA Policy specification does not provide a means to define intents which
are mutually exclusive. This is a noticeable omission when considering the
intents in the SCA Transaction specification which are mutually exclusive by
nature (managedTransaction vs. noManagedTransaction, propagatesTransaction vs.
suspendsTransaction). There is a need to be able to define intents which are
mutually exclusive and for the exclusion to be checked by the SCA runtime to
avoid the error of specifying exclusive intents on a single artifact. In
addition, there should be rules defined for the handling of mutually exclusive
intents which are attached at different levels of a composite or a hierarchy of
composites.
I have attached a patch to provide the capability to define mutually exclusive
intents. This is achieved using a new @excludes attribute on the <intent/>
element in definitions.xml. For example:
<intent name="propagatesTransaction" constrains="implementation"
excludes="suspendsTransaction"/>
@excludes is a list of intents which are mutually-exclusive with the named
intent. In order to be effective, a reciprocal definition needs to be made as
shown below.
<intent name="suspendsTransaction" constrains="implementation"
excludes="propagatesTransaction"/>
The patch makes no assumptions about the relationship of qualified intents to
the base intent. Therefore exclusive relationships between qualified intents
need to be spelled out.
<intent name="noManagedTransaction" constrains="implementation"
excludes="managedTransaction managedTransaction.global
managedTransaction.local"/>
A key part of the patch is that there now are two types of intent inheritance
with respect to exclusive intents. There is a "default" inheritance between
certain hierarchical elements within a composite. For example consider this
snippet from a composite:
<component name="C1" requires="propagatesTransaction">
<reference name="r1"/>
<reference name="r2"/>
<reference name="r3" requires="suspendsTransaction"/>
</component>
In this case the first two references inherit the default intent
"propagatesTransaction" from the component element. However the third
reference does not inherit it because it specifies an exclusive intent
"suspendsTransaction" which overrides the component-level default.
The second type of inheritance is used when inheriting intents from an
implementation (e.g. introspected Java code, or an implementation composite).
In this case the intents of the implementation cannot be overridden. Consider
this example:
<component name="D1">
<implementation.composite name="CZ1"/>
<reference name="r1" requires="suspendsTransaction"/>
</component>
Let's assume CZ1 contains the component C1 shown earlier and that it promotes
the component reference C1/r1 as r1. C1/r1 has the intent
"propagatesTransaction". This intent is considered a requirement of the
implementation and it cannot be overridden by the using composite. Therefore
D1 is in error.
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