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Subject: Issue 82 - resolution
12/15/2004 04:52 PM
Abstract BPEL definition:
A BPEL abstract process is a partially specified process that is not intended to be executed. Executable and abstract BPEL processes share the same expressive power, while the latter allows process definitions that are capable of abstracting away operational details _ either through omission or explicit opacity. Whereas executable processes are fully concretized and thus can be executed, an abstract process lacks some of the required concrete operational details expressed by or when mapping it to a fully executable artifact. An abstract BPEL process must be explicitly declared as 'abstract'.
Abstract processes serve a descriptive role, with more than one possible purpose. A BPEL abstract process can define the publicly visible behavior of some or all of the services it offers ("myRole" in its partnerLinks), which may include its interactions along its partnerLinks with other services. Alternatively, a BPEL abstract process can define a process "template" embodying domain-specific best practices, encoded in a platform-neutral and portable way by both enterprises and vertical standards organizations. The process "template" captures some essential process logic while leaving out operational details that will be concretized by the enterprises and vertical standards organizations when mapping the partial process specification to a fully executable artifact.
The different uses of abstract BPEL require
different levels of opacity and restrictions or relaxations on which elements
of a corresponding executable artifact are omitted or hidden. To
cleanly enable different usages of abstract processes, an extensible approach
is taken using a set of basic, minimal requirements (base) for all abstract
processes. From this base, "usage profiles" can be defined. On
its own, the base lacks well-defined semantics - the only constructs that
have defined meaning are the constructs of executable processes, and that
meaning is defined by prescribed behaviour of the executable process as
a whole, and is thus not directly available outside an executable process.
The semantics of an abstract process are therefore derived from those of
the executable process it could be mapped to. To define these semantics,
the range of corresponding executable processes can be constrained by a
usage profile, placing constraints on what is opaque or omitted. An
abstract process MUST
identify the usage profile that
defines its meaning with a URI. Profiles
may be defined by anyone - in separate standardization efforts, internally
in organizations or for proprietary usage, etc.
[It is left open, at the moment, whether the base definition itself is to be considered as a profile. If it is, it will be assigned a URI to identify it]
[The TC will rework the 1.1 AP definition into a profile with a defined URI. The placement of this profile and other questions concerning the structure of the ultimate specification and how many documents there will be will be left open and dealt with as part of 158]
Semantics of Abstract Processes:
 Although it might contain complete information that would render it executable as specified for executable BPEL, its abstract status states that any concrete realizations of it may perform additional processing steps that are not relevant to the audience to which it has been given. The minimum criteria for an abstract process is defined in this specification while completeness is left to the usage profile a particular abstract process definition belongs to.
 Abstract processes permit the use of all of the constructs of executable processes. Thus there is no fundamental expressive power distinction between abstract and executable processes.
 An abstract process may omit operational details that are mandatory for BPEL executable processes through the use of "opaque" entities of various types (i.e. elements, attributes, etc.) or through the omission of specific BPEL elements or attributes that are allowed to be implicitly opaque. This omission is treated as a syntactic shortcut equivalent to the attribute or expression being in the omitted location with an opaque value.
The semantics and consistency constraints of executable BPEL are clearly defined. The semantics of each construct in an abstract process is derived from that of the same construct in executable BPEL. The difference is strictly a consequence of the opacity used in that construct (missing information) and other parts of the process affected by it ( For example, opacity in a link source element may affect the link target element).
Addionally, an abstract process is required to be syntactically valid using the executable schema extended with the opaque entities.
 In this base definition, to avoid absurd constructs and to clarify opacity, the minimial requirement is that for any abstract process, there exists at least one syntactic completion that yields a *valid executable process* by
a) replacing each opaque token by a concrete entity or removing the opaque token completely, and
b) adding new BPEL XML elements anywhere in the process.
 For this base definition, there are no requirements on how concretized, executable realizations of abstract process should be implemented (ie: language, platform, etc) (c.f. analogy to WSDL); nor are specific relationships with such realizations specified.
 Abstract processes are *incomplete* and non-executable by definition, whether or not they contain opaque entities. Therefore the semantics of the non-opaque constructs they contain cannot be understood in isolation from the relationship of the abstract process with the executable *completions* it permits. The reason being that the semantics of those constructs actually exists only in the possible executable completions. As an edge case, a permitted completion may sometimes be identical to the abstract process syntactically, but this is the exception rather than the rule.
Chief Scientist, Choreology Ltd
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