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Subject: RE: [wsbpel] Issue - 2- requirements for a sub function solution
Interesting. I want to make sure I am still following the main thread. Are we saying that:
Close? ++harvey
-----Original
Message-----
Yaron, 1. Discovering that an <invoke> isn't "real" (involving a proper partner) -- there is no clear way to distinguish such service invocations from subprocesses, except by tricks like naming conventions. 2. Decoding the initial <assign> to figure out what the input parameters are for the subprocess. 3. Decoding the trailing <assign> to figure out what the output parameters are for the subprocess. This doesn't sound like
the best of modelling approaches, and certainly makes run-time implementations
more difficult. Can we do better? Here is my view on what the requirements are for a solution to Issue 2. Terminology: Host - This is the process that called the sub-function. Requirements: Integration into host fault/compensation model - The sub-function MUST be able to throw faults that will propagate to the host. The host MUST be able to call compensation handlers defined in the sub-function.
I would add that
sub-functions must integrate into the host's serializable scopes, when shared
variables are passed by reference to the sub-function. Functional Programming - One of the key concepts of functional programming is the idea of isolation. Functions in functional programming systems are expected to be able to run in their own context and to only have contact with the host context through explicit value passing. The sub-function solution MUST enable for isolation. Human Usable - It MUST be reasonably easy for a programmer to read/write sub-function definitions and calls without requiring the aid of a tool.
I believe we established
this as a requirement for BPEL very early on, in the debate over whether <sequence>
was necessary. By Reference - Messages seem only to get larger with multi-megabyte XML messages now commonly seen. Therefore when passing data into a sub-function it is critical that it be possible to hand over the data by reference so that the data does not have to be copied on its way in or out.
While passing by
reference has good, pragmatic foundations in managing the impact of those
ever-growing XML payloads, I believe another line of argument would support the
need for clear, controllable parameter passing to the subfunction, such that
the process author can clearly control and understand how data are passed to
and from the sub-function. Use in Expressions - It MUST be possible to call a sub-function anywhere one could call a XPATH or other type of general/query/date expression and have the sub-function return the desired value.
That is an interesting
idea. Such sub-functions would need to have restricted "signatures"
to map to XPath functions; also, isn't XPath supposed to always be side-effect
free? That would imply that all input variables of the sub-function must be
read-only; a single output variable would be nominated as the function result. Import - A mechanism is needed to allow sub-functions to be defined in stand alone files which can then be referenced by multiple BPEL processes.
Agreed. |
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