问题
I've heard from someone that they´re using a business process automation tool (like Weblogic Integration) as a programming language (what sounds like something kind of stupid) to make things declarative. Then they put all the logic inside a process, every single if and while.
But, isn´t a process a how to step-by-step entity to reach a target?
For me it makes a process completely imperative. What do you think?
回答1:
It is definitely not what people usually mean when they talk about declarative programming, even if it some sense can be called declarative.
回答2:
Orchestration languages are in fact imperative scripting languages with conditionals, looping and other traditionally imperative constructs, typically expressed through a flowchart-based user interface. They certainly do not (in my experience) implement tail-recursive functional programming, backward chaining or any other paradigm that might reasonably described as declarative in the generally accepted sense.
MS Workflow Foundation is advertised as having a rules engine, but this is fairly simplistic and doesn't really do forward chaining, except in a somewhat roundabout way. ILOG actually makes an adaptor for their rules engine specifically to drop it into MS workflow foundation.
Other workflow tools have better rule engines and a proper forward chaining system that could be viewed as declarative. However, once you get into the workflows themselves with looping and conditional branches you are most definitely in the territory of imperative programming.
However, some systems also implement a petri-net or state change based markup system for workflow, which might reasonably be described as declarative, but they still have an imperative mode of interaction with the underlying system. They still update variables and have side-effects.
I have seen one or two applications (for example TOAD for data anlaysis) actually using MS Workflow Foundation as a scripting language. As such it allows you to add a scripting facility to the application that (at least for marketing purposes) doesn't require programming skill to use.
In practice, a tool designed for writing, editing and running SQL queries being fitted with a scripting framework for 'non-programmers' makes one wonder what audience it's really aimed at. As a scripting language, workflow modelling tools are fairly clumsy and offer very limited opportunities for abstraction; in practice a .Net based scripting language such as IronPython or Boo, particularly in conjunction with a decent templating mechanism, would be a very powerful addition to such a tool.
One point about graphical languages of this sort is that they do not scale well with complexity. A similar issue applies with ETL tools as well. I have seen a provisioning application (see below) that was done (ironically) with Crossworlds (now known as Websphere Integrator). Within a month of starting on the application it became obvious that the graphical workflow language was not going to scale with the complexity of the application and it was re-built, based on a custom rules engine written in Java and a fairly large body of bespoke java code.
This type of issue is not uncommon with EAI and Orchestration systems and is one of the reasons that SOA is hard to implement in practice. What you are doing is actually pushing business logic into a very clumsy programming environment that is not being officially acknowledged as such. This will work in a simple case but is hard to make work on a complex system - this is sort of a guilty secret in SOA circles.
Coda: A provisioning application is a system that takes plans for telecommunication services contracts (in this case for a mobile phone network) and pushes configuration information based on rules out to various switches, billing applications and other applications. They tend to be fairly complex. When you buy a mobile phone plan with so many minutes and so many texts per month, a provisioning application is pushing out configuration information to the rest of the system about your access and billing rules.
来源：https://stackoverflow.com/questions/325721/is-a-process-design-really-declarative-programming