Jun Tatemura wrote:
Dear team,
In the F2F discussion with OGSA team, we had a question on our specs. We promised that we will discuss in the WG.
[Q] Can a component have a parameter such that (1) sometimes the user want to specify a value but (2) sometimes the user want the component to specify the value dynamically (as a default value).
In fact, it is not obvious from the current component model or CDL.
A default value of a component property can be introduced, for example, as param2 in the following component description, Component1. If the user extends this template without specifying a value on param2, 100 is used as a default value. However, how can a dynamic value be default?
<Component1> <cmp:CommandPath .../> <cmp:CodeBase .../> <param1 cdl:lazy="true"/> <param2>100</param2> <Component1>
For a dynamic value, we use @cdl:lazy. When param1 is assigned by the component, we should specify @cdl:lazy so that any reference to this property is resolved at run time.
However, specifying @cdl:lazy at a property does not mean that the component MUST/SHOULD assign a value to the property. It does not mean that the user MUST NOT/SHOULD NOT assign a value either. In fact, CDL does not have any notion on a role who assigns values (such as users, systems, components, etc).
For CDL, it is fine if the user assign the value to a property with @cdl:lazy. CDL only cares when a reference to the property is resolved. In this sense, the question to the answer can be 'yes' - the user can assign the value to the above param1. But we do not know whether the value is used by the component (or just ignored). In CDL, we cannot say if a component allows the user to specify a specific property.
In fact, in the early design discussion, we were discussing @cdl:use="readonly" for that purpose: <param1 cdl:lazy="true" cdl:use="readonly" /> For some reason (I don't remember), we have removed this from the spec. This means that (1) any reference to this property is resolved at run time (2) you cannot override this property by inheritance -- in a component model, this means that the value can be assigned only by the component (the user is not allowed to assign the value)
A possible solution would be one of the following:
[1] CDL-level solution: add "readonly" as a value of @cdl:use [2] Component-level solution: extend the component model specification so that one can specify how a property is used by a component. [3] Application-level solution: just let it unspecified (as is) and think this as a semantic level matter the component model does not care.
I dont particularly see why it is an issue. Yes, you can declare a value for something that is dynamic. In that case, the static value is used until the runtime sets the dynamic value. Accordingly, it does not make much sense to declare a value for a dynamic value, because it will get lost by the time anything does a lazy read of the value. Anything that does a non-lazy resolve of the value, well, they get whatever is there. The other thing to consider is that an implementation of a component may choose to implement its own policy. That is, it may have a lazy value by default, but if one is set then that one takes precendence. That is trivial to implement, at least in the smartfrog runtime. I think our TempFile component may do some of that; anything you leave out gets some random value picked out the air, while stuff you fill in gets used as parameters for the operation. -steve