Saturday, October 15, 2005

REST and OO Abstraction

I'm still getting over just how simple REST is. In my first post on REST I was wondering where a protocol like IIOP fits in. Mark's comment kindly explained, that as far a REST is concerned it doesn't. So I've been thinking how does abstraction fit into REST. In REST a datum is data that is exposed by a component. Which makes sense, since data that you want to communicate to another component cannot be hidden. So the datums exposed by my object are resources that I can lookup:

// client side lookup
datum1 = connector.GET(
"protocol://myServer/myServiceObject/Datum1");

OK this is pretty fine grained, what if I want to get a lot of data at once. Well I can think of two options. Option 1: Expose all of the data in my object and send it through the connector to my client. This is what IIOP does through marshalling. What IIOP goes on to do is to un-marshall my data back into an object at the client end which breaks the REST constraints on a connector.

So without unmarshalling we have:

// client side lookup for a data structure
struturedDatumXML =
connector.GET( "protocol://myServer/myServiceObject/ObjectDataAsXML");

// data structure access
structuredDatum = new XMLObject(struturedDatumXML)
datam1 = struturedDatum.getElement("datum1").getValue();

But we've broken encapsulation, so the messaging is no longer Object Orientated. Going back to IIOP my structuredDatum would be a data transfer object (DTO), with a set of getters for each data element. But a DTO is really just a data struture also, so IIOP doesn't help. So how can we do this in REST, and not break encapsulation. Option 2: How about mobile objects:

// client side lookup for a real Object
objectDatumSerialised =
connector.GET( "protocol://myServer/myServiceObject/Serialise);

// client side use of object
objectDatum = ObjectLoader.load(objectDatumSerialised);
datum1 = objectDatum.getDatum1();

Treating an object as a resource should be possible with VM based languages.

So after my fears about abstraction and encapsulation, it turns out that the REST constraints do not preclude abstraction at all. I'm new to this REST stuff, so the above could be very wrong and all comments are welcomed. In particluar I'm interested in finding out more about "self describing data" as it sounds like a mechanism that will help abstraction.

So it looks like I don't need to loose Object Orientated abstraction to use REST. Until someone explains otherwise.

1 comment:

Mark said...

You're not breaking encapsulation, you're breaking data hiding. Encapsulation is preserved because the data is still "owned" by the resource.

But yah, I see REST as very much "OO" in the sense you describe in that second example. Hence why I've stuck with my email address from my CORBA days.