Question

What is the benefit of Connectedness as defined by Resource Oriented Architecture (ROA)? The way I understand it, the crux of Connectedness is the ability to crawl the entire application state using only the root URIs.

But how useful is that really?

For example, imagine that HTTP GET http://example.com/users/joe returns a link to http://examples.com/uses/joe/bookmarks.

Unless you’re writing a dumb web crawler (and even then I wonder), you still need to teach the client what each link means at compile-time. That is, the client needs to know that the ‘bookmarks URI’ returns a URI to Bookmark resources, and then pass control over to special Bookmark-handling algorithms. You can’t just pass links blindly to some general client method. Since you need this logic anyway:

1. What’s the difference between the client figuring out the URI at runtime versus providing it at compile-time (making http://example.com/users/bookmarks a root URI)?

2. Why is linking using http://example.com/users/joe/bookmarks/2 preferred to id='2'?

The only benefit I can think of is the ability to change the path of non-root URIs over time, but this breaks cached links so it’s not really desirable anyway. What am I missing?

Answer 1

You are right that changing Uris is not desirable but it does happen and using complete Uris instead of constructing them makes change easier to deal with.

One other benefit is that your client application can easily retrieve resources from multiple hosts. If you allowed your client to build the URI’s the client would need to know on which host certain resources reside. This is not a big deal when all of the resources live on a single host but it becomes more tricky when you are aggregating data from multiple hosts.

My last thought is that maybe you are oversimplifying the notion of connectedness by looking at it as a static network of links. Sure the client needs to know about the possible existence of certain links within a resource but it does not necessarily need to know exactly what are the consequences of following that link.

Let me try an give an example: A user is placing an order for some items and they are ready to submit their cart. The submit link may actually go to two different places depending on whether the order will be delivered locally or internationally. Maybe orders over a certain value need to go through an extra step. The client just knows that it has to follow the submit link, but it does not have compiled in knowledge of where to go next. Sure you could build a common ‘next step’ type of resource so the client could have this knowledge explicitly but by having the server deliver the link dynamically you introduce a lot less client-server coupling.

I think of the links in resources as placeholders for what the user could choose to do. Who will do the work and how it will be done is determined by what uri the server attaches to that link.