How would one model resources that are hierarchical? Assume for example that a person has a "message board" and that a "message" is a resource. Let's say that the "message" can have replies so that it forms a discussion thread. How does one model the notion of a thread?
Does the "message" include it's children? Is the "thread" it's own resource? Finally, what kind of REST URI would work in this instance?
How about:
# View message with all replies GET messages/id/ # Create a new reply to a message with form data POST /messages/id/replies/ # View a reply, possibly in the context of its original message GET /replies/id/ # Delete comment spam DELETE replies/id/
and so on...
Modelling a situation in which every reply can be replied to is more complicated, but the idea would be the same.
There's also Railscasts # 139, which shows one way to flatten nested resources. It's Rails-specific, but the basic approach can be used anywhere.
If you consider that all replies to a message are also messages I would give an id to every one of them and use these URIs:
#message {id} (only the message, no replies)
/messages/{id}
#replies to the {id} message (a list of the id's of the replies)
/messages/{id}/replies
To create a new message do a post to the uri
/messages
To create a reply to a message {id} do a post to
/messages/{id}/replies
Update
I will modify here my previous answer. Hopefully this time in a correct Restful style.
You have an entry point uri, let's call it {messages}.
GET {messages} -> list of uris of all the messages, {message1}, {message2}, etc.
GET {message1} -> responds with the message1 document, for example in xml it could be:
<message responses="{link to message1 responses}">
<date>...</date>
<body>...</body>
</message>
{link to message1 responses} is the link that the client has to follow to get the list of uris of the responses.
If a message is a response to another it will include that in his content, for example
<message responses="{link to message1 responses}" inResponseTo="{uri}" >
<date>...</date>
<body>...</body>
</message>
Now to add new messages just post it to the original {messages} uri. If the message is a response to other message just include it in its content (note that this is an efective change to the inital answer where you post the replies to an special uri).
To modify some message do a PUT to its uri.
All the uris coudl follow thats of the first part of the answer, but this is not necessary.
So far every response has not been RESTful at all. REST doesn't need to be hierarchical. Just have an entry point like /threads/ which gives full URIs of each thread resource, and each thread resource would respond with the URIs of each message in itself, or the top message plus the URIs of its replies, and so on. The way these URIs are generated is immaterial as long as they are discoverable via hypertext from an entry point.