http - Paging in a Rest Collection

Question

I'm interested in exposing a direct REST interface to collections of JSON documents (think CouchDB or Persevere). The problem I'm running into is how to handle the GET operation on the collection root if the collection is large.

As an example pretend I'm exposing StackOverflow's Questions table where each row is exposed as a document (not that there necessarily is such a table, just a concrete example of a sizable collection of 'documents'). The collection would be made available at /db/questions with the usual CRUD api GET /db/questions/XXX, PUT /db/questions/XXX, POST /db/questions is in play. The standard way to get the entire collection is to GET /db/questions but if that naively dumps each row as a JSON object, you'll get a rather sizeable download and a lot of work on the part of the server.

The solution is, of course, paging. Dojo has solved this problem in its JsonRestStore via a clever RFC2616-compliant extension of using the Range header with a custom range unit items. The result is a 206 Partial Content that returns only the requested range. The advantage of this approach over a query parameter is that it leaves the query string for...queries (e.g. GET /db/questions/?score>200 or somesuch, and yes that'd be encoded %3E).

This approach completely covers the behavior I want. The problem is that RFC 2616 specifies that on a 206 response (emphasis mine):

The request MUST have included a Range header field (section 14.35) indicating the desired range, and MAY have included an If-Range header field (section 14.27) to make the request conditional.

This makes sense in the context of the standard usage of the header but is a problem because I'd like the 206 response to be the default to handle naive clients/random people exploring.

I've gone over the RFC in detail looking for a solution but have been unhappy with my solutions and am interested in SO's take on the problem.

Ideas I've had:

• Return 200 with a Content-Range header! - I don't think that this is wrong, but I'd prefer if a more obvious indicator that the response is only Partial Content.
• Return 400 Range Required - There is not a special 400 response code for required headers, so the default error has to be used and read by hand. This also makes exploration via web browser (or some other client like Resty) more difficult.
• Use a query parameter - The standard approach, but I'm hoping to allow queries a la Persevere and this cuts into the query namespace.
• Just return 206! - I think most clients wouldn't freak out, but I'd rather not go against a MUST in the RFC
• Extend the spec! Return 266 Partial Content - Behaves exactly like 206 but is in response to a request that MUST NOT contain the Range header. I figure that 266 is high enough that I shouldn't run into collision issues and it makes sense to me but I'm not clear on whether this is considered taboo or not.

I'd think this is a fairly common problem and I'd like to see this done in a sort of de facto fashion so I or someone else isn't reinventing the wheel.

What's the best way to expose a full collection via HTTP when the collection is large?

Answer 1

I don't really agree with some of you guys. I've been working for weeks on this features for my REST service. What I ended up doing is really simple. My solution only makes a sense for what REST people call a collection.

Client MUST include a "Range" header to indicate which part of the collection he needs, or otherwise be ready to handle a 413 REQUESTED ENTITY TOO LARGE error when the requested collection is too large to be retrieved in a single round-trip.

Server sends a 206 PARTIAL CONTENT response, with the Content-Range header specifying which part of the resource has been sent, and an ETag header to identify the current version of the collection. I usually use a Facebook-like ETag {last_modification_timestamp}-{resource_id}, and I consider that the ETag of a collection is that of the most recently modified resource it contains.

To request a specific part of a collection, the client MUST use the "Range" header, and fill the "If-Match" header with the ETag of the collection obtained from previously performed requests to acquire other parts of the same collection. The server can therefore verify that the collection hasn't changed before sending the requested portion. If a more recent version exists, a 412 PRECONDITION FAILED response is returned to invite the client to retrieve the collection from scratch. This is necessary because it could mean that some resources might have been added or removed before or after the currently requested part.

I use ETag/If-Match in tandem with Last-Modified/If-Unmodified-Since to optimize cache. Browsers and proxies might rely on one or both of them for their caching algorithms.

I think that a URL should be clean unless it's to include a search/filter query. If you think about it, a search is nothing more than a partial view of a collection. Instead of the cars/search?q=BMW type of URLs, we should see more cars?manufacturer=BMW.