REST - Modify Part of Resource - PUT or POST

Question

I'm seeing a good bit of hand-waving on the subject of how to update only part of a resource (eg. status indicator) using REST.

The options seem to be:

1. Complain that HTTP doesn't have a PATCH or MODIFY command. However, the accepted answer on http://stackoverflow.com/questions/1672025/http-modify-verb-for-rest does a good job of showing why that's not as good an idea as it might seem.

2. Use POST with parameters and identify a method (eg. a parameter named "action"). Some suggestions are to specify an X-HTTP-Method-Override header with a self-defined method name. That seems to lead to the ugliness of switching within the implementation based on what you're trying to do, and to be open to the criticism of not being a particularly RESTful way to use POST. In fact, taking this approach starts to feel like an RPC-type interface.

3. Use PUT to over-write a sub-resource of the resource which represents the specific attribute(s) to update. In fact, this is effectively an over-write of the sub-resource, which seems in line with the spirit of PUT.

At this point, I see #3 as the most reasonable option.

Is this a best practice or an anti-pattern? Are there other options?

Answer 1

There are two ways to view a status update.

1. Update to a thing. That's a PUT. Option 3

2. Adding an additional log entry to the history of the thing. The list item in this sequence of log entries is the current status. That's a POST. Option 2.

If you're a data warehousing or functional programming type, you tend to be mistrustful of status changes, and like to POST a new piece of historical fact to a static, immutable thing. This does require distinguishing the thing from the history of the thing; leading to two tables.

Otherwise, you don't mind an "update" to alter the status of a thing and you're happy with a PUT. This does not distinguish between the thing and it's history, and keeps everything in one table.

Personally, I'm finding that I'm less and less trustful of mutable objects and PUT's (except for "error correction"). (And even then, I think the old thing can be left in place and the new thing added with a reference to the previous version of itself.)

If there's a status change, I think there should be a status log or history and there should be a POST to add a new entry to that history. There may be some optimization to reflect the "current" status in the object to which this applies, but that's just behind-the-scenes optimization.

Answer 2

Option 3 (PUT to some separated sub-resource) is your best bet right now, and it wouldn't necessarily be "wrong" to just use POST on the main resource itself - although you could disagree with that depending on how pedantic you want to be about it.

Stick with 3 and use more granular sub-resources, and if you really do have a need for PATCH-like behavior - use POST. Personally, I will still use this approach even if PATCH does actually end up as a viable option.

Answer 3

HTTP does have a PATCH command. It is defined in Section 19.6.1.1 of RFC 2068, and was updated in draft-dusseault-http-patch-16, currently awaiting publication as RFC.

Answer 4

PATCH is fine for patch or diff formats. Until then it's not very useful at all.

As for your solution 2 with a custom method, be it in the request or in the headers, no no no no and no, it's awful :)

Only two ways that are valid are either to PUT the whole resource, with the sub data modified, or POST to that resource, or PUT to a sub-resource.

It all depends on the granularity of your resources and the intended consequences on caching.