Ok, I know the difference in purpose. GET is to get some data. Make a request and get data back. POST should be used for CRUD operations other than read I believe. But when it comes down to it, does the server really care if it's receiving a GET vs. POST in the end?
According to the HTTP RFC, GET should not have any side-effects, while POST may have side-effects.
The most basic example of this is that GET is not appropriate for anything like a purchase-transaction or posting an article to a blog, while POST is appropriate for actions-that-have-consequences.
By the RFC, you can hold a user responsible for actions done by POST (such as a purchase), but not for GET actions. 'Bots always use GET for this reason.
From the RFC 2616, 9.1.1:
9.1.1 Safe Methods
Implementors should be aware that the
software represents the user in
their interactions over the Internet,
and should be careful to allow the
user to be aware of any actions they
might take which may have an
unexpected significance to themselves
or others.
In particular, the convention has
been established that the GET and
HEAD methods SHOULD NOT have the
significance of taking an action
other than retrieval. These methods
ought to be considered "safe". This
allows user agents to represent other
methods, such as POST, PUT and
DELETE, in a special way, so that the
user is made aware of the fact that
a possibly unsafe action is being
requested.
Naturally, it is not possible to
ensure that the server does not
generate side-effects as a result of
performing a GET request; in fact,
some dynamic resources consider that a
feature. The important distinction
here is that the user did not request
the side-effects, so therefore
cannot be held accountable for them.
It does if a search engine is crawling the page, since they will be making GET requests but not POST. Say you have a link on your page:
http://www.example.com/items.aspx?id=5&mode=delete
Without some sort of authorization check performed before the delete, it's possible that Googlebot could come in and delete items from your page.
Since you're the one writing the server software (presumably), then it cares if you tell it to care. If you handle POST and GET data identically, then no, it doesn't.
However, the browser definitely cares. Refreshing or clicking back to a page you got as a response to a POST pops up the little "Are you sure you want to submit data again" prompt, for example.
GET has data limit restrictions based on the sending browser:
The spec for URL length does not dictate a minimum or maximum URL length, but implementation varies by browser. On Windows: Opera supports ~4050 characters, IE 4.0+ supports exactly 2083 characters, Netscape 3 -> 4.78 support up to 8192 characters before causing errors on shut-down, and Netscape 6 supports ~2000 before causing errors on start-up
If you use a GET request to alter back-end state, you run the risk of bad things happening if a webcrawler of some kind traverses your site. Back when wikis first became popular, there were horror stories of whole sites being deleted because the "delete page" function was implemented as a GET request, with disastrous results when the Googlebot came knocking...
"Use GET if: The interaction is more like a question (i.e., it is a safe operation such as a query, read operation, or lookup)."
"Use POST if: The interaction is more like an order, or the interaction changes the state of the resource in a way that the user would perceive (e.g., a subscription to a service), or the user be held accountable for the results of the interaction."
source
You be aware of a few subtle security differences. See my question
GET versus POST in terms of security?
Essentially the important thing to remember is that GET will go into the browser history and will be transmitted through proxies in plain text, so you don't want any sensitive information, like a password in a GET.
Obvious maybe, but worth mentioning.
By HTTP specifications, GET is safe and idempotent and POST is neither. What this means is that a GET request can be repeated multiple times without causing side effects.
Even if your server doesn't care (and this is unlikely), there may be intermediate agents between your client and the server, all of whom have this expectation. For example proxies to cache data at your ISP or other providers for improved performance. THe same expectation is true for accelerators, for example, a prefetching plugin for your browser.
Thus a GET request can be cached (based on certain parameters), and if it fails, it can be automatically repeated without any expecation of harmful effects. So, really your server should strive to fulfill this contract.
On the other hand, POST is not safe, not idempotent and every agent knows not to cache the results of a POST request, or retry a POST request automatically. So, for example, a credit card transaction would never, ever be a GET request (you don't want accounts being debited multiple times because of network errors, etc).
That's a very basic take on this. For more information, you might consider the "RESTful Web Services" book by Ruby and Richardson (O'Reilly press).
For a quick take on the topic of REST, consider this post:
http://www.25hoursaday.com/weblog/2008/08/17/ExplainingRESTToDamienKatz.aspx
The funny thing is that most people debate the merits of PUT v POST. The GET v POST issue is, and always has been, very well settled. Ignore it at your own peril.
GET has limitations on the browser side. For instance, some browsers limit the length of GET requests.
I think a more appropriate answer, is you can pretty much do the same things with both. It is not so much a matter of preference, however, but a matter of correct usage. I would recommend you use you GETs and POSTs how they were intended to be used.
Technically, no. All GET does is post the stuff in the first line of the HTTP request, and POST posts stuff in the body.
However, how the "web infrastructure" treats the differences makes a world of difference. We could write a whole book about it. However, I'll give you some "best practises":
Use "POST" for when your HTTP request would change something "concrete" inside the web server. Ie, you're editing a page, making a new record, and so on. POSTS are less likely to be cached, or treated as something that's "repeatable without side-effects"
Use "GET" for when you want to "look at an object". Now, such a look might change something "behind the scenes" in terms of caching or record keeping, but it shouldn't change anything "substantial". Ie, I could repeat my GET over and over and nothing bad would happen, except for inflated hit counts. GETs should be easily bookmarkable, so a user can go back to that same object later on.
The parameters to the GET (the stuff after the ?, traditionally) should be considered "attributes to the view" or "what to view" and so on. Again, it shouldn't actually change anything: use POST for that.
And, a final word, when you POST something (for example, you're creating a new comment), have the processing for the post issue a 302 to "redirect" the user to a new URL that views that object. Ie, a POST processes the information, then redirects the browser to a GET statement to view the new state. Displaying information as a result of a POST can also cause problems. Doing the redirection is often used, and makes things work better.
Should the user be able to bookmark the resulting page? Another thing to think about is some browsers/servers incorrectly limit the GET URI length.
Edit: corrected char length restriction note - thanks ars!
It depends on the software at the server end. Some libraries, like CGI.pm in perl handles both by default. But there are situations where you more or less have to use POST instead of GET, at least for pushing data to the server. Large amounts of data (where the corresponding GET url would become too long), binary data (to avoid lots of encoding/decoding trouble), multipart files, non-parsed headers (for continuous updates pre-AJAX style...) and similar.
The server technically couldn't care one way or the other about what kind of request it receives. It will blindly execute any request coming across the wire.
Which is the problem. If you have an action that destroys or modifies data in a GET action, Google will tear your site up as it crawls through indexing.
The server usually doesn't care. But it's mostly for following good practices, as you mentioned. The client side also matter - as mentioned you cannot bookmark a POST'd page usually, and some browsers have limits on the length of the URL for really long GET queries.
Since GET is intended for specifying resource you wanna get, depending on exact software on the server side, the web server (or the load balancer in front of it) may have a size limit on GET requests to prevent Denial Of Service attacks...
Be aware that browsers may cache GET requests but will generally not cache POST requests.
Yes, it does matter. GET and POST are quite different, really.
You are right in that normally, GET is for "getting" data from the server and displaying a page, while POST is for "posting" data back to the server. Internally, your scripts get the same data whether it's GET or POST, so no, the server doesn't really care.
The main difference is GET parameters are specified in URLs, while POST is not. This is why POST is used for signup and login forms - you don't want your password in a URL. Similarly, if you're viewing different pages or displaying a specific view of some data, you normally want a unique URL.
It really does matter. I have gathered like 11 things you should know abut them.
11 things you should know about GET vs POST
No, they shouldn't except for #jbruce2112 answer and uploading files require POST.
Related
I see many discussions arround the topic, but seen to always post's method get a better place...
"Get is a insecure method" (for some propurses)
is little bit faster and smaller than post, but for real is arround in top max 9 kb difference and even in great projects i don't see the problem nowadays.
Get can't handle as much data than Post, so for big request's you will use post.
I know the patterns, i know how to use Get and Post and etc, i still use it, but the real question is why?
Why use this?
GET and POST are literally just different verbs. There's no fundamental difference between a GET and a POST request on a technical level. Most POST requests contain additional data in the body that GET requests don't; but that's mostly because of the primary difference between them: they mean different things. You don't need to send body data in a GET request because you're requesting data, not sending any.
So this leaves the primary difference, again: they're different verbs that mean different things and are treated differently by clients and servers. GET requests are for requesting information from a server, and can be highly cached, which is a primary reason HTTP is very scalable. POST requests send data to the server and request the data to be processed; that should not and is not cached and is treated differently in everything from the client through middleware to the server.
There's no "better" or "more secure", you simply use different verbs for different semantics.
For a while I was (wrongly) thinking that a RESTful API just exposed CRUD operation to persisted entities for a web application. When you code something up in "the real world" you soon find out that this is not enough. For example, a bank account transfer doesn't have to be a persisted entity. It could be a transient resource where you POST to /transfers/ and in the payload you specify the details:
{"accountToCredit":1234, "accountToDebit":5678, "amount":10}
Using POST here makes sense because it changes the state on the server ($10 moves from one account to another every time this POST occurs).
What should happen in the case where it doesn't affect the server? The simple first answer would be to use GET. For example, you want to get a list of savings and checking accounts that have less than $100. You would then call something like GET to /accounts/searchResults?minBalance=0&maxBalance=100. What happens though if your search parameter need to use complex objects that wouldn't fit in the maximum length of a GET request.
My first thought was to use POST, but after thinking about it some more it should probably be a PUT since it isn't changing the state of the server, but from my (limited) understanding I always though of PUT as updating a resource and POST as creating a resource (like creating this search results). So which should be used in this case?
I found the following links which provide some information but it wasn't clear to me what should be used in the different cases:
Transient REST Representations
How to design RESTful search/filtering?
RESTful URL design for search
I would agree with your approach, it seems reasonable to me to use GET when searching for resources, and as said in one of your provided links, the whole point of query strings is for doing things like search. I also agree that PUT fits better when you want to update some resource in an idempotent way (no matter how many times you hit the request, the result will be the same).
So generally, I would do it as you propose. Now, if you are limited by the maximum length of GET request, then you could use POST or PUT, passing your parameters in a JSON, in a URI like:
PUT /api/search
You could see this as a "search resource" where you send new parameters. I know it seems like a workaround and you may be worried that REST is about avoiding verbs in the URIs. Well, there are few cases that it's still acceptable and RESTful to use verbs, e.g. in cases where calculation or conversion is involved to generate the result (for more about this, check this reference).
PS. I think this workaround is still RESTful, but even if it wasn't, REST isn't an obsession and an ultimate goal. Being pragmatic and keeping a clean API design might be a better approach, even if in few cases you are not RESTful.
As stated in http://www.boutell.com/newfaq/misc/urllength.html, HTTP query string have limited length. It can be limited by the client (Firefox, IE, ...), the server (Apache, IIS, ...) or the network equipment (applicative firewall, ...).
Today I face this problem with a search form. We developed a search form with a lot of fields, and this form is sent to the server as a GET request, so I can bookmark the resulting page.
We have so many fields that our query string is 1100 bytes long, and we have a firewall that drops HTTP GET requests with more than 1024 bytes. Our system administrator recommends us to use POST instead so there will be no limitation.
Sure, POST will work, but I really feel a search as a GET and not a POST. So I think I will review our field names to ensure the query string is not too long, and if I can't I will be pragmatic and use POST.
But is there a flaw in the design of RESTful services? If we have limited length in GET request, how can I do to send large objects to a RESTful webservice? For example, if I have a program that makes calculations based on a file, and I want to provide a RESTful webservice like this: http://compute.com?content=<base64 file>. This won't work because the query string has not unlimited length.
I'm a little puzzled...
HTTP specification actually advises to use POST when sending data to a resource for computation.
Your search looks like a computation, not a resource itself. What you could do if you still want your search results to be a resource is create a token to identify that specific search result and redirect the user agent to that resource.
You could then delete search results tokens after some amount of time.
Example
POST /search
query=something&category=c1&category=c2&...
201 Created
Location: /search/01543164876
then
GET /search/01543164876
200 Ok
... your results here...
This way, browsers and proxies can still cache search results but you are submitting your query parameters using POST.
EDIT
For clarification, 01543164876 here represents a unique ID for the resource representing your search. Those 2 requests basically mean: create a new search object with these criteria, then retrieve the results associated with the created search object.
This ID can be a unique ID generated for each new request. This would mean that your server will leak "search" objects and you will have to clean them regularly with a caching strategy.
Or it can be a hash of all the search criteria actually representing the search asked by the user. This allows you to reuse IDs since recreating a search will return an existing ID that may (or may not) be already cached.
Based on your description, IMHO you should use a POST. POST is for putting data on the server and, in some cases, obtain an answer. In your case, you do a search (send a query to the server) and get the result of that search (retrieve the query result).
The definition of GET says that it must be used to retrieve an already existing resource. By definition, POST is to create a new resource. This is exactly what you are doing: creating a resource on the server and retrieving it! Even if you don't store the search result, you created an object on the server and retrieved it. As PeterMmm previsouly said, you could do this with a POST (create and store the query result) and then use a GET to retrive the query, but it's more pratical do only a POST and retrieve the result.
Hope this helps! :)
REST is a manner to do things, not a protocol. Even if you dislike to POST when it is really a GET, it will work.
If you will/must stay with the "standard" definition of GET, POST, etc. than maybe consider to POST a query, that query will be stored on the server with a query id and request the query later with GET by id.
Regarding your example:http://compute.com?content={base64file}, I would use POST because you are uploading "something" to be computed. For me this "something" feels more like a resource as a simple parameter.
In contrast to this in usual search I would start to stick with GET and parameters. You make it so much easier for api-clients to test and play around with your api. Make the read-only access (which in most cases is the majority of traffic) as simple as possible!
But the dilemma of large query strings is a valid limitation of GET. Here I would go pragmatic, as long as you don't hit this limit go with GET and url-params. This will work in 98% of search-cases. Only act if you hit this limit and then also introduce POST with payload (with mime-type Content-Type: application/x-www-form-urlencoded).
Have you got more real-world examples?
The confusion around GET is a browser limitation. If you are creating a RESTful interface for an A2A or P2P application then there is no limitation to the length of your GET.
Now, if you happen to want to use a browser to view your RESTful interface (aka during development/debugging) then you will run into this limit, but there are tools out there to get around this.
This is an easy one. Use POST. HTTP doesn't impose a limit on the URL length for GET but servers do. Be pragmatic and work around that with a POST.
You could also use a GET body (that is allowed) but that's a double-whammy in that it is not correct usage and probably going to have server problems.
I think if u develop the biz system, encounter this issue, u must think whether the api design reasonable, if u GET api param design a biz_ids, and it too long.
u should think about with UI or Usecase, whether use other_biz_id to find biz_ids and build target response instead of biz_ids directly or not.
if u old api be depended on, u can add a new api for this usecase, if u module design well u add this api may fast.
I think should use protocols in a standard way as developer.
hope help u.
This question already has answers here:
When should I use GET or POST method? What's the difference between them?
(15 answers)
Closed 9 years ago.
I've only recently been getting involved with PHP/AJAX/jQuery and it seems to me that an important part of these technologies is that of POST and GET.
First, what is the difference between POST and GET? Through experimenting, I know that GET appends the returning variables and their values to the URL string
website.example/directory/index.php?name=YourName&bday=YourBday
but POST doesn't.
So, is this the only difference or are there specific rules or conventions for using one or the other?
Second, I've also seen POST and GET outside of PHP: also in AJAX and jQuery. How do POST and GET differ between these 3? Are they the same idea, same functionality, just utilized differently?
GET and POST are two different types of HTTP requests.
According to Wikipedia:
GET requests a representation of the specified resource. Note that GET should not be used for operations that cause side-effects, such as using it for taking actions in web applications. One reason for this is that GET may be used arbitrarily by robots or crawlers, which should not need to consider the side effects that a request should cause.
and
POST submits data to be processed (e.g., from an HTML form) to the identified resource. The data is included in the body of the request. This may result in the creation of a new resource or the updates of existing resources or both.
So essentially GET is used to retrieve remote data, and POST is used to insert/update remote data.
HTTP/1.1 specification (RFC 2616) section 9 Method Definitions contains more information on GET and POST as well as the other HTTP methods, if you are interested.
In addition to explaining the intended uses of each method, the spec also provides at least one practical reason for why GET should only be used to retrieve data:
Authors of services which use the HTTP protocol SHOULD NOT use GET based forms for the submission of sensitive data, because this will cause this data to be encoded in the Request-URI. Many existing servers, proxies, and user agents will log the request URI in some place where it might be visible to third parties. Servers can use POST-based form submission instead
Finally, an important consideration when using GET for AJAX requests is that some browsers - IE in particular - will cache the results of a GET request. So if you, for example, poll using the same GET request you will always get back the same results, even if the data you are querying is being updated server-side. One way to alleviate this problem is to make the URL unique for each request by appending a timestamp.
A POST, unlike a GET, typically has relevant information in the body of the request. (A GET should not have a body, so aside from cookies, the only place to pass info is in the URL.) Besides keeping the URL relatively cleaner, POST also lets you send much more information (as URLs are limited in length, for all practical purposes), and lets you send just about any type of data (file upload forms, for example, can't use GET -- they have to use POST plus a special content type/encoding).
Aside from that, a POST connotes that the request will change something, and shouldn't be redone willy-nilly. That's why you sometimes see your browser asking you if you want to resubmit form data when you hit the "back" button.
GET, on the other hand, should be idempotent -- meaning you could do it a million times and the server will do the same thing (and show basically the same result) each and every time.
Whilst not a description of the differences, below are a couple of things to think about when choosing the correct method.
GET requests can get cached by the browser which can be a problem (or benefit) when using ajax.
GET requests expose parameters to users (POST does as well but they are less visible).
POST can pass much more information to the server and can be of almost any length.
POST and GET are two HTTP request methods. GET is usually intended to retrieve some data, and is expected to be idempotent (repeating the query does not have any side-effects) and can only send limited amounts of parameter data to the server. GET requests are often cached by default by some browsers if you are not careful.
POST is intended for changing the server state. It carries more data, and repeating the query is allowed (and often expected) to have side-effects such as creating two messages instead of one.
If you are working RESTfully, GET should be used for requests where you are only getting data, and POST should be used for requests where you are making something happen.
Some examples:
GET the page showing a particular SO question
POST a comment
Send a POST request by clicking the "Add to cart" button.
With POST you can also do multipart mime encoding which means you can attach files as well. Also if you are using post variables across navigation of pages, the user will get a warning asking if they want to resubmit the post parameter. Typically they look the same in an HTTP request, but you should just stick to POST if you need to "POST" something TO a server and "GET" if you need to GET something FROM a server as that's the way they were intended.
The only "big" difference between POST & GET (when using them with AJAX) is since GET is URL provided, they are limited in ther length (since URL arent infinite in length).
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?
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.
My gut feeling is that the HTTP range extensions aren't designed for your use case, and thus you shouldn't try. A partial response implies 206, and 206 must only be sent if the client asked for it.
You may want to consider a different approach, such as the one use in Atom (where the representation by design may be partial, and is returned with a status 200, and potentially paging links). See RFC 4287 and RFC 5005.
You can still return Accept-Ranges and Content-Ranges with a 200 response code. These two response headers give you enough information to infer the same information that a 206 response code provides explicitly.
I would use Range for pagination, and have it simply return a 200 for a plain GET.
This feels 100% RESTful and doesn't make browsing any more difficult.
Edit:
I wrote a blog post about this: http://otac0n.com/blog/2012/11/21/range-header-i-choose-you.html
If there is more than one page of responses, and you don't want to offer the whole collection at once, does that mean there are multiple choices?
On a request to /db/questions, return 300 Multiple Choices with Link headers that specify how to get to each page as well as a JSON object or HTML page with a list of URLs.
Link: <>; rel="http://paged.collection.example/relation/paged"
Link: <>; rel="http://paged.collection.example/relation/paged"
...
You'd have one Link header for each page of results (an empty string means the current URL, and the URL is the same for each page, just accessed with different ranges), and the relationship is defined as a custom one per the upcoming Link spec. This relationship would explain your custom 266, or your violation of 206. These headers are your machine-readable version, since all of your examples require an understanding client anyway.
(If you stick with the "range" route, I believe your own 2xx return code, as you described it, would be the best behavior here. You're expected to do this for your applications and such ["HTTP status codes are extensible."], and you have good reasons.)
300 Multiple Choices says you SHOULD also provide a body with a way for the user agent to pick. If your client is understanding, it should use the Link headers. If it's a user manually browsing, perhaps an HTML page with links to a special "paged" root resource that can handle rendering that particular page based on the URL? /humanpage/1/db/questions or something hideous like that?
The comments on Richard Levasseur's post remind me of an additional option: the Accept header (section 14.1). Back when the oEmbed spec came out, I wondered why it hadn't been done entirely using HTTP, and wrote up an alternative using them.
Keep the 300 Multiple Choices, the Link headers and the HTML page for an initial naive HTTP GET, but rather than use ranges, have your new paging relationship define the use of the Accept header. Your subsequent HTTP request might look like this:
GET /db/questions HTTP/1.1
Host: paged.collection.example
Accept: application/json;PagingSpec=1.0;page=1
The Accept header allows you to define an acceptable content type (your JSON return), plus extensible parameters for that type (your page number). Riffing on my notes from my oEmbed writeup (can't link to it here, I'll list it in my profile), you could be very explicit and provide a spec/relation version here in case you need to redefine what the page parameter means in the future.
Edit:
After thinking about it a bit more, I'm inclined to agree that Range headers aren't appropriate for pagination. The logic being, the Range header is intended for the server's response, not the applications. If you served 100 megabytes of results, but the server (or client) could only process 1 megabyte at a time, well, thats what the Range header is for.
I'm also of the opinion that a subset of resources is its own resource (similar to relational algebra.), so it deserve representation in the URL.
So basically, I recant my original answer (below) about using a header.
I think you answered your own question, more or less - return 200 or 206 with content-range and optionally use a query parameter. I would sniff the user agent and content type and, depending on those, check for a query parameter. Otherwise, require the range headers.
You essentially have conflicting goals - let people use their browser to explore (which doesn't easily allow custom headers), or force people to use a special client that can set headers (which doesn't let them explore).
You could just provide them with the special client depending on the request - if it looks like a plain browser, send down a small ajax app that renders the page and sets the necessary headers.
Of course, there is also the debate about whether the URL should contain all the necessary state for this sort of thing. Specifying the range using headers can be considered "un-restful" by some.
As an aside, it would be nice if servers could respond with a "Can-Specify: Header1, header2" header, and web browsers would present a UI so users could fill in values, if they desired.
You might consider using a model something like the Atom Feed Protocol since it has a sane HTTP model of collections and how to manipulate them (where insane means WebDAV).
There's the Atom Publishing Protocol which defines the collection model and REST operations plus you can use RFC 5005 - Feed Paging and Archiving to page through big collections.
Switching from Atom XML to JSON content should not affect the idea.
I think the real problem here is that there is nothing in the spec that tells us how to do automatic redirects when faced with 413 - Requested Entity Too Large.
I was struggling with this same problem recently and I looked for inspiration in the RESTful Web Services book. Personally I don't think 206 is appropriate due to the header requirement. My thoughts also led me to 300, but I thought that was more for different mime-types, so I looked up what Richardson and Ruby had to say on the subject in Appendix B, page 377. They suggest that the server just pick the preferred representation and send it back with a 200, basically ignoring the notion that it should be a 300.
That also jibes with the notion of links to next resources that we have from atom. The solution I implemented was to add "next" and "previous" keys to the json map I was sending back and be done with it.
Later on I started thinking maybe the thing to do is send a 307 - Temporary Redirect to a link that would be something like /db/questions/1,25 - that leaves the original URI as the canonical resource name, but it gets you to an appropriately named subordinate resource. This is behavior I'd like to see out of a 413, but 307 seems a good compromise. Haven't actually tried this in code yet though. What would be even better is for the redirect to redirect to a URL containing the actual IDs of the most recently asked questions. For example if each question has an integer ID, and there are 100 questions in the system and you want to show the ten most recent, requests to /db/questions should be 307'd to /db/questions/100,91
This is a very good question, thanks for asking it. You confirmed for me that I'm not nuts for having spent days thinking about it.
One of the big problems with range headers is that a lot of corporate proxies filter them out. I'd advise to use a query parameter instead.
With the publication of rfc723x, unregistered range units do go against an explicit recommendation in the spec. Consider rfc7233 (deprecating rfc2616):
"New range units ought to be registered with IANA" (along with a reference to a HTTP Range Unit Registry).
You can detect the Range header, and mimic Dojo if it is present, and mimic Atom if it is not. It seems to me that this neatly divides the use cases. If you are responding to a REST query from your application, you expect it to be formatted with a Range header. If you are responding to a casual browser, then if you return paging links it will let the tool provide an easy way to explore the collection.
Seems to me that the best way to do this is to include range as query parameters. e.g., GET /db/questions/?date>mindate&date<maxdate. Upon a GET to the /db/questions/ with no query parameters, return 303 with Location: /db/questions/?query-parameters-to-retrieve-the-default-page. Then provide a different URL by which whomever is consuming your API to get statistics about the collection (e.g., what query parameters to use if s/he wants the entire collection);
While its possible to use the Range header for this purpose, I don't think that was the intent. It seems to have been designed for handling flaky connections as well as limiting the data (so the client can request part of the request if something was missing or the size was too large to process). You are hacking pagination into something that is likely used for other purposes at the communication layer.
The "proper" way to handle pagination is with the types you return. Rather than returning questions object, you should be returning a new type instead.
So if questions is like this:
<questions>
<question index=1></question>
<question index=2></question>
...
</questions>
The new type could be something like this:
<questionPage>
<startIndex>50</startIndex>
<returnedCount>10</returnedCount>
<totalCount>1203</totalCount>
<questions>
<question index=50></question>
<question index=51></question>
..
</questions>
<questionPage>
Of course you control your media types, so you can make your "pages" a format that suits your needs. If you make is something generic, you can have a single parser on the client to handle paging the same for all types. I think that is more in the spirit of the HTTP specification, rather than fudging the Range parameter for something else.