Why do I have to use POST for REST queries?

Question

I am trying to understand the logic with the SharePoint client.svc (_api) Web service.

For some queries, for example List GetItems, or search queries, SharePoint requires the use of POST in ajax calls. Queries are just read operations, without server side updates, so why is that?

Using POST is heavy, as you need to retrieve a request digest token first. I'd rather use a simple GET.

For an example of query that requires POST, see the answer on this page: http://social.msdn.microsoft.com/Forums/sharepoint/en-US/92cccd65-ba4c-4870-a858-7cd0e38a0482/how-can-i-use-caml-queries-with-the-rest-api

Answer 1

From the example you have supplied it seems that you only need to use POST if you are using CAML queries. You can use a simple GET if you use the OData querying method see OData in SharePoint 2013 with jQuery and SharePoint 2013 – CRUD on List Items Using REST Services & jQuery for fuller explanations and some simple examples.

See MSDN - Use OData query operations in SharePoint REST requests for the OData query syntax used.

The quote and code below is from SharePoint 2013 – CRUD on List Items Using REST Services & jQuery.

REST SERVICES – GET LIST ITEMS Getting List Items using the SharePoint REST Services is probably the simplest operation of them all. An example of how to do this is below:

// Getting list items based on ODATA Query
function getListItems(url, listname, query, complete, failure) {

    // Executing our colors ajax request
    $.ajax({
        url: url + "/_api/web/lists/getbytitle('" + listname + "')/items" + query,
        method: "GET",
        headers: { "Accept": "application/json; odata=verbose" },
        success: function (data) {
            complete(data); // Returns JSON collection of the results
        },
        error: function (data) {
            failure(data);
        }
    });    
}

Answer 2

I didn't find a definitive answer, but as I was exploring the REST search api I found some elements that could explain the need for POST.

The search api allows both GET and POST. POST requests are recommended in the following scenarios:

• When you'll exceed the URL length restriction with a GET request.
• When you can't specify the query parameters in a simple URL. For example, if you have to pass parameter values that contain a complex type array, or comma-separated strings, you have more flexibility when constructing the POST request.
• When you use the ReorderingRules parameter because it is supported only with POST requests.

My assumption: with requests that include CAML queries, like GetItems, there's a significant risk to exceed the URL length restriction (well, at least in older browsers), and that's why GET requests are not allowed.

[Update] the url limitations for Internet Explorer are detailed here: http://support.microsoft.com/kb/208427

Excerpt:

the POST method is not limited by the size of the URL for submitting name/value pairs. These pairs are transferred in the header and not in the URL.