Enable CORS not possible on SharePoint authenticated application without HttpModule

On 1st research, enabling CORS on SharePoint (on-premisse) web applications seems to be easy: just configure it in the web.config.

Source: enable Cross-Origin Resource Sharing - CORS on IIS7:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
 <system.webServer>
   <httpProtocol>
     <customHeaders>
       <add name="Access-Control-Allow-Origin" value="*" />
     </customHeaders>
   </httpProtocol>
 </system.webServer>
</configuration>

(also given as answer on stackoverflow: Enable CORS in SharePoint 2013)

However, on validating the effect of the config change, it turns out that the web.config based approach is insufficient in case of authenticated SharePoint application. You encounter 2 issues with only the configuration based approach:

1. In the web.config you can only specify either the '*' or explicit client domain for the Access-Control-Allow-Origin header. To interoperate with an authenticated application, you need to pass authorization headers and set Access-Control-Allow-Credentials to true. The issue is that according to the CORS specification (see also explanation on MDN, HTTP Access Control), if Access-Control-Allow-Credentials is set to true, Access-Control-Allow-Origin cannot contain *, to disallow just any client making requests with credentials attached. Also on CORS specification level, it is documented that in case of specific domain list, it actually means 1 single specific domain (https://www.w3.org/TR/cors/#access-control-allow-origin-response-header). This limits enable CORS to a single client application.
2. First call in the CORS protocol, is HTTP OPTIONS to establish whether CORS is allowed between client host domain and server host domain. A web.config enabled SharePoint application responds successful on the OPTIONS call with CORS-allow headers. However, SharePoint / IIS returns for authenticated webapplication the OPTIONS response with status code 401; and as result the preflight handling is stopped in browsers that respect the CORS specification.

Conclusion:
Enable CORS for a SharePoint / IIS authenticated webapplication, cannot be done via configuration only. Resolving both issues strict on SharePoint level is possible, but requires a custom HttpModule. If such server-side deployment is not allowed, another option is to resolve it on infra level, via a Reverse Proxy setup.

Background resources:

• Mozilla Developer Network - HTTP Access Control
• Fetch - CORS protocol
• Enable CORS in SharePoint 2013
• SharePoint 2013 - Integration Challenges - #4 Same Origin Policy & Authentication - CORS
• How to make GET CORS request with authorization header
• Cross-Origin Resource Sharing with Credentials

Approach to performant display from a SPList with multiple Lookups + Person fields

Issue: business users complain that the page loading of a SharePoint listview takes a long time, up to a minute. During page loading + rendering, the browser is totally unresponsive (Chrome even pops up a dialog about page unresponsive).

In the analysis for the root cause of this structural slow performance I observe that the list contains multiple Lookup fields to other lists in the site, and also a 'Person or Group' field - which is a specific type of Lookup, to the hidden UserInformationList.

This multitude of Lookups in the list is the most significant cause for the slow performance. However, also on user experience / functional level, it is wrong designed: the usage of the page is to first load + display all the items from the list, and next the end-user must filter to select the relevant items.

I therefore first aligned with business owner on another functional approach: let the end-user start with specifying the relevant filter(s) [can be a combination of filters for multiple columns], and then select + retrieve only the items from the list that satisfy the filter-conditions. For the technical design, the Search tool must be setup future-proof, aka cloud ready. Thus interoperating with SharePoint either via CSOM or via REST services. I decided to utilize REST, so that data is returned in JSON data-format, and can be directly data-binded in client-side UI (I used Knockout.js; but same holds for other clientside UI frameworks as Angular, Ember).

In SharePoint REST one uses the $expand parameter to include referred lookup values in the result set. But similar as for XsltListView, this quickly destroys any performance:

So I needed a way to avoid the Lookup-expands, while still being able to filter on and select the values of the Lookup fields. Totally getting rid of the Lookup columns is not an option: for consistent data-management it is a Must-Have that one can select only from the values maintained in the Lookup list. And it holds even more for the Person field: selecting via the PeoplePicker ensures a user-friendly selection + validation.

The approach I decided to is to 'flatten' the lookup values in additional columns. Functional management can still manage the data-items using the lookup functionality, and on data-selection + retrieval I avoid the need to $expand.

The elements of the approach

1. Per lookup value that is needed in the selection and/or display, add a single-line of text column to the list. Set them to hidden in the Item content type, so that their existense is invisible for functional management in the New/Edit/View list-forms;
   For the 'Person or Group' column, include 4 new fields, for
   1. Person Name,
   2. Photo,
   3. Department,
   4. and the UserId in the UserInformationList
2. Create a SharePoint Designer Workflow on the list that activates on ItemCreated + ItemModified events. Design the workflow to propagate ('flatten') the lookup values to their respective flattened counter-field;
3. Realize the bulk flattening of the existing list items through the same workflow, via javascript start the workflow on every item.

Impression of the solution setup + result

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Some handy resources:

• Hide list column on new, edit item-OOTB
• Extract the department from a Person/Group field
• Handling Lookup Columns in a SharePoint Designer Workflow
• Start Workflow using JavaScript or Ribbon Button