CQWP, PictureLibrary and Blobcache

ContentByQueryWebPart is a very usable ‘tool’ to display all types of SharePoint data in a page. Listdata, document libraries and also picture library. But be aware that CQWP has some strange default behaviour wrt picture library. CQWP by default displays the preview images, instead of the actual images.

This also has several performance ramifications:

1. On server side: actual images can be cached in blobcache, but preview images and also thumbnails are by SharePoint design not cached in blobcache. The reasoning is that blobcache is for content that is multiple times (often) retrieved, while preview images are typically only of interest for content manager upon functional management of the picture library contents. As result, each request for a preview image means that SharePoint needs to retrieve it from the content database.
2. On client, network + server side: browser cache can be applied to avoid the browser over and over requesting same image. But the browser then still needs to query the server whether the cached resource is unmodified at the server (response 304). For images / static resources that do not change (often), even this request can be avoided: minimizing request/response handling between client (browser), server, and the network transfer. Browsers support this via ‘max-age’ setting. SharePoint supports this ‘max-age’ setting for SharePoint content, via… Blobcache. For SharePoint content retrieved outside blobcache, as thus preview images, the max-age value is not set in the http response. As result, the browser will query the remote SharePoint server whether the image is unmodified, and the server responds with '304 NotModified'. And this can end-up in some noticeable latency, dependent on how busy the SharePoint server is:

Solution is to modify the CQWP configuration to retrieve and display the actual image. This comprises of 2 parts:

1. In the ItemStyle.xsl, change the rendering specification to display ‘EncodedAbsUrl’ iso ‘ImageUrl’;

2. And you need to modify the ‘CommonViewFields’ specification of CQWP instance, to also include ‘EncodedAbsUrl’.