This is a proposal for GSoC 2013: create a Java/OSGi backend that allows Sling to use an Apache Solr server for content storage.
Interested students are welcome to get in touch with the Sling community via its mailing lists to discuss the project, see http://sling.apache.org/project-information.html#mailing-lists
Recent additions to the Sling resource management support using any existing content store as a backend for content storage, and creating an example ResourceProvider  that uses Apache Solr as its backend would be a good example of that.
This support for arbitrary content stores is relatively new in Sling, so we might discover some missing features or incomplete implementations along the way. The student will need to interact closely with the Sling community to discuss any improvements needed to reach this project's goals.
As Solr is probably not well suited for storing large binaries, a hybrid store will probably be needed, where "small" data items are stored directly in Solr and large, binary values stored on a filesystem-based structure like the Jackrabbit DataStore.
Advanced Java skills are required, as is good knowledge of HTTP protocols and clients and knowledge of automated testing tools to be able to validate the results.
The following pages give more information about GSoC @apache: