SharePoint 2013 doesn't allow publishing content directly into it's index. It does provide a way to create connectors that it can use to crawl non native content called SharePoint Business Data Connectivity (BDC).
In previous versions we offered a WCF based BDC connector to index content into SharePoint search. This component used a File System based intermediate repository, which brought a series of inconveniences regarding performance, security and scalability.
With the creation of STageR (the mongodb and node.js based staging repository) it was now possible to create a more compact solution that would leverage all the capabilities of STageR when indexing content into SharePoint Search.