There are two separate solr thread pools for indexing, one serial and one parallel. The reason for the serial service is a bug in mycore, that can occur while uploading files. It is possible that the transaction is not closed before a file is send to solr. This results in possible empty file sizes. With the new onCommit feature in MCRSession this behaviour can be avoided.
The goal of this ticket is to get rid of the serial executor and have just one parallel. The other goal is to rewrite the MCRSolrIndexEventHandler to use the onCommit feature.