In previous releases of Citrix's XenApp solution, every server in the Farm was required to download all of the Farm's data to its Local Host Cache when joining.  This process consumes large amounts of bandwidth, which on a WAN connection can be especially problematic.  Not to mention, the XenApp servers were forced to devote resources to the overhead processes involved with maintaining numerous read/write operations with the  IMA datastore.

To resolve this issue Citrix has created a separate "Worker" role for servers joining a XenApp 6.5 Farm.  With this role installed, the servers operate in a "Session-only" or "Session-host" mode.  These servers operate with the one task of hosting user sessions, and as a result only need to sync a subset of data to their LHCs.  Meaning that these servers operate with much less overhead than a traditional "Controller" mode server allowing them to join the Farm much faster, consume less bandwidth, and consume less resources overall. On the flip side, these hosts are unable to serve as Zone Data Collectors, perform XML brokering tasks, and/or participate in IMA elections.

The server mode is selected when first joining the machine to the XenApp Farm.  To switch modes, the server must leave then re-join the Farm.  A well planned XenApp 6.5 deployment with the right quantity of Controller and Worker servers allow for a significant performance improvement over environments with a previous version of XenApp installed.