Setting DSS (SwitchToOldAssembler) to true will ensure that the Parse XML (Rule-Parse-XML) rules will use pre 7 assembler which is a robust, proven assembler with zero side effects.
You can go into production with the DSS SwitchToOldAssembler set to true. There are other customers in production who are running with this DSS setting set to true.
There are no performance impacts.
Parse XML rules are assembled. The way these rules are assembled, has changed in 7.1.
Difference between the two assemblies is that in pre 7.1 the entire incoming XML used to get parsed and mapped to the clipboard page (1stcopy), usually in the Integration layer having a data model different from the Application layer data model, and then customers would have to map data from Integration layer clipboard page to Application layer clipboard page (2nd copy), using, say, a data transform. This poses a performance problem if the XML contains several hundred nodes while you are actually interested in only a handful of them.
In 7.1, a new assembler was written, which avoids the 1st copy of XML onto Integration layer clipboard page, by wrapping the XML with a Java object and attaching that Java object to the clipboard page. XML data from this Java object is retrieved on-demand.
When you open Rule-Parse-XML class in PRPC, on the advanced tab, you will see two assembly aspects - model (the new assembly which is the primary aspect) and action (the old one). When a Parse XML rule gets saved, both the assemblies get generated. By default, at runtime the model aspect gets used. The DSS "SwitchToOldAssembler” when set to true forces the runtime to use the action aspect.