Thank you very much for your reply. I'm aware of the Summit and was expecting some delay, so no need to apologize (I hope that the Summit was a great event and you have enjoyed it). I wanted to clarify the scenario I was using to execute the example workflows, but instead of duplicating it here, I will point you to my corresponding question on StackOverflow, which I posted while waiting for answers on this site. My question and (as of now) two answers (thank you so much, Gabor and Rosaria!) can be found here: http://stackoverflow.com/q/35645896/2872891.
For better clarity, I'm including KNIME's full output from executing my test script below:
Knime: Cannot open display:
GTK+ Version Check
CompilerOracle: exclude javax/swing/text/GlyphView.getBreakSpot
INFO main BatchExecutor ===== Executing workflow . =====
ERROR main BatchExecutor Workflow is locked by another KNIME instance
INFO main BatchExecutor ========= Workflow did not execute sucessfully ============
JVM terminated. Exit code=3
I was not running any KNIME GUI (explicitly), as I'm interested in running KNIME on server in batch mode only. Moreover, per my understanding, specifying "-application org.knime.product.KNIME_BATCH_APPLICATION" explicitly tells KNIME not to initiate an interactive (GUI) session (assuming that, the message "Knime: Cannot open display:" looks suspicious.
Now, a couple of words in regard to my goals. I thought (before reading Gabor's comment and answer) that it is possible to export the whole workflow (analytical model(s), pre-processing and workflow graph per se) as PMML. I was confused by reading materials I mention on StackOverflow, in particular, related to modular PMML. Anyway, my goal is to allow users to create, modify and sometimes execute research workflows, using their preferred environments (i.e., using KNIME GUI locally, that is, on their desktops or laptops), whereas main execution tasks and workflow sharing to be done on the server side in batch mode only. Since workflow sharing is an important feature, I expected that users would be able to export their workflows (not only analytical models) in a common tool-independent format (which I though PMML is) and share them via server, which, in turn, would allow other users to import those workflows and execute and/or modify them, using their preferred tools (not necessarily KNIME). It seems that PMML currently doesn't offer this functionality. In his excellent answer on StackOverflow, Gabor presented the basic approach on how to create a workflow, exportable as PMML. However, it is seems to me that such workflows would still require KNIME to import and run them. Either that or my uderstanding is wrong and those workflows would be portable across all tools, supporting various open source PMML-based workflows execution environments (KNIME in batch mode, Kamanja, Cascading + Pattern, Augustus, Openscoring / JPMML, R, Weka, Spark). Please advise on these aspects (here or as an answer on StackOverflow). Your clarifications and/or help will be much appreciated.