Moving from VMS to Unix

Question

Once upon a time, a team of guys sat down and wrote an application in C, running on VMS on a VAX. It was a rather important undertaking and runs a reasonably important back-end operation at LargeCo. This whole shebang works so well that twenty-five years later it's still chugging along and doing it's thing.

Time passes and people retire and it so happens that the Last Man Standing has turned over the keys to a new generation who - we might imagine - are less than thrilled to find themselves caretakers of a system old enough to be their younger brother. Yet, as underwhelmed as they are by the idea of dealing with Ultra Legacy Systems, they can't justify the cost of replacing the venerable application.

LMS discovered that I habla unix and put this question to me. And since I habla unix but don't speak the C I shall summarize and put it to you. Long Story Short:

LMS wants to port LegacyApp, written in C. from VMS to unix. Resources? Any books he can read? People he can talk to?

Answer 1

1. The first question I'd need to ask is why, and I'd be leading the conversation in the direction of "Do you really need to port it off of VMS". There are a number of things worth mentioning about VMS:

   -> VMS is still actively developed and maintained by HP. They just release V8.4 for Field Test last week (see http://h71000.www7.hp.com/openvmsft/).

   -> VMS is available on new hardware; specifically HP's Integrity servers based on the Itanium processor.

   -> VMS is also available on virtual platforms via the Charon Emulation products.

   -> Popular estimates are that there are about 300,000 VMS systems still in active use today. LMS may be the last man at LargeCo, but he's far from the last man standing worldwide.

   -> Lots of information out there, see openvms.org for example, to see lots of current information on VMS, all from current users.

2. OK - you still want to port off of VMS. How do you do it? Well, it depends on lots of stuff.

   -> As others have said, how standard is the code? Chances are, not very. The more VMS-isms, the more difficult the job. 'nuff said.

   -> What is the database? If it's Oracle, probably not too tough to move to Oracle on some other platform. If it's some sort of custom DB based on RMS index files, then you've got more work to do, you'll need to re-create that pseudo DB, or, understand it enough to replace it with some relational DB.

   -> Besides C, what else is used to create the application? What's on the front end? DECforms? FMS? Is there a transaction engine, e.g. ACMS? RTR? These things will have a huge impact on the feasibility and effort required to port to UNIX.

   -> What other products are involved? Are there any 3rd party libraries being used? Are there 3rd party products in use that are critical to the application or functionality?

   -> Is this system clustered? If so why? You'll need to meet those same goals with the UNIX box.

   -> There are companies out there that will help you do it, and claim to have tools to make it easier, but my experience is that these companies tend to be selling you more services than products (i.e. you need to hire them to use the tools. It'll be expensive).

3. The book UNIX for OpenVMS Users will give the VMS novice some help in understanding VMS, but, as the title says, the book is really intended for the opposite purpose.

Answer 2

Everything written on VMS uses lots of VMS specific stuff it was just so convenient.

There are a few companies that sell compatibility libs to make the port easier - they wont be cheap though, VMS tended to be used where reliability mattered more than cost.

The other option is to run openVMS on some modern hardware, possibly in a VM.

Answer 3

I am sure Brian has made his decision by now, but for my sins of working for many years in DEC OpenVMS language support (yes, some people had this dubious honour) the real question I would have asked a customer such as Brian is: is it a real-time application or not? If it is the former, then it would be heavily dependent on many VMS system services which would rule out a 'port' and indicate a re-write. If it were the latter then the frequency of VMS system services should (possibly) be limited and make a port viable.

The acid test for me, would be to SEARCH *.c "SYS$", "LIB$" i.e. to search all of the C source files for "SYS$" and "LIB$" tags which prefix VMS system services. If the count for these are in the 10s then a port is probably likely, between 10 and 100 makes it possibly likely, but over a 100 makes a successful port highly unlikely.

Hope this helps

Answer 4

You have several choices.

1. Get the OpenVMS source, and continue to maintain Open VMS as if it were a Linux distribution. Some folks don't mind keeping up with Linux distributions and OpenVMS distributions. It can be done.

2. Try to recompile the VMS C into Linux. This can be trivial if the C used only standard libraries. This can be very, very difficult if the C used a lot of VMS libraries.

   Once you have facts at your fingertips, you can reevaluate this course of action. Since you didn't list a bunch of VMS library methods this program uses, it's impossible to tell how entangled it is with the OS.

   This may be trivial or impossible. It's difficult to tell without analysis of the source.

3. Write bridge libraries from VMS to Linux. If your program only does a few VMS things, this isn't very difficult. If your program does extensive VMS things, this is craziness.

   The bridge -- in the long run -- is a terrible idea. Managers love it, however.

   An alternative is to replace the VMS library calls with proper, portable Linux calls rather than write bridges. This is better in the long run, because it excises the non-portable features of the program.

4. Rewrite it from scratch in Python. That is usually simpler than trying to port the C code. It will be shorter, cleaner, simpler, and portable.

Answer 5

If you're willing to keep running VMS in a VM, you can look into CHARON-VAX ( http://www.charon-vax.com/ ). As previously mentioned, the ease of porting really depends a lot on how much of the VMS extensions were used; searching the source code for $ characters embedded in strings (usually with a 3-character leading substring, such as lib$gettime or dsc$descriptor or sys$foobar etc) will give you at least a basic idea of what VMS system functions are called and how likely they are to be portable, if the name is reasonably obvious.