Moving from VMS to Unix_问答_开发者_运维开发者技术经验分享

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 appl开发者_如何学编程ication.

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?

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.
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).
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.

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.

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

You have several choices.

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.
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.
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.
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.

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.

If it ain't broke, don't fix it! Why port it or migrate the app if you don't have to? Why not run it on a current install of OpenVMS running on an HP Itanium server; that is assuming you wish to upgrade the hardware, which may not even be necessary if your VAX hardware is still running strong.

To learn C, you might as well drag it from the horse's mouth: "The C Programming Language" by its inventors, Kernighan and Ritchie.

I can recommend "The UNIX programming environment" by (again) Brian Kernighan; a more authoritative source you'll hardly find, and it teaches you both Unix/C idioms and a bit of C programming at the same time.

For more depth and detail on C, I heartily enjoyed a book by Peter van der Linden: "Expert C Programming - Deep C Secrets".

You'll also want to wrestle LMS for a library documentation of VMS-specific C functions with (of course) special emphasis on those actually used in the app. That's where your porting effort will be.

The job could be easy or difficult, depending on how much machine-specific cleverness and bit-twiddling is done, and how many VMS-specific system calls are used. It would be very good if word size was equal (in other words, if your VMS box has a word size of 32 bits, don't run the code on a 64 bit version of Unix!)

Brian, I'm not sure if LMS specified/cared to port C-code or the WHOLE process. As too often people think of languages out of scope of systems.

If there're was a process built on VMS, most likely it used at least scheduling/batch facilities, which are often scripted in DCL (rather simple and clear language, unlike shell or perl scripting).

So the cost of porting the whole process may be higher than originally perceived by your LMS. Add here the reliability aspect, given your crunches with C, which is nothing impossible, of course, with enthusiasm and determination.

If you want simply give the C-code a try, as previously posted, search it for the "$" hits. Or just cc it with all headers present, the basics of compile-link command should be enough.

Alternatively, this looks like a consultant's call, as indeed such jobs were abundant at the "exodus" time. All said VMS remains quite a robust platform (24x7 is a norm!), unless the harware dies, then there're still tons of "exodus" spares. GOOD LUCK!

About a year and a half later, maybe you've already figured out what to do. My organization has recently decided to stick with OpenVMS instead of switching to Linux even though the old guard recently left. We just couldn't argue with what we felt was a very stable and reliable system. We are currently switching from Alpha servers to Integrity servers for end of life reasons. HP has been very helpful with our transition.

For that matter, there may be Linux vendors out there who can help with the transition. Ask your new hardware vendor if they have any recommendations.

Depending on what languages you already know, C is not that hard to learn. I taught myself C in the course of learning C++ after finally prying myself loose from Pascal. (VAX Pascal, plus Rdb/VMS, plus DCL formed a combination that was hard to beat.)

If the software is typical C, you'll spend more time learning the library functions than learning the language.

It's pretty lightweight stuff, but I went through the online tutorials for C++ that Microsoft makes available in conjunction with the express edition of Visual Studio for C++.

Here's the beginner's tutorial:

http://msdn.microsoft.com/en-us/beginner/cc305129.aspx

It's probably worth making the effort to ask why LMS wants to port the application to Unix. The answer may seem obvious, but properly exploring the reasons has its benefits. I would assume:

OpenVMS is an "ultra legacy platform", and for that reason alone is something that is not worth running an application on anymore;
It's tough to find anyone who is willing to maintain an application that runs on OpenVMS these days;
The hardware on-which OpenVMS runs is threatening to become moribund.

We have a similar challenge, but in our case the application in question not only runs on OpenVMS but is also written in COBOL. I would have to say that your situation is rosy in comparison given that your application is written in a cross-platform language.

In any case, I think if you're about to make a big decision like moving from OpenVMS to Unix it would be prudent to do a little due diligence. In your case, try to assess just how portable the code is--only then will you know what the scale of the effort is (worst case could quite easily be a multiple of best case). In C, code portability is mostly a function of the dependencies--are they "standard" or are they VMS-specific?

Our enquiries revealed that HP would be supporting OpenVMS on Itanium until at least 2022. There isn't necessarily a need to rush to another platform--perhaps you could keep things on OpenVMS whilst embarking on an effort to prepare the application for porting (make it less dependent on OpenVMS specifics).

VMS has a surprisingly healthy community and if it's the lack of Unix that's the issue, then maybe GNV could help bridge the gap?

Well u have a few options. if this code needs to be ported rather quickly, i would write a bridge library to emulate the vms libs. whener you get it back up and running on a *nix, then go through replacing the vms library calls with native/portable calls for *nix.

Also if there is a lot of optimizations in the code ie inline assembly and bit twiddling. then you will have to rewrite thi code, which will take an understanding of the VAX arch. also. be sure to check word size differences and endian differences