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Emulate the mainframe and make a modern abstraction interface over the platform. We investigated this option for some IBM 360 COBOL code for a project I was ru

Emulate the mainframe and make a modern abstraction interface over the platform. We investigated this option for some IBM 360 COBOL code for a project I was running. We were a very small team and were market consumers (not implementers) of the original system which got open sourced in a panic. We eventually chose not too - but seriously considered it. If I were the owners (the Fed) I would have.

dpark 9y ago
This doesn't help if part of your goal is to have a system not written in COBOL. Emulating the older hardware and OS gives you an even more complex system that's harder to hire people to work on.
- binarymax 9y ago
  It helps for the first stage - getting the platform onto a more sustainable environment. Once the interface exists and has full coverage, parts of the backend can begin migration without a doomed-to-fail 'big bang' rewrite.
  - dpark 9y ago
    Does it really help? What does it cost to build a trustworthy emulator for an ancient system that you don't own the source for? What would it cost to just migrate your legacy applications to a newer mainframe that IBM supports?
    - binarymax 9y ago
      I wasnt suggesting writing an emulator from scratch. I was suggesting emulating the mainframe. Here is what we, specifically, were looking at: https://en.wikipedia.org/wiki/Hercules_%28emulator%29 https://en.wikipedia.org/wiki/Hercules_%28emulator%29
      - dpark 9y ago
        Don't you then need to pay IBM for the OS anyway? Will they license the OS for this use? Or did your system actually include the OS source?
        binarymax 9y ago
        For 360, it is now in the public domain: https://en.wikipedia.org/wiki/OS/360_and_successors https://en.wikipedia.org/wiki/OS/360_and_successors
        dpark 9y ago
        Interesting. Thank for the details.
ethbro 9y ago
One of the problems with attempting to write interface is the opaque source/consumer problem. E.g. I'm working on a system that I could hypothetically abstract (I've got access to it, can poke with enough tests and test data, etc). However. What I don't have is access to code or test injection into any of my sources / consumers. Both of which are expecting all the corner case quirks to be exactly identical and may actually have accreted software that depends on a specific quirk. A specific quirk that I have no way of knowing about. Or may send me something I've never seen and am not expecting because it's a 1:1,000,000 corner case, and we don't have any logging of an example that came through production. I haven't worked on too much of the heavyweight stuff like you have, but I tend to take the perspective that "a 100% compatible rewrite is impossible." 95%+ maybe, but we're going to have to deal with the <= 5% after it goes to production. Did you ever pursue writing and dropping a new tailored load balancer / router type application on the incoming data stream such that you could divert a specific portion onto new system(s)?
- pc86 9y ago
  > we don't have any logging of an example that came through production. If it's something that's never come through production, it is not already documented anywhere, and it has an occurrence rate of approximately 0.001%, is it really a feature that needs to be replicated?
  - tenken 9y ago
    Sure, because clients will say anything short of 100% success is failure. (Currently migrating a site of 125k pages of content with oodles of edge-cases)

dpark 9y ago

This doesn't help if part of your goal is to have a system not written in COBOL. Emulating the older hardware and OS gives you an even more complex system that's harder to hire people to work on.

binarymax 9y ago
It helps for the first stage - getting the platform onto a more sustainable environment. Once the interface exists and has full coverage, parts of the backend can begin migration without a doomed-to-fail 'big bang' rewrite.
- dpark 9y ago
  Does it really help? What does it cost to build a trustworthy emulator for an ancient system that you don't own the source for? What would it cost to just migrate your legacy applications to a newer mainframe that IBM supports?
  - binarymax 9y ago
    I wasnt suggesting writing an emulator from scratch. I was suggesting emulating the mainframe. Here is what we, specifically, were looking at: https://en.wikipedia.org/wiki/Hercules_%28emulator%29 https://en.wikipedia.org/wiki/Hercules_%28emulator%29
    - dpark 9y ago
      Don't you then need to pay IBM for the OS anyway? Will they license the OS for this use? Or did your system actually include the OS source?
      - binarymax 9y ago
        For 360, it is now in the public domain: https://en.wikipedia.org/wiki/OS/360_and_successors https://en.wikipedia.org/wiki/OS/360_and_successors
        dpark 9y ago
        Interesting. Thank for the details.

ethbro 9y ago

One of the problems with attempting to write interface is the opaque source/consumer problem. E.g. I'm working on a system that I could hypothetically abstract (I've got access to it, can poke with enough tests and test data, etc). However. What I don't have is access to code or test injection into any of my sources / consumers. Both of which are expecting all the corner case quirks to be exactly identical and may actually have accreted software that depends on a specific quirk. A specific quirk that I have no way of knowing about. Or may send me something I've never seen and am not expecting because it's a 1:1,000,000 corner case, and we don't have any logging of an example that came through production. I haven't worked on too much of the heavyweight stuff like you have, but I tend to take the perspective that "a 100% compatible rewrite is impossible." 95%+ maybe, but we're going to have to deal with the <= 5% after it goes to production. Did you ever pursue writing and dropping a new tailored load balancer / router type application on the incoming data stream such that you could divert a specific portion onto new system(s)?

pc86 9y ago
> we don't have any logging of an example that came through production. If it's something that's never come through production, it is not already documented anywhere, and it has an occurrence rate of approximately 0.001%, is it really a feature that needs to be replicated?
- tenken 9y ago
  Sure, because clients will say anything short of 100% success is failure. (Currently migrating a site of 125k pages of content with oodles of edge-cases)