Mainframe modernization

Question

Hello,
We plan to move a 20+ years old Mainframe (CICS, Cobol and DB2) application to Windows platform with SQL.
The business request is to maintain uninterrupted operations while development is in progress and to incrementally release completed work into production.
There are tones of articles on the web and we identified “Rehostong”, “Regenrationg” and “Rewriting” as possible strategies: each of them have disadvantages and risks.
Would someone who went through Mainframe migration process share from their experience and make some suggestions on strategies to use?
If we were to choose the “Rewrite” options, during the development that will take years, we need to have data synchronised between Mainframe and SQL so the business can use the Mainframe and the new system.
Does anybody have any experience on how to achieve this? 
Thank you,
Gabriela

Answer 1

First, why would a single application take years to rewrite?  Especially if it's a DB2 application.  If you resist the urge to make the .NET/SQL app fancy, and perform straightforward ports of DB2 > SQL.  Reimplement the COBOL programs
in either TSQL stored procedures, or C# code.  Replace the screens with ASP.NET screens.  It may not take all that long.  The key thing to remember in rebuilding an application is that you have a working design (which is the hardest part of
application development).  It may be an _old_ design, and it may not be that good.  But it works.  So use it.  Slavishly copy the design  of the tables, programs and screens.  This makes implementation, testing and migration much
easier than trying to improve the system during the port.  Leave that for later releases after the migration. 
Also resist the temptation to translate COBOL code to .NET.  That's a false economy.  Don't use an automatic transator, and don't do a line-by-line copy of the COBOL code.  Instead understand what the program is doing, and do the same thing
in a .NET/SQL best-practice way.  If you copy the COBOL logic, you'll end up with tons of cursors and nested loops. 
But assuming the rewrite does take years, and you want to migrate incrementally, you need some good integration between the mainframe and Windows.  And you will need COBOL programmers to write new CICS programs.
Check out HIS
Microsoft Host Integration Server
https://msdn.microsoft.com/en-US/library/dn148340.aspx
Which is an extremely mature and capable integration server for mainframes.  You can integrate at the database layer, the CICS layer or at the UI layer (LU2 screen scraping).  You need to divide the application into functional units that can be
migrated independently, with their data replicated back to the host.  This is often difficult or impossible, so you need to call CICS transactions from your .NET app, and vice versa, and often to do this you will need to build or modify CICS transactions
(thus the need for the COBOL programmers). 
So it's do-able.  But overall it's significantly more complex and difficult than a simple rebuild.
David
David http://blogs.msdn.com/b/dbrowne/