If so, create a new database on instance A and then use GBLOCKCOPY to copy from the existing database to the new one.   Then just move the new database to instance B.

That can help sometimes. Thank you. Just move - you mean unmount and download cache.dat file?

Is this a one-time migration of data from instance A to instance B?

My question is a request for general  approaches.  But my task now is to extract some part of consistent data from the large database to use it as test data in my local database for development purposes.

I would add my 5 cents. 

We have DeepSeeWeb projects which is Angular.js+gulp+Twitter Bootstrap thing.

It operates with DeepSee using MDX2JSON REST API.

You can see how it works e.g. in this GS Contest dashboard.

With Caché REST API and JSON as a transferring format for the data there a lot of pretty similar approaches for building UI:

React.JS, Angular2, Zen Mojo for web UI and .NET for the desktop.

In Catch block I usually execute Log method of Exception 

try {
                // App logic
}
 catch e
    {
                 d e.Log()
                 // Error handling logic
         }

And then find this in Application Error Log journal with all the stack.

E.g. link to the bugs in USER Namespace

http://localhost:57772/csp/sys/op/UtilSysAppErrorNamespaces.csp?$NAMESPACE=USER