During my time working as a systems integration consultant in the financial services sector, I had to do a lott of integration with these core banking systems.

During my time working as a systems integration consultant in the financial services sector, I had to do a lott of integration with these core banking systems. Most of the time you try to reuse existing integration points from previous projects, as negotiating completely new interfaces had a lott of friction, both technical and business/compliance related, and could easily set your project back for more than a year or two. Integrations are usually delivering structured documents before a certain time in the evening for overnight batch processing. The existing documents with data based on offsets have by design regions of not yet assigned space in them to accomodate future updates and use. You negotiate over the bytes of those that can be assigned to your needs if new info is required. For data extraction you will sometimes find more 'modern' api's, but do not expect too many fine grained REST stuff. You'll often find yourself in meetings negotiating with regulation and compliance looking for ways to avoid costly new developments. On a sidenote: I often found compliance people a lott more pragmatic and solution oriented than IT. Having to convince my team that 'the letter' of a regulation did not mean you have to interpret litterally in the most restrictive way possible what is written was often more a challange than getting a deal with compliance. Now imagine hundreds of projects over many decennia of these strata of integrations, and you will start to get the first glimpse of why replacing these core systems is such a challange.