Where to connect interfaces in a 4 system landscape for testing?

[ThomasHiesener]

Dear all,

I have a question I cannot find a really good approach for.

We have a 1 tier and 4 system landscape and soon 2 tier.

Current:
DEV-->QAS-->PRE-->PROD

Soon:
Maintenance
DEV-->QAS-->PRE-->PROD

Project
DEV-->QAS

Tests:

Functional integration tests (FIT) and user acceptance tests (UAT) in general, based on transports of copies, in the QAS system, happen.

Functional integration tests (FIT) and regression tests (RT) in general, based on released transports requests, in the PRE system, happen.

So far. so good. But when it comes to the point of Interfaces to (third-party) predecessor/successor systems, for example, MES, we run into big challenges with testing.

Challenge:

For example, MES landscapes and their interfaces to S4HANA or ECC systems are deeply integrated. Let's assume to have more than 60 interfaces to S4HANA or ECC systems.

When it comes to testing we must connect the interfaces to QAS system for functional integration tests.
But this is a bit risky because in QAS system is also the transport of copies. So finally not the best choice for integrative testing but a good start.

The final functional integration tests should be in the PRE system executed.

Questions:

But if it's not possible to connect 1 (third-party) predecessor/successor test system to both, QAS and PRE systems at the same time.
On the other hand, if you can only do valid functional integration tests with (third-party) predecessor/successor test system, what is the use case for QAS?
To connect all (third-party) predecessor/successor test systems to both, QAS and PRE system at the same time, is a huge effort (costs and administrative.

Does anybody solve this challenge and how?

And especially in a 2tier landscape, we have to connect 2 QAS systems and 1 PRE system to all (third-party) predecessor/successor test systems?
How does this work?

Can you help?

Best regards

Thomas