Regarding the understanding of Northbound interface and Sounthbound interface of a system, which one will be correct?

  • A

In understanding A, the traffic from/to higher level system is northbound interface (blue), while that from/to lower level system is southbound interface (orange).


  • B

In understanding B, the traffic from lower to higher is northbound (blue), while that from higher to lower is southbound (orange).


  • 3
    Blue vs. orange, up vs. down, north vs. south, A vs. B. All of these are arbitrary; they are merely stand-ins for other concepts, which depends entirely on context.You have given absolutely zero context. You've essentially asked us the relationship between stand-ins without knowing what they stand for.
    – John Wu
    Feb 7, 2020 at 9:27
  • It's option A, not B; It's the role and the positioning, not the direction of flow.
    – Erik Eidt
    Feb 7, 2020 at 15:59

1 Answer 1


The traffic itself is not the interface. The interface is, in one sense, represented by the rules and conventions of how one system talks to another, and in a different (but related) sense, it is a piece of code on the periphery of a system that implements those rules, making the communication possible.

When two systems communicate, data can flow in either direction, depending on what exactly these systems do. For example, the internal code of some system may make calls to its northbound interface layer in order to talk to a higher level system; the northbound interface will then communicate with the southbound interface of the other system, which will in turn interact with the internals of that higher level system.

So it's more like this:

enter image description here

The interfaces define the way the communication happens, making the internals of each system decoupled from each other.

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