Sometimes synchronous calls between APIs are necessary. Therefore, a decision dealing this issue is required. All APIs must use the same way of synchronous communication.
No other alternatives are seriously considered.
To take this decision some research has to be done. No other evaluation is required.
In oder to make synchronous calls the domains must call the API. The API should already offer all required cases of synchronous calls. If it is not the case, the API has to implement it.
Synchronous Calls should not be used. However sometimes they are necessary. Because the APIs should already cover all cases of synchronous communication, the resolution is fairly simple.