Note we also need to plan the xApp adaptation effort related to this item and coordinate/inform other projects of needed work.

Make xApps independent of E2 ASN.1. This would be, e.g., removal of SubscrptionRequest ASN.1-encocded RMR messages in the xApp interface. So that xApps only depend on the ASN.1 they actually understand. E.g. an S1 xApp only needs S1 ASN.1. Most likely this means Protobuf + Go/C++ library on top of that. E.g if E2SM uses only JSON encoding, then xApp must be implementable without ever touching an ASN1 compiler.

Since subscriptions are not that performance relevant, we could handle this also via REST, but this will make xApps use REST (behind the scenes of an API) and RMR.

Remember a goal of xApps is that they can be implemnted as event polling loop (epoll). And xApps might  have to issue subscriptions also outside of their startup logic. I.e., subscriptions should be possible to issue at the same time as processing RMR insert messages.