@pukkamustard recently announced a significant change of direction for the #software:openengiadina project, where at least the GeoPub client will be based on XMPP instead of ActivityPub. A similar decision for the CPub generic AP server has not yet been made. Here are some snippets quoted from the announcement as for the reasons to make this change:
The network protocol we have been using is ActivityPub. This seemed to be a perfect fit. We believe that managing knowledge is a social activity and ActivityPub is primarily a protocol for social interactions. Furthermore, ActivityPub can be extended to allow arbitrary RDF content via the JSON-LD serialization.
Unfortunately, we have come to realize that using ActivityPub is considerably harder than we expected:
Using JSON-LD as an RDF serialization is very complicated. It requires the usage of algorithms (e.g. the Expansion Algorithm or the Framing Algorithm) that are incomprehensible and just pure madness. JSON-LD maybe was really just not intended to be an RDF serialization and trying to use it as such is painful.
There are practically no implementations of the ActivityPub Client-to-Server protocol (C2S). This made developing and testing the client and server more time-consuming as we had to develop the protocol in lockstep on client and server. At the end we were still only compatible with our own software.
ActivityPub is not a complete specification and many additional protocols need to be implemented (e.g. WebFinger) in specific ways in order to be compatible with existing servers.
The fate of CPub - the generic ActivityPub server - is still undecided. We might re-purpose it for an XMPP to ActivityPub bridge or continue development as is.
With the new direction the intent is to retain compatibility and interop with the ActivityPub network:
For openEngiadina we intend to use RDF/XML (an XML serialization of RDF) over XMPP. In fact, we intend to use the ActivityStreams vocabulary serialized as XML over XMPP. This allows, at least in theory, a very clean interoperability with the existing ActivityPub network.