Patches leading to https://git.opendaylight.org/gerrit/c/controller/+/104745 encapulate Kryo so users are not directly exposed to it, while defanging the implementation. At that patch Kryo is an implementation detail and can be replaced by providing an alternative to KryoJournalSerdes – which really boils down to:

• keeping track of registration IDs and their manifestation in the serialized format
• implementing EntryInput/EntryOutput is a binary compatible way, of which
• perhaps readObject()/writeObject() are the most bothersome, but should boil down to Externalizable serialization, which in turn sal-akka-segmented-journal can enforce

Also, the @VisibleForTesting methods can actually be replaced with either a plain DataOutput.writeInt(), or perhaps WritableObjects.writeLong() – it is just testing, we do not actually need that.

In terms of us implementing EntryInput/EntryOutput in a binary compatible way, we'll probably need to directly reuse the implementation from com.esotericsoftware.kryo.io.Output and Input classes. Basically a copy-paste into our EntryInput/EntryOutput classes. I don't like this approach. Do we have any other choice? 

Depends on how much of that we are really using in the codepaths we are taking.