Doug, Let me restate, so you can tell me if I'm wrong. "Writable is used instead of Serializable, because it provides for more compact stream format and allows for easier random access. They have different semantics, but don't have a major impact on versioning."
In my experience, using Serialization instead of DataInput/DataOutput streams has a major impact on versioning. Serialization keeps a lot of metadata in the stream. This makes detecting format changes very easy, but can really complicate backward compatibility. Also, serialization is geared toward preserving the connections of an object graph, which is behind a lot of the differences you mentioned. You didn't address the interoperability advantage of using standard Java classes instead of WritableS. As I mentioned, while using serialization would provide this benefit, it isn't necesary for it. You could provide a mechanism for Writers to be registered for classes. So, instead of IntWriteable, users could just use a normal Integer. The stream would be byte-for-byte identical to what it is now, but users could work with standard types. - Curt
