[jboss-dev-forums] [Design of JBoss Remoting, Unified Invokers] - Wire Versioning at the Application-level Needed

[joe.marques@jboss.com]

I feel that wire versioning IS a very important feature, because if people are using JBoss Remoting as the framework on which they are building distributed applications, then it will be of the utmost importance for many to be able to upgrade one node at a time to ensure zero downtime when upgrading the entire enterprise.

However, I think that only solves half of the problem.  Wire versioning in 2.0.0 only addresses the infrastructure, but there will still be issues at the application-level.  More precisely, regardless of whether the cooperating nodes' remoting capabilities can be upgraded independently, if the application code running on top of that infrastructure can't be updated in a similar "rolling" fashion, then much of the potential utility for wire versioning is being restrained.  

That being said, let's take a look at some of the use cases this application level scheme needs to handle.

Essentially, if I have two participants sending and receiving the following definition for Person:


public class Person
  | {
  |      private String name; // version 1 only has a name
  | 
  |      // appropriate getters/setters
  | }

And one of the participants gets upgraded with a new definition, say:

public class Person
  | {
  |      private String name;
  |      private int age; // version 2 also has age
  | 
  |      // appropriate getters/setters
  | }

Then there needs to be a mechanism to handle the graceful conversion of this one class definition to the other, so that enterprises can upgrade each node's definition in a "rolling" fashion.

The semantics of specifically what happens should perhaps be flexible within each remoting node's configuration, but somethings need to be defined that will intercept the call and handle the graceful transformation.

For instance, mabye extra attributes from an inconsistent class definition are simply dropped.  In the reverse scenario, missing attributes might get simply get type-defined default values.

This example assumed that the change in the newest version of the class was a simple addition.  What happens in more complex cases?  Let?s say from v2 to v3, Person had changes that looked like:

public class Person
  | {
  |      private String fullName; // refactor name to fullName
  |      private Date birthday; // change age type: int->Date 
  | 
  |      // appropriate getters/setters
  | }

Is it possible to design this application-level wire versioning in a way so that it executes a translation when the object is being deserialized / unmarshalled on the receiving end, so that I can transform names into fullNames (and vice versa), and birthdates into ages and ages into (albeit not perfectly accurate) birthdates?

I'm thinking a solution might have to use something other than Java serialization because by default there would be serialization exceptions all over the place unless the exact same class definition existed on both ends.  But maybe I'm mistaken here?  Maybe someone knows some clever trick that we could use to implement this indirectly using Java serialization, such as pushing out some metadata in front of each serialized attribute, and inspecting that metadata on the receiving end by the intercepting transformer?

Another approach that might was something I saw while reading through the source code for some of the unmarshallers.  It appears it's possible to use a remote classloader to instantiate the serialized object when the class definition doesn't exist on the receiving end.  So maybe this could be adapted slightly to instantiate the object when the class definitions differ, and then use some clever tricks with reflection to "copy and paste" the data from the remote object type to the local one (e.g. something along the lines of local.setAttr(remote.getAttr)).

Keep in mind that an overarching concern here is performance.  So, I want to keep that consideration clearly on the table as possible implementation strategies for this are discussed further.

-joseph

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