1. Extending Java RMI for Dynamic Reconfiguration
Presentation for ECE 8813
Georgia Institute of Technology
Spring 2003
Christophe Levand

2. Java RMI
CLIENT
RMI Registry ?
myObj
SERVER

3. Java RMI
RMI Registry
Naming.bind(“hello”, myObj)
SERVER
myObj

4. Java RMI Naming.lookup(“hello”) lookup returns the stub of myObj.
CLIENT
Naming.lookup(“hello”)
RMI Registry
lookup returns the stub of myObj.
The stub code resides on the client.

5. Java RMI Stub on client / Skeleton on server
Stub/Skeleton responsible for parameters marshalling/unmarshalling.
Client uses the stub through an interface. The interface is implemented by the class of myObj.
The stub is following the proxy design pattern.
Consequence: Client uses the remote object as if it was “local”.

6. Dynamic reconfiguration: Motivations
RMI
RMI
Server A
Server B
Server C

7. Dynamic reconfiguration: Motivations
RemoteException !
Server A
Server B
Server C
RMI ?

8. Dynamic reconfiguration: Motivations
Solutions?
The client intercepts the Exception and makes a new lookup
Error handling is not transparent to the application.
Before moving the target component, one has to make sure that there is no ongoing transactions.
Extend the RMI middleware  XRMI.
From a static to a dynamic approach.

9. RMI vs XRMI XRMI RMI Abstraction Client Server RMI Registry RMI Stub
lookup (1)
uses (3)
forwards to (4)
RMI Registry
creates (2)
RMI Stub
RMI
Client
Server
Component
Manager
Virtual Stub
lookup (1)
creates (2)
uses (6)
RMI lookup (3)
RMI Registry
creates (4)
RMI Stub
uses (5)
forwards to (7)
XRMI
Abstraction

10. XRMI: General perspective
Attributes of the virtual stub:
- ClientName / ServerName provided by the CM
A target reference provided by the CM.
Dependency list:
(client, server)
- lock flag: to block new invocations
invocation counter
Client
Server
lookup (1)
uses (6)
Component
Manager
Virtual Stub
creates (2)
forwards to (7)
RMI lookup (3)
uses (5)
RMI Registry
creates (4)
RMI Stub
XRMI

11. XRMI: Scenario 1
Client needs to calls a method on the remote server object.
Lock flag is false, so a call can be made.
Virtual stub adds a dependency.
Virtual stub delegates the call to rmi stub (target reference), which forwards to server.
After the server sends the result back, Virtual stub removes the dependency.
Virtual stub returns the result to the Client.
Client
Server
lookup (1)
uses (6)
Component
Manager
Virtual Stub
creates (2)
forwards to (7)
RMI lookup (3)
uses (5)
RMI Registry
creates (4)
RMI Stub
XRMI

12. XRMI: Scenario 2 XRMI Server needs to be moved to another location.
The CM on Server’s location broadcasts a query to all CMs in the network.
Each CM gathers a list of all its Virtual stubs that point to server.
For each Virtual stub of that list, set the lock flag to true: to block any new requests.
Each Virtual stub of that list monitors its dependency list and sends a signal to the CM when the list is empty.
Each CM sends a signal to the CM in Server’s location.
Client
Server
lookup (1)
uses (6)
Component
Manager
Virtual Stub
creates (2)
forwards to (7)
RMI lookup (3)
uses (5)
RMI Registry
creates (4)
RMI Stub
XRMI

13. XRMI: Scenario 2 XRMI For each virtual stub used by server:
set lock to true
monitor the dependency list.
send a signal to the CM when the list is empty.
the CM moves the server to another location.
the CM broadcasts a signal to every CMs.
Each CM makes an RMI look up for the remote server and update the target reference in their virtual stubs that point to the old location.
Client
Server
lookup (1)
uses (6)
Component
Manager
Virtual Stub
creates (2)
forwards to (7)
RMI lookup (3)
uses (5)
RMI Registry
creates (4)
RMI Stub
XRMI

14. XRMI: Improvement XRMI remote server XRMI local server no RMI lookup
Client
Server
Client
Server
Component
Manager
Virtual Stub
lookup (1)
creates (2)
uses (3)
forwards to (4)
XRMI local server
lookup (1)
uses (6)
Component
Manager
Virtual Stub
creates (2)
forwards to (7)
no RMI lookup
no parameter marshalling
RMI lookup (3)
uses (5)
RMI Registry
creates (4)
RMI Stub
XRMI remote server

15. XRMI: Improvement XRMI Attributes of the virtual stub:
- ClientName / ServerName provided by the CM
A target reference provided by the CM.
Dependency list:
(client, server)
- lock flag: to block new invocations
invocation counter
 when high, move client to the same location as server.
Client
Server
lookup (1)
uses (6)
Component
Manager
Virtual Stub
creates (2)
forwards to (7)
RMI lookup (3)
uses (5)
RMI Registry
creates (4)
RMI Stub
XRMI

16. RMI / XRMI cost comparison
Runtime environment
Middleware
Time (ms)
Caller and callee on same computer
RMI
1.01
XRMI
0.06
Caller and callee on different computers
2.79
2.83

17. Resources
Xuejun Chen, Extending RMI to Support Dynamic Reconfiguration of Distributed Systems. Proceedings of the 22nd International Conference on Distributed Computing Systems, page(s): 401 – 408, 2002.