1. Advanced Comm. between Distributed Objects 1 Advanced Communication Among Distributed Objects  Outline  Request synchronization  Request multiplicity  Request reliability

2. Advanced Comm. between Distributed Objects 2 Request Synchronization  Request synchronization  Synchronous  Wait for the results  Oneway  Do not expect any results or exceptions  Request returns control to client as soon as the middleware accepts the request  Server and Client execute concurrently  Deferred synchronous  Dynamic  Explicit request object  Invoke more than one concurrent operations with single thread  Asynchronous  Callback from the server to client  Non-synchronous requests can be achieved by synchronous requests and multi-threading  COBRA provide direct support for all 4 different requests

3. Advanced Comm. between Distributed Objects 3 Oneway Request  Using thread  Spawn a child thread to make synchronous request  Terminate the child thread once the request is finished  Performance penalty

4. Advanced Comm. between Distributed Objects 4 Oneway Request Class PrintSquas { static void main(string[] args){ Team team; Date date; // initialization.. OnewayReq a = new OnewayReq (team, date); a.start(); // continue … } Class OnewayReq extends Thread { Team team; Date date; OnewayReq (Team t, Date d){ team = t; date = d;} public void run(){ team.print(date); // call remote method } Class PrintSquas { static void main(string[] args){ Team team; Date date; // initialization.. OnewayReq a = new OnewayReq (team, date); a.start(); // continue … } Class OnewayReq extends Thread { Team team; Date date; OnewayReq (Team t, Date d){ team = t; date = d;} public void run(){ team.print(date); // call remote method }

5. Advanced Comm. between Distributed Objects 5 Oneway Request  Using thread  Spawn a child thread to make synchronous request  Terminate the child thread once the request is finished  Performance penalty  CORBA  Static and dynamic invocation support oneway request  IDL defines oneway operations  Precondition: –No out, or inout parameter –Viod return type –No type-specific exceptions  Static invocation, Server decides this synchronization scheme  Dynamic invocation, client can choose oneway request (review the Request object, send operation.)

6. Advanced Comm. between Distributed Objects 6 Oneway Request  CORBA  IDL defines oneway operations  Static invocation, Server decides this synchronization scheme  Dynamic invocation, client can choose oneway request interface Team { oneway void mail_timetable (in Date date); … }; interface Team { oneway void mail_timetable (in Date date); … }; : Client : Server r:Request r =create_request() send_oneway delete() Op()

7. Advanced Comm. between Distributed Objects 7 Oneway Request  Comparison  Thread  Less efficient (Create/delete thread)  IDL (oneway)  Less flexible  Type safe  DII  Flexible using different flag with send  Type unsafe  Less efficient (Create/delete request object)

8. Advanced Comm. between Distributed Objects 8 Deferred Request  Using thread  Spawn a child thread to make synchronous request  Wait for termination of the child request thread  Obtain the result  Performance penalty

9. Advanced Comm. between Distributed Objects 9 Deferred Request --- Using Thread class PrintSquad { public void print (Team team, Date date) { DefsyncReqPrintSquad a = new DefSyncReqPrintSquad (team, date); // do something here. a.join(this) ;// wait for request thread to die System.out.println(a.getResult()); } class DefSyncReqPrintSquad extends Thread { … DefsyncReqPrintSquad (Team t, Date d); Public String getResult(); Public void run() { strings; s = team.asString(date); // call remote method and die } class PrintSquad { public void print (Team team, Date date) { DefsyncReqPrintSquad a = new DefSyncReqPrintSquad (team, date); // do something here. a.join(this) ;// wait for request thread to die System.out.println(a.getResult()); } class DefSyncReqPrintSquad extends Thread { … DefsyncReqPrintSquad (Team t, Date d); Public String getResult(); Public void run() { strings; s = team.asString(date); // call remote method and die }

10. Advanced Comm. between Distributed Objects 10 Deferred Request  CORBA  Only dynamic invocation interface support deferred synchronous request  Cannot be issued using client stub  get_response: no-wait returns control back to client with indication if the result is ready : Client : Server r:Request r =create_request() send_defeered() delete() Op() get_response()

11. Advanced Comm. between Distributed Objects 11 Asynchronous Request  Using thread  Similar to deferred synchronous request  Request thread invokes a call back operation  Request thread join the main thread

12. Advanced Comm. between Distributed Objects 12 Asynchronous Request --- Using Thread Interface Callback { public void result (String s); } class PrintSquad implements Callback { public void print (Team team, Date date) { AsyncReqPrintSquad a = new AsyncReqPrintSquad (team, date, this); a.start(); // continue to do some work here } public void result (String s) {// callback System.out.println(s); } class AsyncReqPrintSquad extends Thread { Team team; Date date; Callback call; AsyncReqPrintSquad (Team t, Date d, Callback c); Public void run() { string s; s = team.asString(date); // call remote method call.result(s); // pass result to parent thread } Interface Callback { public void result (String s); } class PrintSquad implements Callback { public void print (Team team, Date date) { AsyncReqPrintSquad a = new AsyncReqPrintSquad (team, date, this); a.start(); // continue to do some work here } public void result (String s) {// callback System.out.println(s); } class AsyncReqPrintSquad extends Thread { Team team; Date date; Callback call; AsyncReqPrintSquad (Team t, Date d, Callback c); Public void run() { string s; s = team.asString(date); // call remote method call.result(s); // pass result to parent thread }

13. Advanced Comm. between Distributed Objects 13 Asynchronous Request  CORBA : Client : Server r:Request r =create_request() send() Op() Callback ()

14. Advanced Comm. between Distributed Objects 14 Asynchronous Request  Using Message Queues ClientServer Request Queue Reply Queue enter removeenter remove

15. Advanced Comm. between Distributed Objects 15 Request Multiplicity  Request multiplicity  Unicast (from one client to one server)  Group request  same operations from multiple server objects  Group requests are anonymous  Group composition is managed by a channel  Group membership is unknown to the request producer  Group members are request consumers  Multiple request  Different operations from different server objects that are known to the client  Requested operations are independent with each other with no precedence relation  Client can collect results as they become available

16. Advanced Comm. between Distributed Objects 16 Request Multiplicity  Implementing group request  Channel (Example 7.5)  CORBA Event channel (Example 7.6)  Implementing multiple request  Thread  Tuple space (Example 7.8) –Out –In (blocking call) –Rd (blocking call)  Performance and development overhead  DII (Example 7.9)

17. Advanced Comm. between Distributed Objects 17 Request Reliability  Unicast Request Reliability  Exactly-once  Highest reliability  Depends on middleware, fault-tolerant, hardware, os platforms  Expensive  Atomic  Avoid side-effects when a failure occurs  Transactions  At-least-once  Middleware guarantees the execution  Multiple execution of a single request due to reply lose  If request modify server’s state, this semantics is not desirable  At-most-once  If not executed, errors are raised  Maybe  No guarantee of execution  Client doesn’t know the failure  Oneway request in CORBA

18. Advanced Comm. between Distributed Objects 18 Request Reliability  Group and Multiple Request Reliability  K-reliability  Totally ordered  Best effort  Trade off between performance and Reliability