Advanced Communication Among Distributed Objects

1. Advanced Communication Among Distributed Objects • Outline • Request synchronization • Request multiplicity • Request reliability 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 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 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 } } 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.) 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); … }; :Server :Client r =create_request() r:Request send_oneway Op() delete() 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) 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 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 } } 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 :Server :Client r =create_request() r:Request send_defeered() Op() get_response() delete() 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 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 } } Advanced Comm. between Distributed Objects

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

14. Asynchronous Request • Using Message Queues Request Queue enter remove Client Server remove enter Reply Queue 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 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) 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 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 Advanced Comm. between Distributed Objects