Quick Messaging Service John Sung

Objective Use Demeter/Java to the fullest –Use COOL for coordination –Use RIDL for RMI Ability to send messages “instantly” Ability to handle 1 server fault at a time

QMS Implementation Implemented in 3.1 phases –Phase 1: Client/Server database with accessor traverversals and testing infrastructure –Phase 2: Client’s GUI –Phase 2.1: Client/Server communication –Phase 3: Server/Server communication for Fault-Tolerance

QMS System Architecture Client Server User Server Notifications Logon/Logoff/ Messages

Client’s View of System User Logs on and off it’s server Sends message with source and destination to the server Server looks up in the database to find the destination client Another server will notify if the current server is unreachable

Server’s View of System User Message Delivery –Find destination client in the database –Send the message directly to the client Notifications to other servers in the system –Client Logon/Logoff – New Server Entering System –Server Failure Recovery

Server Failure Recovery Detection –Attempt to connect to all of the servers in the system at a regular interval Recovery –All servers broadcast timestamp of failure detection time –Each server determines the server with the lowest timestamp to take over –Failed Clients notified of change by the winning server –All servers update their databases

Fault-Tolerance Fault-Tolerance  Robustness Able to handle 1 server fault at a time Possible Failing Cases –Client logon/logoff during server failure recovery –New server attempting to enter system during server failure recovery

Screen Shots

COOl Example selfex processServerFailureNotification; mutex {serverFailureRecoveryStart, processServerFailureNotification,getWonServer} condition finishedRecovery = false; processServerFailureNotification { on exit { notificatoinReceived++; if (notificationExpected != -1 && notificationExpected == notificationReceived) finishedRecovery = true; } } getWonServer requires(finishedRecovery) { on exit { finishedRecovery = false; notificationExpected = -1; }

RIDL Example // the portal for the qmsServer object portal qmsServer { public void clientLogOn(ClientAddress newClientAddr) newClientAddr: copy; public void sendMessage(qmsUserMessage newMsg) newMsg: copy; public void clientLogOff(ClientAddress client) client: copy; public ServerList newServerEnterSystemRequest(ServerAddress newSrv) newSrv: copy return: copy ; public void newServerEnteredNotification(ServerAddress newSrv) newSrv: copy; public void newClientLogOnNotification(ServerAddress srvAddr, ClientAddress newClientAddress) srvAddr: copy newClientAddress: copy ; public void oldClientLogOffNotification(ServerAddress srvAddr, ClientAddress oldClientAddress) srvAddr: copy oldClientAddress: copy ; public Boolean isAlive() return: copy; } // qmsServer

Conclusion All of the basic functionality works –Client’s GUI –Client can send messages to server –Server can deliver messages from the client –New server can enter the system –Notifications of clients and servers –Very Limited Single Fault Tolerance There are still many bugs hidden in there!

Conclusion Found bugs in RIDL and COOL Demeter/Java Pros –Great for accessing databases and traversing trees/graphs –Great for testing parts of the program Demeter/Java Cons –Performance of code generator/compilation –Robustness of different features