1. Tzu-Han Wu15524883 Yi-Chi Chiang54641046 Han-Yang Ou59283702

2.  Motivation  System overview  Basic functions  Advanced functions  Summary

3.  Problems in single server ◦ Non-scalable ◦ Non-durable  Solution: Design a seat-booking system in distributed environment ◦ Scalability: Multi-server ◦ Durability: Fully replication

4.  Available  Temp. Reserved  Temp. Occupied  Unavailable  Reserved Client 1 Client 3 Client 2 Server 1 Server 2

5.  Available  Temp. Reserved  Temp. Occupied  Unavailable  Reserved

6.  Seat information ◦ Status  Available  Temporary reserved  Temporary occupied  Unavailable  Reserved

7.  Seat information ◦ Status  Available  Reserved  The client ID which is processing (Temp. Occupying) ◦ Version  To avoid updating old version status ◦ Flag  The clients which connected to this server cannot request the seat if the flag is set to true

8.  Server list (A list includes servers in the system) ◦ Server ID ◦ Server address ◦ Server load (Main)  Number of clients connected to the server.  Use for load balancing  Client list (A list includes clients connected to it) ◦ Client ID  Based on the server ID and sequence number ◦ Client address ◦ A list of seats which the client has requested

9.  Request a seat  Release a seat  Session commit  Session abort

10.  Reserve an available seat temporary  Problem: Several clients request the same seat at the same time ◦ Clients connect to same server  Solution: Flag ◦ Clients connect to different servers  Solution: Ricart-Agrawala algorithm

11. Client  Send request message Server  Check the flag of the seat ◦ If the flag is false then set the flag true then continues; else reject the request  Send seat request messages to other servers ◦ Continue if gets OK messages from all other servers; otherwise break  Set the seat status to client ID and update version  Add the seat to the client in the client list Time

12. Client  Modify the icon if request successfully Server  Send seat status update messages to other servers  Send seat status update messages to the clients Time

13. S1 S2 M S3 C101 C102 C202 C204 C205C306 C301 C003 C001 C005 The other server set flag 5 to trueThe other servers send OK to S2 Flag 5: True Assume seat 5 is available C204 send seat 5 request message to S2 Flag 5: false S2 set flag 5 to true S2 send seat request to other server Flag 5: true S2 sends update notification to other serversServers sends update notification to clients

14.  Release a seat which is reserved temporary before  No mutual exclusion problem

15. Client  Send release message  Modify the icon if request successfully Server  Set the seat status to be available and update version  Remove the seat from the client in the client list  Set the flag of the seat to false  Send seat status update messages to other servers  Send seat status update messages to the clients Time

16. S1 S2 M S3 C101 C102 C202 C204 C205C306 C301 C003 C001 C005 Assume seat 7 is occupied by C202 Flag 7: true C202 sends release message to S2S2 sets flag 7 to false Flag 7: false S2 sends update notification to other serversServers send update notifications to clients Flag 7: false Other servers set their flag 7 to false

17.  Commit all the seats which are reserved temporary before  The client cannot make any modification after commitment  No mutual exclusion problem

18. Client  Send commit message  Modify the icon if request successfully Time Server  For each seats of the client in the client list ◦ Set the seat to be reserved and update the version ◦ Send seat status update messages to other servers ◦ Send seat status update messages to the clients  Remove the client from the client list  Notify main server the load decrease

19. S2 sends seat 2 update notifications messages to other servers S2 sends seat 3 update notifications messages to other servers Servers send seat 2 update notifications to clients S1 S2 M S3 C101 C102 C202 C204 C205C306 C301 C003 C001 C005 Assume C205 has requested seat 2 and 3 Flag 2: true Flag 3: true Flag 2: true Flag 3: true C205 sends commit message to S2Servers send seat 3 update notifications to clients

20.  Release all the seats which are reserved temporary before and end the session

21.  Add a new server  Add a new client

22.  Add a server to the system dynamically  Get seat status from the main server  Problem: Other servers may update seat status at the same time ◦ Solution: status version  Compare status version before update

23. New server  Connect to main server  Query seat status  Query server information Server  Main server notifies other servers the new coming server  Add to its own server list  Send seat status  Send server information Time

24. S1 S2 M S3 N Notify new server Send other servers info and status of seats Notify other servers new server coming At the same time, the other server may send seat status update messages The new server should compare the version and decide to update or ignore the messages

25.  Add a client to the system dynamically  Which server should a client connect to? ◦ Strategy 1: All clients connect to the same server  Disadvantage: Non-scalable ◦ Strategy 2: Maintain a server list in clients  Disadvantage: Inappropriate in the environment which servers join/leave dynamically

26. ◦ Strategy 3: Every client connects to the main server first. Main server assigns the client to a server based on round-robin.  Advantage: Transparency  Disadvantage: The load of the servers may not be balanced ◦ Strategy 4: Every client connects to the main server first. Main server assigns the client to a server based on load of each server.  Use number of transactions in each server as load measurement  It’s the strategy the we use

27. Client  Connect to main server  Connect to assigned server  Get the client ID  Query seat status Server  Main server find the server with the lowest load  Return the address of that server  Generate a client ID to the client  Notify main server the load increasing  Send seat status Time

28. ID: 103 Connection Request Client address Ask appropriate server S1 S2 M S3 C101 C102 C202 C204 C205C306 C301 C003 C001 C005 N Reply address of S1 C103 Ask status of seats Reply status of seats

29.  Client ◦ Show seats status in real-time  Server ◦ Multi-server processing in distributed environment ◦ Fully replication ◦ Load balancing ◦ Add servers dynamically  Communication ◦ JAVA RMI