Software Engineering 3156

1. Software Engineering 3156 24-Sep-01 #6: Concurrency and Project Phil Gross

2. Administrivia • Groups are more or less set • HW1 is out • ACM • ACM Programming • Research 2

3. Concurrency • Humans perform tasks in parallel • Computers need to do so, too • In reality, not with 1-CPU computers: “multitasking” simulates • How do we do it? • How do we deal with the problems? 3

4. First cut • One of the modern operating system’s tasks is to support job or process control • Allow multiple programs (“processes”) to run at the same time • ps command on UNIX; CTRL-SHIFT-ESC on WinNT/2k/XP • Protect one process from another • Not always true: RTOS – to be discussed later 4

5. Process control • First-level solution: run multiple processes • “&” in UNIX, “start” in Win32 OS’s, fork( ) • Works surprisingly well – many tasks are discrete and can be run separately • Problem • How to share information between processes? • Often overkill 5

6. Threads • Essentially, lightweight processes • Ability to run concurrent sets of code in one process • Access to the same variables, methods: information sharing easy 6

7. Threads in Java • Class must extend Thread or implement Runnable • Implement run( ) method • Start by using .start( ), not .run( ) • Example • Suhit covering multithreaded server in recitation this week: perfect example of why you want this • Handler class executed in Thread: one controller, amny handlers 7

8. Life is easy…? • But wait: • Say both threads modify the same variable • Who wins? • We do not know • This is very, very bad – if we want randomness, we use java.util.Random! 8

9. Why is it not predictable? • You’re not the only thing in the system • System calls are affected • One system call may take longer than another, identical system call • There exist RTOS’s which are predictable in such situations 9

10. Why this is bad • Banking at IdiotBank™ • You’re depositing money • The bank is depositing interest • Bank software reads your balance to calculate interest • You deposit • Bank software adds 3% interest, saves your balance back • You’ve just lost $20,000! (Well, ~ $19,997) • (They’re not the idiots, you are) 10

11. Solution • Obviously, only one task should be able to access the balance at a time • Set a lock on bank account • Can only write if you have the lock • synchronized(account) method in Java (example) • When bank starts interest checker, it acquires the lock • Your deposit waits (“blocks”), since it doesn’t have the lock and can’t acquire it • Bank interest done, release lock • You acquire lock, deposit, release • Perfect! (Except you wanted to deposit before the interest calculation…) 11

12. Still not foolproof • IdiotBank™’s programmers use synchronize, but move process to night • Interest process requires access to interest percent table • They forgot that loan payment deductions occur at night, and needs to access interest percent table too 12

13. IdiotBank™ is doomed • Interest credit process locks account • Loan program runs, locks interest table • Interest credit process blocks on interest table • Loan program tries to subtract from your account, blocks there • Each process is waiting to get the other’s lock • Deadlock 13

14. Solving deadlock • Many tricks • Finer-granularity locks: lock balance • No read locks, just write locks: loan program doesn’t lock interest table • Priorities • Automatic reboot of computer • Interesting: no way to guarantee there won’t be deadlock • We’ll discuss more in Concurrency II 14

15. Project Stuff • Specific enough? • Lots of info in the XML schema • Unfortunately, you can’t read an XML schema • Hopefully fixing some of that today 15

16. XML Schema • Define some global complex types • Use these as you define top level elements • Two top level elements in current schema • May be split into two separate schema • One: static structure: the Map element • Two: dynamic messages: the GameEvent element 16

17. XML Elements • Have a name and a type • Can have a simple type • Often prefixed with xs: • xs:string, xs:int, xs:boolean, etc. • Or can be a complex type • Arbitrarily complex hierarchy 17

18. Element Children • Three types • Sequence: these elements in this order • Choice: any one of these elements • All: all of these elements in any order • Less common (tough to parse, weird restrictions) • Note that an element is never a direct child of another element 18

19. Map Element • Contains all the information necessary to describe a game map, including graphics for the client and simple scripts for the AI • Basic entities: • Tiles • Objects • Bots • Actors (user players) 19

20. Map Element Parts • Metadata – basic info on map • {tile | item | bot}set • For tile and item, unique ID plus URL to graphic • For bot, initial state plus scripts • More on scripts later • {tile | item | bot | actor}instances • An instance of a class at a particular location on the board and with its own instance data • Item instances have behaviors 20

21. Actors • Bots are an extension of this • Have hp, xp, name, speed attribute • And a status word • 32 bit value • Each bit represents some accomplishment in the game • Interactions with objects and bots can set bits • Setting bit 31 ends game • Covers both inventory and plot progression 21

22. Bots • Have a state, and sometimes a target, plus possibly a master • State: current activity, e.g. attacking, fleeing, wandering, conversing, assisting • State says what to do, but it’s up to the AI to actually do it • Target: for certain states, e.g. attacking • Master: If assisting, this is the character to assist 22

23. Bot Scripts • For a given state, describe what to do if someone is seen, conversed with, or interacted with • Filter: check any of my and their instance variables • Result: if filter passed, these effects happen • Seeing only allows effect on self (other may be far away) 23

24. The GameEvent Element • A message between components • AIs and clients request moves • Server evaluates them and sends deltas out to all connected clients/AIs 24

25. Move And Chat • Movement requests • Checked for validity and timing, and either granted or rejected • Start/EndChat • Enter conversation mode • Send/ReceivedText • Exchange chat messages 25

26. Attacks And Change • Attacked • Gives result of an assault • ReplaceObject • Bot with a corpse, closed chest with open chest, etc. • ChangeStats • Flip a status bit, give health, hp, xp, gold, etc. • StatusMessage • Get a message from the server 26

27. Game Entry/Exit • Enter/Quit game • Client requests • KickGame • Server boots you • ExplicitSave • Client requests that current actor state be sent to repository 27

28. Editor • Get data from Editor mode of client to server • SendObject, LoadMap • Plus confirmations • More work to be done here… 28

29. Data Transfer • GetMapData • Send absolute info • MapData • The absolute info • MapDelta • Changes since last delta • Deltas have a sequence number for detection of dropped events 29

30. Exceptions • ExMoveForbidden, ExNoConversation, ExKicked, ExNoPermession, ExUnknown 30