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

Software Engineering 3156

24-Sep-01

#6: Concurrency and Project

Phil Gross

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Administrivia

Groups are more or less set HW1 is out ACM ACM Programming Research

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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?

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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

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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

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Threads

Essentially, lightweight processes Ability to run concurrent sets of code in one

process Access to the same variables, methods:

information sharing easy

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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

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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!

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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

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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)

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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…)

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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

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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

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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)

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The GameEvent Element

A message between components AIs and clients request moves Server evaluates them and sends deltas out to

all connected clients/AIs

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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

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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

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Game Entry/Exit

Enter/Quit game– Client requests

KickGame– Server boots you

ExplicitSave– Client requests that current actor state be sent to

repository

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Editor

Get data from Editor mode of client to server SendObject, LoadMap

– Plus confirmations– More work to be done here…

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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

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Exceptions

ExMoveForbidden, ExNoConversation, ExKicked, ExNoPermession, ExUnknown