TIVDM2 Introduction, AST extensions and Concurrency in VDM++

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Introduction to VDM2, Simple AST extensions and Concurrency in VDM++

Professor Peter Gorm Larsen

(pgl@iha.dk)

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Agenda

Administrative information about the course

• Simple AST Extensions

• Concurrency primitives in VDM++

• Example: POP3 Server

• Example: MSAWConc

• Concurrency and Overture

• Feedback on abstraction test and VDM1 reports

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Teaching Material

• John Fitzgerald, Peter Gorm Larsen, Paul Mukherjee, Nico Plat and Marcel Verhoef: Validated Designs for Object-oriented Systems. Springer Verlag, 2005. (second part)

• Development Process of Distributed Embedded Systems using VDM, Peter Gorm Larsen et al, 2010.

• The Overture tool (v0.2) used during the course

• Enterprise Architect

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• All information concerning this course including lecture notes, assignments announcements, etc. can be found on the TIVDM2 web pages http://kurser.iha.dk/eit/tivdm2/

• You should check this site frequently for new information and changes. It will be your main source of information for this unit. The layout of the WebPages should be fairly self explanatory

• Campus WebPages will be used only for mailing information

TIVDM2 web pages

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• Confrontation with the teacher• Thursdays 8:00 – 16:00 in Room 316• Read in advance of each lecture

• Combination of • Lessons teaching theory • Strategy for lessons: quick intro to concepts and then usage in

larger examples• Continuation of projects where theory is turned into practice• Using Overture for projects

• Exam form• 15 minutes oral examination without preparation + 5 minutes for

evaluation• Oral examination will be centered around projects performed• Projects reused from TIVDM1 and extended further• Exam will be on the 10th of June 2010

Education Form

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Learning Objectives

The participants must at the end of the course be able to:• Discuss and compare different abstract models

(sequential, concurrent and distributed) in relation to each other.

• Explain constructs and concepts in the concurrent, real-time and distributed part of the modelling language VDM++.

• Define and explain syntax and semantics for the concurrent, real-time and distributed subset of VDM++.

• Apply VDM++ and UML with the associated tool support for either the description of semantics of a programming language or for abstract and precise modelling and validation of concurrent, real-time and distributed systems.

• Evaluate practical use of VDM++ for the validation of concrete distributed and embedded systems.

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Evaluation principles• The project reports will be evaluated against:

• Mastering abstraction• Testability of the VDM++ models• Use of basic VDM++ structures• Use of VDM++ concurrency principles (for RT projects)• Use of VDM++ real-time primitives (for RT projects)• Exploration of alternative distributive architectures (RT)• Using VDM for prog lang semantics issues (for AST projects)• VDM++/UML model quality considerations• Refection of pros and cons with the VDM-RT approach (RT)• Report quality including the use of logfile analysis (RT)

• The exam will be evaluated against:• The project report• The project presentation by the student• The students ability to show understanding of VDM++/UML

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Focus in this course

• Focus is on • Abstract modeling of realistic systems• Understanding the VDM concepts for AST handling,

concurrency, real-time and distribution• Learning how to read models made in VDM++/UML• Learning how to write models in VDM++/UML• Learning how to move from a sequential model, via a

concurrent model to a real-time distributed model• Learning how to validate all these models

• Focus is not on• Toy examples• Implementation

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Why have this course?

• To understand the underlying primitives for being able to model complex concurrent and distributed computer systems

• To understand how language semantics can be described using VDM++ concepts

• To be able to comprehend the formulation of important desirable properties precisely

• To be able to express important desirable properties (including real-time) precisely

• To enable the formulation of abstract models in an industrially applicable formal notation

• To validate those models to increase confidence in their correctness and eliminate potential architectural bottlenecks

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Structure of the course

Week

1. Introduction, Concurrency in VDM++, AST simple extensions (chap 12)

2. Real-time and distributed modelling in VDM++ (additional notes)

3. Model Quality (chap 13)

4. More about semantics about different language constructs in VDM++

5. Model Structuring and Combining Views (chap 9+10)

6. Report writing

7. Course evaluation and repetition (if desired)

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Anticipated Plan with Projects

• Week 1: Revisit the model from TIVDM1• Week 2: Add concurrency to the model and validate it• Week 3: Add real-time and distribution to the model +

Present initial ideas and get feedback on distributed real-time models. Validate the model and experiment with alternative distribution architectures

• Week 4: Work on your project• Week 5: Present final distributed real-time models• Week 6: Write report for the project• Week 7: Project report is handed in to the teacher.

Evaluation of insight gained by using the VDM-RT extensions to VDM++

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Agenda

Administrative information about the course

Simple AST Extensions

• Concurrency primitives in VDM++

• Example: POP3 Server

• Example: MSAWConc

• Concurrency and Overture

• Feedback on abstraction test and VDM1 reports

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Extensions of Simple

• Introduction of operations• Notion of state

• Block statements

• Assignment statements

• Operation calls and return statements

• Introduction of set expressions• Both enumeration and comprehensions

• Also set type in type system

• Introduction of higher-order functions• Functions that yield functions as results

• Typically used in functional programming languages

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Introduction of operations

• Notion of state:• <state definition> ::= <identifier> ":" <type> ":=" <expression>

• Operation definition: • <operation definition> ::=

<identifier> <paramlist> "==" <statement>• Statements:

• <statement> = <block statement> |<assignment statement> |

<call statement> | <return statement>• <block statement> ::= <statement> | "(" [ { <statement> } ] ")"• <assignment statement> ::= <identifier> ":=" <expression>• <call statement> ::=• <identifier> "(" [ <expression> {"," <expression>} ] ")"• <return statement> ::= "return" <expression>

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Introduction of set expressions

• Set enumerations:• <set enumeration> ::=

"{" [ <expression> ] {"," <expression> } "}"

• Set Comprehensions:• <set comprehension> ::=

"{" <expression>

"|" <identifier> "in set" <expression>

"&" <expression> "}”

• Set types:• <type> ::= "real" | "int" | "nat" | "bool" | "set of" <type> |

<identifier>

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Introduction of higher-order functions

• Higher order functions:• <function definition> ::=

<identifier> <paramlist> { <paramlist> } "==" <expression>

• Parameter lists:• <paramlist> ::= "(" <parameter> {"," <parameter>} ")"

• Example of a higher order function:• Add(x : int)(y : int) == x + y

• Here ”Add(7)” is similar to a new function like:• lambda y : int & 7 + y

• Which is a new function that can be passed around as an argument or called with other arguments

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Agenda

Administrative information about the course

Simple AST Extensions

Concurrency primitives in VDM++

• Example: POP3 Server

• Example: MSAWConc

• Concurrency and Overture

• Feedback on abstraction test and VDM1 reports

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Concurrency Primitives in VDM++

• Concurrency in VDM++ is based on threads• Threads communicate using shared objects• Synchronization on shared objects is specified using

permission predicates

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Threads

• Modelled by a class with a thread sectionclass SimpleThreadthread IO`print(”Hello World”)end SimpleThread

• Thread execution begins using start statement with an instance of a class with a thread definitionstart(new SimpleThread())

• Note that execution ends when debug thread is finished

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Thread Communication

• Threads operating in isolation have limited use.• In VDM++ threads communicate using shared

objects.

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A Producer-Consumer Example

• Concurrent threads must be synchronized• Illustrate with a producer-consumer example• Produce before consumption …• Assume a single producer and a single consumer• Producer has a thread which repeatedly places data

in a buffer• Consumer has a thread which repeatedly fetches data

from a buffer

Producer

Produce() : seq of char

Consumer

Consume(d : seq of char)

Buffer

data : [seq of char] = nil

Get() : seq of charPut(newData : seq of char)

-b-b

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The Producer Classclass Producer

instance variables

b : Buffer

operations

Produce: () ==> seq of charProduce() == ...

thread while true do b.Put(Produce())end Producer

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The Consumer Classclass Consumer

instance variables

b : Buffer

operations

Consume: seq of char ==> ()Consume(d) == ...

thread while true do Consume(b.Get())end Consumer

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The Buffer Classclass Buffer

instance variables

data : [seq of char] := nil

operations

public Put: seq of char ==> ()Put(newData) == data := newData;

public Get: () ==> seq of charGet() == let oldData = data in ( data := nil; return oldData )

end Buffer

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Permission Predicates

• What if the producer thread generates values faster than the consumer thread can consume them?

• Shared objects require synchronization.• Synchronization is achieved in VDM++ using

permission predicates.• A permission predicate describes when an

operation call may be executed.• If a permission predicate is not satisfied, the

operation call blocks.

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Permission Predicates

• Permission predicates are described in the sync section of a classsync

per <operation name> => predicate

• The predicate may refer to the class’s instance variables.

• The predicate may also refer to special variables known as history counters.

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History Counters

Counter Description

#req op The number of times that op has been requested

#act op The number of times that op has been activated

#fin op The number of times that op has been completed

#active op The number of active executions of op

#waiting op

The number of waiting executions of op

#active op = #act op - #fin op

#waiting op = #req op - #act op

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The Buffer Synchronized

• Assuming the buffer does not lose data, there are two requirements:• It should only be possible to get data, when the

producer has placed data in the buffer.• It should only be possible to put data when the

consumer has fetched data from the buffer.• The following permission predicates could model

these requirements:• per Put => data = nil• per Get => data <> nil

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The Buffer Synchronized (2)

• The previous predicates could also have been written using history counters:

• For exampleper Get => #fin(Put) - #fin(Get) = 1

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Mutual Exclusion

• Another problem could arise with the buffer: what if the producer produces and the consumer consumes at the same time?

• The result could be non-deterministic and/or counter-intuitive.

• VDM++ provides the keyword mutex• mutex(Put, Get)

• Shorthand for• per Put => #active(Get) = 0• per Get => #active(Put) = 0

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Agenda

Administrative information about the course

Simple AST Extensions

Concurrency primitives in VDM++

Example: POP3 Server

• Example: MSAWConc

• Concurrency and Overture

• Feedback on abstraction test and VDM1 reports

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Example: POP3 Server

• Protocol which allows email clients to fetch messages from a server.

• Server contains a collection of messages for a number of users.

• Server is typically capable of communicating with multiple clients concurrently.

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Overall Class Structure

POP3Server POP3ClientHandler

-parent

MessageChannel

-msgChannel

MessageChannelBuffer

-connChannel

0..1

-data

0..1

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Behaviour

• The server listens constantly for clients.• When a client initiates contact, a client handler is

spawned.• The client handler then responds to commands from

the client, until the client terminates the session.

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The POP3Server Instance Variables

class POP3Server

instance variables maildrop : MailDrop; passwords : map POP3Types`UserName to POP3Types`Password; locks : map ClientHandlerId to POP3Types`UserName; serverStarted : bool := false;

types

public MailDrop = map POP3Types`UserName to MailBox;public MailBox :: msgs : seq of POP3Message locked : bool;

public ClientHandlerId = nat;

end POP3Server

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The POP3Server Basic Synchronizationclass POP3Server

operations

SetUserMessages: POP3Types`UserName * seq of POP3Message ==> ()SetUserMessages(user, newMsgs) == maildrop(user) := mu(maildrop(user), msgs |-> newMsgs)pre user in set dom maildrop;

GetUserMail: POP3Types`UserName ==> MailBoxGetUserMail(user) == return maildrop(user)pre user in set dom maildrop;

sync mutex(SetUserMessages); mutex(SetUserMessages, GetUserMail)

end POP3Server

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Synchronization of Lockingclass POP3Serveroperationspublic AcquireLock: ClientHandlerId * POP3Types`UserName ==> ()AcquireLock (clId, user) == locks := locks ++ { clId |-> user}pre clId not in set dom locks;

public ReleaseLock: ClientHandlerId ==> ()ReleaseLock(clId) == locks := {clId} <-: lockspre clId in set dom locks;

public IsLocked: POP3Types`UserName ==> boolIsLocked(user) == return user in set rng locks;

syncmutex(AcquireLock);mutex(ReleaseLock);mutex(AcquireLock, ReleaseLock, IsLocked);end POP3Server

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Client Handler States

• Three states:• Authorisation: The client has not yet been authorised by the

client handler.• Transaction: The client is able to interrogate the client handler

about the contents of the mail box for the client's user. • Update:The client has ended the session; messages marked for

deletion are actually deleted from the server, then the thread dies.

class POP3ClientHandler

ServerState = <Authorization> | <Transaction> | <Update>;

end POP3ClientHandler

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Starting a Client Sessionclass POP3Server

threadwhile true do( let msgChannel = connChannel.Get() in CreateClientHandler(msgChannel); serverStarted := true;)

instance variables connChannel: MessageChannelBuffer;

operations

CreateClientHandler: MessageChannel ==> ()CreateClientHandler(mc) == start(new POP3ClientHandler(self, mc));

end POP3Server

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Interacting with the Client 1class POP3Types

types

public ClientCommand = StandardClientCommand | OptionalClientCommand;public StandardClientCommand = QUIT | STAT | LIST | RETR | DELE | NOOP | RSET;public OptionalClientCommand = TOP | UIDL | USER | PASS | APOP;

public RETR :: messageNumber : nat;…public ServerResponse = OkResponse | ErrResponse;public OkResponse :: data : seq of char;public ErrResponse :: data : seq of char;

end POP3Types

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Interacting with the Client 2class POP3ClientHandlerthread( dcl cmd: POP3Types`ClientCommand; id := threadid; cmd := msgChannel.ServerListen(); while (cmd <> mk_POP3Types`QUIT()) do ( msgChannel.ServerSend(ReceiveCommand(cmd)); cmd := msgChannel.ServerListen() ); msgChannel.ServerSend(ReceiveCommand(cmd));)operations ReceiveCommand: POP3Types`ClientCommand ==> POP3Types`ServerResponse ReceiveCommand(c) == cases c: mk_POP3Types`QUIT() -> ReceiveQUIT(c), mk_POP3Types`STAT() -> ReceiveSTAT(c), mk_POP3Types`LIST(-) -> ReceiveLIST(c), mk_POP3Types`RETR(-) -> ReceiveRETR(c), mk_POP3Types`DELE(-) -> ReceiveDELE(c), mk_POP3Types`NOOP() -> ReceiveNOOP(c), mk_POP3Types`RSET() -> ReceiveRSET(c), mk_POP3Types`TOP(-,-) -> ReceiveTOP(c), mk_POP3Types`UIDL(-) -> ReceiveUIDL(c), mk_POP3Types`USER(-) -> ReceiveUSER(c), mk_POP3Types`PASS(-) -> ReceivePASS(c) endend POP3ClientHandler

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Interacting with the Client 3class POP3ClientHandler

instance variables ss : ServerState; parent: POP3Server; user : POP3Types`UserName; id : POP3Server`ClientHandlerId;

operations

ReceiveRETR: POP3Types`RETR ==> POP3Types`ServerResponseReceiveRETR(retr) == if ss = <Transaction> then if parent.IsValidMessageNumber(user, retr.messageNumber) then let msgText = parent.GetMessageText(user, retr.messageNumber), sizeText = int2string(parent.GetMessageSize(user,

retr.messageNumber)) in return mk_POP3Types`OkResponse(sizeText ^ "\n" ^ msgText) else return mk_POP3Types`ErrResponse(unknownMessageMsg) else return mk_POP3Types`ErrResponse(negativeStatusMsg);

end POP3ClientHandler

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Synchronized Communicationin the POP3 Server 1

class MessageChannel

instance variables data : [POP3Types`ClientCommand | POP3Types`ServerResponse] := nil;

operations

Send: POP3Types`ClientCommand | POP3Types`ServerResponse ==> ()Send(msg) == data := msg;

Listen: () ==> POP3Types`ClientCommand | POP3Types`ServerResponseListen() ==let d = data in ( data := nil; return d );

end MessageChannel

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Synchronized Communicationin the POP3 Server 2class MessageChannel

operationspublic ServerSend: POP3Types`ServerResponse ==> ()ServerSend(p) == Send(p);

public ServerListen: () ==> POP3Types`ClientCommandServerListen() == Listen();…sync per ClientSend => #fin(ServerSend) = #fin(ClientListen) and #fin(ClientSend) = #fin(ServerListen) and #fin(ServerSend) = #fin(ClientSend); per ServerListen => #fin(ClientSend) - 1 = #fin(ServerListen);

end MessageChannel

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Sequence Diagram

client : POP3Test

client : POP3Test

handler : POP3ClientHandler

handler : POP3ClientHandler

buffer : MessageChannel

buffer : MessageChannel

ServerListen( )

ClientSend(POP3Types`ClientCommand)

ClientListen( )

ServerSend(POP3Types`ServerResponse)

ServerListen( )

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Agenda

Administrative information about the course

Simple AST Extensions

Concurrency primitives in VDM++

Example: POP3 Server

Example: MSAWConc

• Concurrency and Overture

• Feedback on abstraction test and VDM1 reports

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The TimeStamp Class (1/3)class TimeStamp

instance variables currentTime : nat := 0;wakeUpMap : map nat to nat := {|->};

operations

public WaitRelative : nat ==> ()WaitRelative(val) == AddToWakeUpMap(threadid, currentTime + val);

public WaitAbsolute : nat ==> ()WaitAbsolute(val) == AddToWakeUpMap(threadid, val);

AddToWakeUpMap : nat * nat ==> ()AddToWakeUpMap(tId, val) == wakeUpMap := wakeUpMap ++ { tId |-> val };

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The TimeStamp Class (2/3)

public NotifyThread : nat ==> ()NotifyThread(tId) == wakeUpMap := {tId} <-: wakeUpMap;

public NotifyAll : () ==> ()NotifyAll() == let threadSet : set of nat = {th | th in set dom wakeUpMap & wakeUpMap(th) <= currentTime } in for all t in set threadSet do NotifyThread(t);

NotifyAndIncTime : () ==> ()NotifyAndIncTime() ==( currentTime := currentTime + 1; NotifyAll(););

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The TimeStamp Class (3/3)

public GetTime : () ==> nat

GetTime() ==

return currentTime;

sync

mutex(NotifyAll);

mutex(AddToWakeUpMap);

mutex(AddToWakeUpMap, NotifyAll);

end TimeStamp

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The ClockTick Class

class ClockTick is subclass of GLOBAL

instance variables

tid : int := -1;

operations

public ClockTick : Time ==> ClockTickClockTick (t) == tid := t;

thread

while true do (World`timerRef.NotifyThread(tid); World`timerRef.WaitRelative(1); )

end ClockTick

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The World Class

class World…public World : () ==> WorldWorld() == ( env := new Environment("scenario.txt"); env.setAirSpace(MSAW`airspace); MSAW`atc.addObstacle(MSAW`militaryZone); MSAW`atc.addRadar(MSAW`radar1); MSAW`atc.addRadar(MSAW`radar2); );public Run : () ==> ()Run() == (start(env); start(MSAW`atc); env.isFinished(); MSAW`atc.isFinished(); env.showResult() ) end World

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The Environment Thread

class Environment…thread (start(new ClockTick(threadid)); while World`timerRef.GetTime() < simtime do (updateFOs(); if updating then (World`timerRef.WaitRelative(0); updating := false); World`timerRef.NotifyAndIncTime(); World`timerRef.Awake(); ); busy := false; )end Environment

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The Environment Synchronisation

class Environment...public isFinished : () ==> () isFinished() == skip;

sync

mutex(handleFOWarningEvent);per isFinished => not busy;

mutex(handleRadarWarningEvent);mutex(handleRadarWarningEvent, handleFOWarningEvent);

end Environment

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Air Traffic Controller (1/2)

class AirTrafficController is subclass of GLOBAL...operations public isFinished : () ==> ()isFinished() == for all r in set radars do r.isFinished(); public Step : () ==> ()Step() == ( for all r in set radars do r.Scan(MSAW`airspace); updateHistory(); findThreats(); busy := false);

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Air Traffic Controller (2/2)

thread

(for all r in set radars do

start(r);

while true do

Step();

)

sync

per isFinished => not busy;

per Step => busy

end AirTrafficController

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The Radar Class Thread & Syncclass Radar ...thread

while true do (let as = MSAW`airspace in (detected := { x.getId() |-> x | x in set as.getAirspace() & InRange(x)}; UpdatePriorityList(); World`timerRef.WaitRelative(TimeStamp`stepLength); World`timerRef.NotifyAll(); World`timerRef.Awake(); ) )sync per isFinished => not busy;mutex(removeNotDetected);mutex(addNewlyDetected);mutex(UpdatePriorityList) end Radar

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Agenda

Administrative information about the course

Simple AST Extensions

Concurrency primitives in VDM++

Example: POP3 Server

Example: MSAWConc

Concurrency and Overture

• Feedback on abstraction test and VDM1 reports

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Concurrency and Overture

• Overture provides• Type checking of thread and sync sections of a class.

• Interpretation of threads in a deterministic fashion.

• All threads are stopped when a breakpoint is reached.

• Threadid and information for the broken thread can be inspected in the debug perspective.

• Note that the scheduler for concurrency is VERY new• VDMTools additionally provide code generation of

thread and sync sections of a class (Java only).

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Agenda

Administrative information about the course

Simple AST Extensions

Concurrency primitives in VDM++

Example: POP3 Server

Example: MSAWConc

Concurrency and Overture

Feedback on abstraction test and VDM1 reports

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Answers for the Libary questions• On the basis of your understanding of the text, which of the following

statements do you agree with?• Multiple copies can exist for the same book

• Do the requirements say anything about the order used for transaction 3?• No

• Can an ordinary borrower return a book?• Yes, if he delegates it to a staff user

• Can new users be added to the library?• Not clear from the text

• Can staff users also be ordinary borrowers?• Not clear from the text

• Is there a limit for how many books a staff user may borrow?• No

• What information can we deduce from the text that we need to record for a book? (check all that apply)• Author• Subject• Copies of it

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Answers for the Student questions

• On the basis of your understanding of the text, which of the following statements do you agree with? • Multiple occurrences of “course occurrences” can exist of the same “course”

• Do the requirements say anything about the order in the list of courses used for function 3?• No

• Can an ordinary student enrol in a course?• Yes, if it is done with the help of a staff user

• Can new students be added to the system?• Not clear from the text

• Can staff users also be students?• Not clear from the text

• Is there a limit on how many enrolments a student may have overall?• No

• What information can we deduce from the text that we need to have for a course or course occurrence? • Teacher(s)• Title• Study program• State of a course

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Student performances

begin end begin end changeLeni Lausdahl lib stud 5.166667 4.75 -0.416667Michael Mortensen lib stud 6.166667 9.5 3.333333José Esparza stud lib 5.5 11.5 6Morten Mikkelsen lib stud 8.666667 5.75 -2.916667Johannes Jensen stud lib 9.25 6.5 -2.75Hans Chr Jørgensen stud lib 4.5 8.25 3.75Jon Nielsen stud lib 4.25 6.75 2.5Allan Kristensen lib stud 8.75 10.5 1.75Jeppe Sørensen stud lib 6.416667 8 1.583333Lasse Rasmussen lib stud 6 10 4Frederik Laulund lib stud 7.166667 7.75 0.583333Christian Jensen stud lib 4 6.667 2.667Kim Bjerge lib stud 7.5 6.5 -1

Average 5.555556 6.8278 1.272244

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Summary

• What have I presented today? • Administrative information about the course• Simple Extensions• Concurrency primitives in VDM++• The POP3 and MSAW Concur examples• Concurrency support in Overture• Feedback on abstraction test and VDM1 reports

• What do you need to do now?• Inspect the electronic copy of the POP3 example• Get new version of Overture installed and start looking at

the language manual• Revisit your project • Present your project selection to all of us

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Quote of the day

Bill Gates

It’s different, in that we take on novel tasks every time.

The number of times civil engineers make mistakes is very

small. And at first you think, what’s wrong with us? It’s

because it’s like we’re building the first skyscraper every

time.