CS452 - Real-Time Programming - Spring 2010

Lecture 21 - Task Structure

Public Interest Announcements

First train tracking assignment
Workers can have code assigned by administrator

Servers and Attendant Tasks

1. Proprietor with a Notifier

2. Using a Courier

3. Using a Warehouse

4. The Secretary

5. Administrator, Worker

6. The Detective

Simple Events

The notifier is a task that waits on events.

AwaitEvent is like Send
It has two features
- a simple, kernel-defined event that it waits on
- hardware/kernel is like Receive/Reply
- can only serve one master
The notifier needs to pass on that the event has happened
- which it does using Send
```
FOREVER {
  AwaitEvent( eventId );
  Send( server );
}
```

You could call a notifier a detective,

who looks around on your behalf,
and let's you know when something you care about happens,
but really it is a detective's worker.

Complex Events

In an application there is likely to be lots of waiting on combinations of events.

form the combinations using Boolean operators

We use the detective to discover that a complex event has occurred.

How does the detective work?

Conjunction

Code could be

FOREVER {
  Send( part1 );
  Send( part2 );
  ...
  Send( master );
}

Disjunction

Code above doesn't work! Try instead

// InitializeDB;
// Create workers and synchronize;
// Synchronize with client;
FOREVER {
  Receive( *requester, request );
  if ( request.type == CLIENT ) {
    parsedRequest = parse( request );
    if ( happened( parsedRequest, DB ) ) Reply( requester );
    else enqueue( parsedRequest );
  }
  else if (request.type == WORKER ) {
    updateDB ( request );
    Reply( requester );
    foreach ( queuedRequest )
      if ( happened( parsedRequest, DB ) ) {
        dequeue( parsedRequest );
        Reply( client );
      }
  }
}

FOREVER {
  Receive
  switch
  case CLIENT: Reply c't or hold
  case WORKER: Reply w'r, Reply held
}

This is the code of a detective.

Not

We can say that an event has not happened yet.

Only at the end of the universe can we say that an event simply has not happened.

Time-outs are needed for NOT

OR with Delay on the clock server.

Typical Time-out code

Time-out Worker

FOREVER {
    Send( detective, <timed-out>, <delay-request> )
    Delay( delay-time );
}

Detective

FOREVER {
    Receive
    switch {
    case CLIENT_TO:
        if ( to-worker available && other-worker available ) {
            Reply( to-worker );
            Reply( other-worker );
            enqueue( client );
        }
        else {
            if ( !to-worker available ) Create( to-worker ) // and initialize
            if // other-worker
    case WORKER_TO:
        if ( !first && client-waiting ) { Reply( client ); enqueue( to-client ) }
        else Reply( to-worker( delay );
        ...
    }
}

Who is the client of the detective

Initiator of a normal action
Housekeeper of the system who will clean up pathologies (Idletask?))

Assignments

1. Tracking & Route Finding

What you are expected to do

One locomotive on the track:
- Give the locomotive a destination & speed at the terminal.
- Train finds the shortest route to the destination and travels to it.
- Your application knows where the train is at all times.
This should be robust against malfunctioning sensors and switches
To do this you need
- Calibration
- Route creation. In route creation routes in which the lomotive changes direction seem more clever.

2. Avoiding

What you are expected to do

Two or three locomotives on the track:
- Give each locomotive a destination & speed at the terminal.
- They find non-conflicting routes to their destinations and travel to them.
- Your application knows where the trains are at all times.
This should be robust against malfunctioning sensors and switches
- Locomotives should not collide
- Locomotives should not fuse into one when they get too close together
To do this you need
- Adaptive route finding
- A track reservation system

Projects

1. Train style

Give trains roles and objectives. For example,

Passenger train travels on a repetitive route meeting a schedule.
Freight train travels to random destinations as fast as possible.
Objective is to deliver as much freight as possible while keeping a passenger train on time.

Another example,

Trains are taxis.
When a load appears they race to see if they can get it

2. Game style

The track is a graph.

Several different ways to choose vertices and edges

Many games are played on graphs

Checkers, snakes & ladders, maze games, etc.
Implement a graph game played on the track graph.

E.g. watchmen and bandit.

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