Concurrent Systems and Processes

Question

I am working on a concurrency problem of a shop who can have no more than 5 people in the store at one time, the shopkeeper can only serve one customer at a time and during that time the customer has exclusive access to the shopkeeper, after the customer has got access to the shopkeeper the shopkeeper will wait until the customer indicates he is ready to make a purchase and will then wait until his items are bagged. I am told to assume that a customer that enters the shop will always purchase an item.
I would appreciate it if someone would look at my code and see if I am going about the problem in the correct way or not, thanks.
/* Note. You can assume that customers entering the shop will queue up in
/* an orderly fashion and begin to be served starting with the one currently
/* at the head of the queue.
Global Invariant: (num_cust_in_shop <= SHOP_CAPACITY)
process main_process
     /* declare and initialise global values/variables
     final int SHOP_CAPACITY = 5;
     int num_cust_in_shop = 0; //initially the shop is empty
     int NUM_CUSTOMERS = 20; // this is the number of potential customers
     boolean lock = false; // this lock ensures exclusive update of global variable
     //set processes running
     co for [j=1 to NUM_CUSTOMERS] customer_process[j] // shopkeeper_process oc
} //end main_process
//customer process
process customer_process [int I=1 to N]
     while(true){
     <await (num_cust_in_shop < SHOP_CAPACITY)>; //wait if shop is full
     {...one customer enters shop...};
     //exclusive update of global variable num_cust_in_shop
     <await (lock==false) lock=true;>
     num_cust_in_shop = num_cust_in_shop + 1;
     <lock = false>;
     }//end while
}//end customer_process
//shopkeeper process
process shopkeeper_process
     while(true){
     <await (num_cust_in_shop >=1)>
     {...serve customers in shop...};
     <await (lock==false)lock=true;>
          num_cust_in_shop:=num_cust_in_shop -1;
     <lock = false>;
     } //end while
} //end shopkeeper_processEdited by: JP_Training on Feb 8, 2010 2:41 PM

Answer 1

Your very best bet is to take this to your instructor and ask him. He knows what he wants. What ever you have there is not valid Java code.