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# CS 511: Homework Assignment 2 solution

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CS 511: Homework Assignment 2

2 Problem Description
You are the owner of a bakery during the pandemic. Health safety experts
recommend that your store occupancy be at or below 50 people. There are
three shelves, one for loaves of rye bread, one for loaves of sourdough bread
and one for loaves of wonder bread. At the start of the day, your store bakes
20 loaves each of rye, sourdough and wonder bread and places them on their
corresponding shelves. Throughout the day people come in and pick up the
loaves of bread from one of the shelves. Customers may buy more than one loaf
of bread of any type, as described below.

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CS 511: Homework Assignment 2

2 Problem Description
You are the owner of a bakery during the pandemic. Health safety experts
recommend that your store occupancy be at or below 50 people. There are
three shelves, one for loaves of rye bread, one for loaves of sourdough bread
and one for loaves of wonder bread. At the start of the day, your store bakes
20 loaves each of rye, sourdough and wonder bread and places them on their
corresponding shelves. Throughout the day people come in and pick up the
loaves of bread from one of the shelves. Customers may buy more than one loaf
of bread of any type, as described below. They then go one at a time through
one of the four cashiers that you employ. Those cashiers count how many people
go through the bakery as well. The customers then leave the store.
The bakery is illustrated in Figure 1. The green arrow depicts the access
door to the bakery and the red arrow the exit door. Only one customer can
remove a loaf from a shelf at a time. Only one customer can be at a register at
a time.
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CS511 – Concurrent Programming 2 PROBLEM DESCRIPTION
Figure 1: Bakery
2.1 The Bakery
Bakery
private static final int TOTAL_CUSTOMERS = 200
private static final int ALLOWED_CUSTOMERS = 50
private static final int FULL_BREAD = 20
private Map<BreadType, Integer availableBread
private ExecutorService executor
private float sales
public void takeBread(BreadType bread)
public void addSales(float value)
public void run()
This bakery has a number of allowed customers and a number of total customers that visit during the day. The bakery has a stock of 20 loaves of each
type of bread with a price given in the stub. Each type of bread is stocked on a
different shelf that only one customer can access at a time. The customers will
pick between one and three random breads from the shelf. When they take an
item from the shelf, it will be taken from the stock. In order for the customer
to take from the shelf:
• The shelf must be available.
• The bread needs to be in stock.
After they get their bread, customers will proceed to one of the four cashiers
and ‘buy’ the item which adds to the sales variable which is shared between the
four cashiers. If all four cashiers are taken, the next customer will have to wait
their turn. The customer then graciously exits the bakery to let more customers
in.
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CS511 – Concurrent Programming 2 PROBLEM DESCRIPTION
When a bread stock reaches 0 stock, an ‘attendant’ comes over and restocks
the shelf. No one can access the shelf for one second while it is being restocked.
The available bread is stored in a ConcurrentHashMap, which is a thread-safe
implementation of a HashMap. We do not want to use the standard HashMap
because its “get” and “put” operations are not atomic.
The implemented takeBread method takes an item off of the stock and handles the restocking process when necessary. Be sure to acquire the proper
semaphore(s) before calling this operation.
2.2 Customer
A customer has a shopping cart called shoppingCart that determines what bread
they take. In addition, they also have a time they spend shopping and a time
they spend at the cashier that is randomized. These random values are determined when a Customer is created.
Customer
private Bakery bakery
private Random rnd
private List<BreadType shoppingCart
private int shopTime
private int checkoutTime
public Customer()
public void run()
public String toString()
private boolean addItem(BreadType bread)
private void fillShoppingCart()
private float getItemsValue()
The operation fillShoppingCart chooses what items the customers want to
buy. The operation addItem takes the loaves from the respective shelves and
puts it into the customers shopping cart (again: if the shelf is empty then the
customer cannot take from the shelf).
2.3 Solution
Model this scenario using semaphores. These semaphores must allow for a
correct use of shared resources. The shared resources in this problem are:
• The shelves: a customer must wait to use the shelf if it is occupied.
• The cashiers: declared in the bakery itself. There are four cashiers available in the bakery that the customers go to check out their items. If the
cashiers are busy the customers have to wait until one of the cashiers is
free.
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CS511 – Concurrent Programming 2 PROBLEM DESCRIPTION
• The sales: the sales variable is maintained by all cashiers and is updated
when a customer checks out.
• The bakery itself is also a shared resource. However rather than using
semaphore for upholding this requirement you are to use the maximum of
TOTAL_CUSTOMERS as the size of your thread pool.
Regarding the simulation of the bakery, the bakery should generate customers
randomly and have them do their shopping. Customers should have between
one and three breads in their shopping list. The shopping itself should take
some time but not too much (in the sense that the simulation doesn’t slow
down too much). Also, the cashiers when checking out should take some time.
The simulation should print out events when a customer starts shopping, takes
an item from stock, when it buys, and and then when it finishes. Each customer
can be uniquely identified using its hashCode. After all customers have finished
shopping, the program should print the total sales from the day before exiting.
2.4 List of Classes
Here is a list of the classes you have to define, all included in a package called
Assignment2
• Bakery
The simulation of the bakery consists in randomly having customers come
in to the bakery. You may assume that there are a total of ALLOWED_CUSTOMERS
that are allowed in the bakery. The run method of Bakery should be in
charge of the simulation itself. The threads corresponding to the customers
should be spawned using a thread pool (as explained below).
• BreadType
Is given in the stub and contains information about the bread and prices.
• Customer
Has a shopping cart that is randomly generated from the requirements
above. Also needs a random value for the time spent shopping and also
the time spent at checkout. Contains a method fillShoppingCart that
generates what the customer wants from the shelves.
• Assignment2 – included in the stub. Starts the program.
1 /* start the simulation */
2 public class Assignment2 {
3 public static void main ( String [] args ) {
4 Thread thread = new Thread ( new Bakery ());
5 thread . start ();
6
7 try {
8 thread . join ();
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CS511 – Concurrent Programming 2 PROBLEM DESCRIPTION
9 } catch ( InterruptedException e ) {
10 e . printStackTrace ();
11 }
12 }
13 }
2.5 A Note on Pool Threads and the Executor Interface
The Executor interface1 or its subinterface ExecutorService (which is the one we
will be using in this assignment) represents an asynchronous execution mechanism which is capable of executing multiple tasks in the background. Such
multiple tasks are referred to as thread pools. An Executor is normally used
instead of explicitly creating threads for managing each of the tasks, i.e., rather
than invoking new Thread(new(RunnableTask())).start() for each of a set of
tasks, you might use:
1 Executor executor = anExecutor ;
2 executor . execute ( new RunnableTask1 ());
3 executor . execute ( new RunnableTask2 ());
4 …
Thread pools are useful when we need to limit the number of threads running
in an application at the same time as there is a performance overhead associated
with starting a new thread (each thread is also allocated some memory for its
stack, etcetera). Instead of starting a new thread for every task to execute
concurrently, the task can be passed to a thread pool. As soon as the pool has
any idle threads the task is assigned to one of them and is then executed.
There are many ways of managing thread pools. One easy way is to use the
static method newFixedThreadPool of the class Executors (not to be confused
with the Executor interface). This method, whose signature is
public static ExecutorService newFixedThreadPool(int nThreads)
creates a thread pool that reuses a fixed number of threads operating off a shared
(unbounded) queue. At any point, at most n threads will be active processing
tasks. If additional tasks are submitted when all threads are active, they will
wait in the queue until a thread is available. If any thread terminates due to
a failure during execution prior to shutdown, a new one will take its place if
needed to execute subsequent tasks. The threads in the pool will exist until it
is explicitly shutdown.
Here is a simple example of ExecutorService:
1 ExecutorService executorService = Executors . newFixedThreadPool (10);
2
3 executorService . execute (new Runnable () {
4 public void run () {
5 System . out . println (“I am an asynchronous task !”);
1Found in java.util.concurrent
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CS511 – Concurrent Programming 3 SUBMISSION INSTRUCTIONS
6 }
7 });
8
9 executorService . shutdown ();
Here is what is happening:
• An ExecutorService is created using the newFixedThreadPool(int) factory
method. This creates a thread pool with 10 threads executing tasks.
• An anonymous class implementation of the Runnable interface is passed to
the execute() method. This causes the Runnable to be executed by one of
the threads in the ExecutorService.
• The ExecutorService is shut down, so the all threads finish running. To
terminate the threads inside the ExecutorService you call its shutdown()
method. The ExecutorService will not shut down immediately, but it will
no longer accept new tasks, and once all threads have finished current
tasks, it shuts down. All tasks submitted to the ExecutorService before
shutdown() is called, are executed.
If you want to shut down the ExecutorService immediately, you can call
the shutdownNow() method. This will attempt to stop all executing tasks
right away, and skips all submitted but non-processed tasks. There are no
guarantees given about the executing tasks. Perhaps they stop, perhaps
the execute until the end. It is a best effort attempt.
3 Submission Instructions
Submit a zip file named Assignment2.zip through Canvas containing the java
source files. Important: Please make sure that your assignment compiles before
submitting.
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