Question

1. Consider a grocery supermarket planning to computerize their inventory management. The items on shelves will be marked with Radio Frequency Identification (RFID) tags and a set of RFID reader-devices will be installed for monitoring the movements of the tagged items. Each tag carries a 96-bit EPC (Electronic Product Code) with a Global Trade Identification number, which is an international standard. The RFID readers are installed on each shelf on the sales floor. The RFID system consists of two types of components (see figure above): (1) RFID tag or transponder, and (2) RFID reader or transceiver. RFID tags are passive (no power source), and use the power induced by the magnetic field of the RFID reader. An RFID reader consists of an antenna, transceiver and decoder, which sends periodic signals to inquire about any tag in vicinity. On receiving any signal from a tag it passes on that information to the data processor.

You are tasked to develop a software system for inventory management. The envisioned system will detect which items will soon be depleted from the shelves, as well as when shelves run out of stock and notify the store management. The manager will be able to assign a store associate to replenish the shelf, and the manager will be notified when the task is completed. Based on the initial ideas for the desired functions of the software system, the following requirements are derived:

REQ1. The system shall continuously monitor the tagged items on the shelves. Every time an item is removed, this event is recorded in the system database by recording the current item count from the RFID reader. The system should also be able to handle the cases when the customer takes an item, puts it in her shopping cart, continues shopping, and then changes her mind, comes back and returns the item to the shelf.

REQ2. The system shall keep track when stock is running low on shelves. It shall detect a “low-stock” state for a product when the product’s item count falls below a given threshold while still greater than zero.

REQ3. The system shall detect an “out-of-stock” state for a product when the shelf becomes empty and the product’s item count reaches zero.

REQ4. The system shall notify the store manager when a “low-stock” or “out-of-stock” state is detected, so the shelves will be replenished. The notification will be sent by electronic mail, and the manager will be able to read it on his mobile phone.

REQ5. The store manager shall be able to assign a store associate with a task to replenish a particular shelf with a specific product. The store associate shall be notified by electronic mail about the details of the assigned task.

REQ6. While the store associate puts items on the shelf, the RFID system shall automatically detect the newly restocked items by reading out their EPC. The system should support the option that customers remove items at the same time while the store associate is replenishing this shelf.

REQ7. The store associate shall be able to explicitly inform the system when the replenishment task is completed. The number of restocked items will be stored in the database record. The item count obtained automatically (REQ5) may be displayed to the store associate for verification. After the store associate confirms that the shelf is replenished, the task status will be changed to “completed,” and a notification event will be generated for the store manager.

To keep the hardware and development costs low, we make the following assumptions:

A1. You will develop only the software that runs on the main computer and not that for the peripheral RFID devices. Assume that the software running the RFID readers will be purchased together with the hardware devices.

A2. The tag EPC is unique for a product category, which means that the system cannot distinguish different items of the same product. Therefore, the database will store only the total count of a given product type. No item-specific information will be stored.

A3. Assume that the RFID system works perfectly which, of course, is not true in reality. As of this writing (2011) on an average 20% of the tags do not function properly. Accurate read rates on some items can be very low, because of physical limitations like reading through liquid or metals still exist or interference by other wireless sources that can disrupt the tag transmissions.

A4. Assume that the item removal event is a clean break, which again, may not be true. For example, if the user is vacillating between buying and not buying, the system may repeatedly count the item as removed or added and lose track of correct count. Also, the user may return an item and take another one of the same kind because she likes the latter more than the former. (A solution may be periodically to scan all tags with the same EPC, and adjust incorrect counts in the database.)

A5. Regarding REQ1, each RFID reader will be able to detect correctly when more than one item of the same type is removed simultaneously. If a customer changed her mind and returned an item (REQ1), we assume that she will return it to the correct shelf, rather than any shelf.

A6. The communication network and the computing system will be able to handle correctly large volume of events. Potentially, there will be many simultaneous or nearly simultaneous RFID events, because there is a large number of products on the shelves and there may be a great number of customers currently in the store, interacting with the items. We assume that the great number of events will not “clog” the computer network or the processors.

Identify the two most important entities of the software-to-be and represent their states using UML state diagrams. Do the following:

(a) List and describe the states that adequately represent the two most important entities

(b) List and describe the events that cause the entities to transition between the states

(c) Draw the UML state diagrams for both entities

Note: Consider all the requirements REQ1 – REQ7.

2. Consider an online auction site, such as eBay.com, with selling, bidding, and buying services. Assume that you are a buyer, you have placed a bid for an item, and you just received a notification that the bidding process is closed and you won it.

(a) Write a single use case that represents the subsequent process of purchasing the item with a credit card. Assume the business model where the funds are immediately transferred to the seller’s account, without waiting for the buyer to confirm the receipt of the goods. Also, only the seller is charged selling fees. Start from the point where you are already logged in the system and consider only what happens during a single sitting at the computer terminal. (Unless otherwise specified, use cases are normally considered only for the activities that span a single sitting.)

(b) Draw use case diagram for above use case.

(c) Draw sequence diagram for above use case.

(d) Draw a UML state diagram showing the possible state changes in the system.

Answer 1

1)

a) All the states:

• State1 - Counting items on shelf
• State2- Low stack
• State3- Out of stack
• State4- Mail/sms to Store Manager
• State5- Mail/sms Store Associate (via Manager)
• State6- Count items which are restored back
• State7- Confirm the status when work is done

b) Events that describe the transition between states:

• From State1 to State2 when the number of items is less than the threshold value( which is still greater than zero)
• State1/State2 to State3 when the count of item is zero.
• State2/State3 to State4: Inform the manager about the low stack.
• State4 to State5 : Inform Store associate
• State5 to State6: The assistant replenishes the shelves.
• State6 to State7: Confirms the status when the work of replenishment is done.

c) UML diagram:

System +RFID tag Product +EPC (96 bit) +RFID tag +RFID Reader +CountItems () +InformManager() Associate Manager +ID +ID +Name +InformAs sociate () +Replenish() +ConfirmStatus () UML STORE MANAGEMENT SYSTEM