Object oriented design for an online auction site

Object oriented design for an online auction site

Typical set of operations on the online auction site from the user perspective are

• Users should register themselves and sign in to participate or start auctions in the online auction site. 
• A user can create a new item category to be auctioned in the site or else he can search ongoing auctions on the item and participate in one or more of these auctions simultaneously. 
• Upon participating in auction he/she can place the bids.
• He/she can come out of the auction at any time.
• If he wins the auction by placing the highest bid, he has to pay the bid price to the payment sub system
• If he is the owner of the auction which was complete with a winner, he should send the items to the delivery sub system.

Based on the above description the main components of the system would be

User -> can act as bidder/seller

Item -> Things which are sold and bought in the system

Auction -> Process in which seller wants to sell the item

Bid -> users quotes for the item being sold

Auctioneer -> one who oversees the process of start, stop and conducts the whole process

PaymentHandler -> component which accepts the payments from winning bidder and transfers the payment to the seller

Based on the above description this is the class diagram of the system:

Object oriented design for a library to borrow and renew books

Object oriented design for a library to borrow and renew books

A typical library has a set of books which the users can borrow for a certain period of time and return back. Users may choose to renew the return date if they feel they need to more time to read the book. We are trying here to design an online library.

The typical user actions with this online library would be

•  sign in/register
•  search books
•  borrow books
•  renew the return date for any of the books borrowed
•  view his profile(check his details, list of books he borrowed )

The online library should support all the above actions. It must keep track of the different books in the library currently available for users to borrow and also the books already borrowed bu users. Put it simply the inventory should be managed.

Going through the above description we can think of these components in the system:

1. User
2. Book
3. BorrowTxn -> record for the event for book b is borrowed by user u
4. InventoryManager -> manages the books in the library. Adds new books, remove books and respond to book search requests
5. LibraryManager-> Allows user interaction with the library like logging in, borrowing, renewal and return.

Below is the class diagram which depicts how these components inter-operate 

The Library Manager(LibManager) interacts with the inventory manager for responding to user search queries. Each borrow operation would result in the creation of a BorrowTxn which encapsulates the book details, borrowing user details and the return date. The inventory state is updated accordingly(i.e. the borrowed book is no longer available for borrowing). Similarly when a book is returned the inventory state is updated.

When the book return date is renewed the inventory state is not changed but the borrow transaction is updated accordingly.

Object Oriented design for Elevator in a multi-storied apartment

Object Oriented design for Elevator in a multi-storied apartment

Below is the design for an elevator in a multi-storied building.

A typical lift has buttons(Elevator buttons) inside the cabin to let the user who got in the lift to select his/her desired floor.Similarly each floor has buttons (Floor buttons) to summon the lift to go floors above and floor below respectively. The buttons illuminate indicating the request is accepted. And the lift reaches the requested floor the button stops illuminating.

Use cases:
User

• presses the floor button to summon the lift
• presses the elevator button to make the lift move to the desired floor

Floor Button/Elevator Button

• illuminates when pressed
• places a elevator request when pressed

Elevator

• Moves up/down as per instruction
• Opens/closes the door

Each button press results in an elevator request which has to be served. Each of these requests is tracked at a global place. ElevatorRequests, the class which stores elevator requests can use different strategies to schedule the elevator requests. The elevator is controlled by a controller class which we call ElevatorController. The elevator controller instructs the elevator what to do and also can shutdown/start up the elevator of the building. The elevator controller reads the next elevator request to be processed and serves it. 

Button is abstract class defining common behavior like illuminate, doNotIlluminate. FloorButton, ElevatorButton extend Button type and define placeRequest() which is invoked when the button is pressed. Both the floor button presses and elevator button presses adds requests at a common place.
ElevatorController runs the show by reading the next request to process and instructing the elevator what to do.


How can we extend this to multiple elevators in a sky scraper

In the single elevator scenario, there is an elevator and an elevator controller and a common area where the floor requests and the elevator button request are stored and processed as per the scheduling algorithm.

To extend this to multiple elevators, each elevator will have a corresponding elevator controller. Floor based requests can be served by any elevator where as elevator button requests will be served by the elevator in which the button is pressed.

FloorButton's placeRequest adds a request to the common area which is accessible to all controller. ElevatorButton's placeRequest adds a request to the controller's internal queue as only one elevator is supposed to serve it.

Each elevator controller runs as a separate thread and checks on if it can process a floor request along with processing requests made by its own elevator button presses.

Object oriented design for a digital watch:

Object oriented design for a digital watch:

A simplistic digital displays Hours of the day, minutes of the hour and the seconds of the minute. If you noticed a digital watch the seconds counts refreshes say twice a second and the keeps updating. When the seconds count is 60 seconds hand is reset and the minute count is incremented. Once the minute hand reaches a count 60 the minutes hand is reset and hours hand is incremented. Once the hours count reaches 24 the hours hand is reset to 0.


From the above description, the components of the system are:
1) WatchController
2) SecondsCounter,MinutesCounter,HoursCounter

Below is the class diagram representation:

We have an abstract Type TemporalCounter which defines the method getCounter() returns the current value of the counter which is common for all the counters.

The remaining methods of the class TemporalCounter are abstract and each sub class should define the behavior/declare itself as an abstract class. The reason they are abstract is each subtype has its own different version of increment. For e.g while incrementing seconds counter, once we reach 60 we have to reset the counter and call minutes hand to increment. Same applies for minutes hand increment except that once it reaches 60 it call hours hand increment.

The main controller class supports methods to start and stop the digital watch. Once started, displayTime is executed by the controller. The sub routine for displayTime should be like this

           public void displayTime() {

               while(!stop){
                    secsHand.increment();
                    hoursHand,display()+":"+minsHand.display()+":"+secsHand.display();
               }
            }

Object Oriented design for a cloud based file storage system like SkyDrive

Object Oriented design for a cloud based file storage system like SkyDrive


The typical characteristics/requirements of a cloud storage system:
1) They are essentially file hosting services
2) Allow users to register/sign in
3) Allow registered users to buy more storage
4) Allow users to access/upload/delete the files on the cloud
5) Interact with a Distributed file system to store/delete/retrieve the files on the cloud

From the above description of the system, the main components of the system to be built are:

1) SkyDriveController-> Main controller of the system. Allows user interaction with the system

2) DFSController-> Stores the file in the cloud and also allows access/deletion

3)OperationValidator -> performs validations before allowing user interactions with SkyDrive

4)StorageSpaceAllocator -> allocates extra space for users who want to but space

Lets put down the use cases in visual form:

Above use cases describe the user actions and the corresponding system side actions. Now lets move to the Class diagram to see how the different components inter operate:

The class diagram should help you understand the basic components of a cloud based storage system and should give you enough material to speak up in an interview.  In an interview scenario you can earn some brownie points if you can talk about the Distributed file system implementation. Checkout http://en.wikipedia.org/wiki/Google_File_System#Design to know about how files might be stored on the cloud.

-(

   )antosh

Ubuntu 12.04 'Precise Pangolin' Officially launched. Some cool new features offered

New offerings in  Ubuntu 12.04 Long Term Support code name 'Precise Pangolin'

Ubuntu has released its 12.04 Long Term Support OS for Desktop, Server, Cloud and Core products. Here are some the cool new features offered.

Heads Up Display (HUD):

The new HUD gives you a fast alternative to the traditional, menu-based way of telling your computer what to do. Instead of clicking on menu items to give it instructions, you just press one key to bring up the HUD and start typing what you want to do. You’ll then get a list of functions, without ever needing to know in which menu they live.

 

The Video Lens:

The new Video Lens makes it easier than ever to find the videos you want to watch. Just click on the icon on the Dash and enter your text to search. Whether it’s stored on your computer, on YouTube or anywhere else, you’ll soon see a list of matching content.

Shotwell:

Shotwell is an image organizer designed to provide personal photo management for the GNOME desktop environment. Shotwell is replacing F-Spot as the default image organizer for Ubuntu.

Being somebody interested in multimedia apps, i thought these are some nice offerings in this version of ubuntu. Please do write back if you have explored any other interesting features in ubuntu.

(

)rinivas

Storage wars in the cloud

Comparison of cloud storage services Google Drive and DropBox 

Having received the activation mail from google regarding google drive, i thought about what are the other free cloud storage services available for users like me. I have lots of ebooks, video material, pics and scanned docs which i would like to store online. So let us see whats on offering for users like me:

Google Drive: Actually i have done a exploration of features of google drive here

DropBox (Free version): Compared to Google Drive the space for free account is less. DropBox offers just 2G space. Files stored in drop box can be accessed using windows, linux, mac, ipad, iphone, andriod and blackberry (is there anything left in the list of ways by which a typical user access the stuff online?) The cool feature as per DropBox is it always transfers the part of the file that changes instead of transferring the whole file everytime. This can give the user a nice quick usage experience and also light on the amount of the data to be transferred. I have not yet explored Google Drive completely, but one cool feature in DropBox is it maintains one month history for the changes you make to the files you store. That a huge amount of history for somebody who frequently make changes to their online docs. Also DropBox says any changes can be undone and any deleted files can be recovered. This is also an important feature. I am no fan of google but i have a feeling that google with its huge user base will slowly overtake Dropbox when it comes to the free version of cloud storage apps. Lets wait and see.

There are still some more free services out there which i am yet to review. They are
Microsoft SkyDrive, icloud. I will come up with the details about these two services too shortly so that we have enough data to choose among the best of these.

Good Luck.

(rinivas.
)

Woohooo...Got my google drive with 5 Gig free storage

I just now saw a mail from google-team  regarding availability of google drive for my google account. With over 5 GB of space it should solve all my online data storage problems. I personally collect lot of ebooks, store some important videos and documents which were download from the web and also store scanned copies of interesting material. So, now this one place where i can have access to all my stuff anywhere( i have an andriod phone Sony Ericsson Xperia X10). I say its cool.

I was just checking the features of google drive. Here are some interesting feature.


1) It offers visually pleasing grid view (basically thumbnail preview) and plain old list view. I liked the grid view

2) Other basic functionality like sort your stuff by last edited, last modified, last opened etc.

3)One very useful information for google drive, ie. How much space you have used so far can be found in Settings -> Storage.

4) You have a predefined list of keyboard shortcuts to ease your interaction with your google drive and you can  define your custom keyboard shortcuts too.

5) One more cool feature of the google drive is that you can add interesting google apps and paint your drives grid view with cool games and apps.

6) Google drive categorizes your stuff in to different buckets and provides you option to directly access the desired document type. You have predefined  document categories like Folders, pdfs, presentations, Drawings,  images, tables, spreadsheets, forms etc and many more.


These are just some of the features i explored in the 10 minutes i got access to google drive. Overall i liked it. This surely will help me as a single place where i can store all my data (videos, songs, documents, presentations, ebooks ) and most interesting of all is access them via my andriod device. Good work google.

Guys, please do write in about any other interesting feature of google drive.


-(rinivas
  )