Project 1b: Entity EJBs

This is the second step of the Project. In this step you will create a database schema and some entity beans.

Due: 19 April 2005.

The Entities

Recall our goal is a distributed Internet gambling site. In our model, customers place wagers on the outcome of events. Each of these three italicized terms corresponds to a real-world entity, hence to an EJB class you will implement.

As you will see, our model is quite simple compared to a real gambling operation.

Customers

A customer is logically just an entity that can place wagers. A customer can pay money to your site, and your site can pay money to a customer. The customer has attributes:

• A (perhaps fictitious) customer name for display purposes.
• An account nuumber at the Bank. (The Bank is an external site with a Web Services interface. It will appear in later parts of this project.)
• A credit limit, the maximum amount of outstanding wagers you (the gambling site) are willing to let the customer make.

You may also want to store state like

• The outstanding wagers this customer has placed on future events.

Presumably the site will enforce a policy like “outstanding wagers may never exceed credit limit.”

Events

An event in our system is a 2-participant winner-take-all contest that takes place atomically at some specified time. Customers can place wagers on the (future) outcome of an event, as described below. Minimally, an event has attributes:

• A description, a human-readable string.
• The time when the event will take place.

Eventually you will probably need to store additional state like

• The total amount that has been wagered on the event;
• The total payout if the first participant wins;
• The total payout if the second participant wins;
• Current odds for first participant (e.g. the amount returned on a $1 wager if the first participant wins);
• Current odds for second participant.

Some of this state could in principle be computed as an aggregate over all the outstanding wagers (see below), but for efficiency and scalability reasons that would not be a really good idea.

Wagers

A wager is, well, a wager. What can I say? It has attributes

• The event and participant for the wager,
• The amount of the wager,
• The payout if the chosen participant wins.

Note that the total amount wagered on an event and the total payout for each possible outcome of the event could be computed by summing over all the outstanding wagers on that event, if you had unbounded computing capacity.

EJBs and Database Schema

In this part of the project you will implement EJBs for each of the above entities. The EJBs will be maintained persistently in your MS SQL Server™ database.

At this point you have to make a choice for your EJB implementation:

• You can use container-managed persistence (CMP) for your Entity EJBs. In this case the code to load and store entities from/to the database is generated automatically for you; but your database schema is pretty strictly determined by the structure of the EJBs.
• Alternatively, you can use bean-managed persistence (BMP) for your Entity EJBs. In this case you can design your database schema more-or-less arbitrarily, but you have to write your own SQL to load and store the entities.

At this point you can make either choice. But before the project is done you will have to do at least some programming using BMP.

What To Build

For this part of the Project, extend the web site you built in Part a as follows.

1. Implement Entity EJBs for the entities described above.
2. Populate your database with at least one customer and at least two events. (You can do this directly using the SQL Server tools -- EM or QA -- rather than writing client code to do this).
3. Write a simple form-based web page that displays the customer's current wagers and allows her to place an additional wager. Use a Session EJB to help with this. (Would a stateful Session EJB be useful here?)

There is no need to worry about login/authentication (yet).

What to Submit

Create a .zip archive called proj1b.zip containing the following:

• the source files of your EJBs and deployment descriptors
• your database schema and contents (these can be screen shots from MS QU or EM).
• two screen shots of your web page showing the result of adding a wager.

Submit the zip file to CMS by the due date.

As in Part a, the due date is not a big deal – we are going to look at these, but not grade them, and I'll set CMS to accept submissions forever. Please do submit this, however.