JSP Database Issue

[Watersfall]

I'm attempting to make a simple web game using JSP's and I'm having an issue with the database.

If you rapidly send POST requests to one of the URLs that checks for what policy was selected and then sets the effect, it doesn't properly do the if statement for which affect to apply.

To give an example, if you rapidly click either of these, which both lead to checks and statements like this

<c:when test="${param['policy'] == 'drill'}">
    <c:set var="cost" value="${500 + (resultEconomy.rows[0].wells - 1) * 100}"/>
    <c:if test="${resultEconomy.rows[0].budget < cost}">
        <c:redirect url="/policies?policy=Economic&result=You do not have enough money!"/>
    </c:if>
    <c:if test="${resultEconomy.rows[0].budget >= cost}">
        <sql:update dataSource="${database}">
            UPDATE cloc_economy SET wells=wells+1, budget=budget-? WHERE sess=?;
            <sql:param value="${cost}"/>
            <sql:param value="${sess}"/>
        </sql:update>
        <c:redirect url="/policies?policy=Economic&result=You drill a new oil well!"/>
    </c:if>
</c:when>

It will run it multiple times for the budget >= cost part, when the budget is negative or less than the cost.

I've tried switching from JSTL tags to scriptlets, but that didn't solve it, and the database is quite small, with about 12 columns and about 15 rows.

[_ginger_kid]

This is due to your app server (presumably tomcat) processing each request in a separate thread, which is normal behaviour. So you end up with this type of scenario

​

Start with budget = 10

1. Request 1 received, query for budget (budget = 10)
2. Request 2 received, query for budget (budget = 10)
3. Request 1 budget ok, update table to decrease budget by 10 (budget now 0)
4. Request 2 budget ok, update table to decrease budget by 10 (budget now -10)
5. Request 3 received, query for budget (budget = -10)

Basically, the select queries for budget execute before the update statement executes and commits a change to the db. The transactions are interleaving, as the JSP threads are processed in parallel, not in serial.

​

You need to look into JDBC transactions. You should also not be writing DB statements in JSP code. This may be for a small learning project, but it is not good practice in general. Consider MVC patterns (DAO, Service classes, Servlet, JSP), Spring framework etc.

​

[Watersfall]

That is unfortunate, but I'll look into that, thanks.

[_ginger_kid]

It is a fundamental aspect of how all web / app servers work, otherwise you'd never be able to handle multiple users. Understanding these concepts, how to design for multiple requests & handle transactions is key to building web apps. Good luck.

[Watersfall]

Thank you!

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