Transactions

With transactions we can make sure our operations are atomic, it also helps performance since we will only need one (disc) sync operation per transaction.

Transactions are maintained within a ThreadLocal, every store and remove operation is stored in the current transaction and applied to the model at commit time.

Please note:

• Running queries will take the current transaction into account, so if you removed an object it will not show up in a query even if you have not committed yet.
• Not calling commit or rollback on a transaction will throw an exception the next time a transaction is started on the same thread. Atm this will not recover on it's own.
• In autocommit mode every store and remove get's it's own transaction. Storing/removing multiple object in one call (dao.store(M ... objects)) will have the effect that they share that transaction.
• When there are no changes in the transaction at commit time the transaction is discarded and no sync operation is performed.

import nl.astraeus.prevayler.Transaction;

public class Test {

    public static void main(String [] args) {
        new Transaction() {
            @Override
            public void execute() {
                Company company = new Company("Company x");
                Employee employee1 = new Employee(company, "Employee x1");
                Employee employee2 = new Employee(company, "Employee x2");
                Employee employee3 = new Employee(company, "Employee x3");
 
                companyDao.store(company, employee1, employee2, employee3);
        
                Company c = companyDao.find(company.getId());
        
                System.out.println("Found company: "+c); // finds company "x"
            }
        };
    }

    public void orDoItYourself() {
        try {
            SimpleStore.begin();

            Company company = new Company("Company x");

            companyDao.store(company);

            Company c = companyDao.find(company.getId());
        
            System.out.println("Found company: "+c); // finds company "x"

            SimpleStore.rollback();
           
            Company c = companyDao.find(company.getId());
        
            System.out.println("Found company: "+c); // null
        } finally {
            if (SimpleStore.hasTransaction()) {
                SimpleStore.rollback();
            }
        }        
    }
}

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