Persistence in Java

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Persistence
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application data and an ApplicantJDBCDao responsible for persisting applicant information Both DAOs use the same DataSource and are invoked by LoanComponent, which is con gured to use a global managed transaction, as shown in Listing 114
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Listing 114 Invoking Multiple DAOs in the Context of a Global Managed Transaction
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import orgosoaannotationsGlobalManagedTransaction; import orgosoaannotationsReference; @GlobalManagedTransaction public class LoanComponent implements LoanService { private LoanApplicationDao loanApplicationDao; private ApplicantDao applicantDao; public LoanComponent(@Reference LoanApplicationDao loanApplicationDao, @Reference ApplicantDao applicantDao) { thisapplicantDao = applicantDao; thisloanApplicationDao = loanApplicationDao; } public LoanResult apply(LoanRequest request) { LoanApplication application = // create the application from the request LoanApplicant applicant = // create the applicant from the request loanApplicationDaosave(application); applicantDaosave(applicant); } // }
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If the LoanApplicationJDBCDao and ApplicantJDBCDao implementations are con gured to use a global managed transaction, the calls to loanApplicationDaosave() and applicantDao save() in Listing 114 will be performed in the same transaction context That is, they will succeed or fail together If, however, the DAO implementations are con gured to use local managed transactions, each call will be performed independently Figure 111 illustrates the difference
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DAOs using a Global Managed Transaction
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Global Transaction Context
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LoanApplicationDao LoanComponent
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DAOs using Local Managed Transactions
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LoanApplicationDao
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Figure 111 Global versus local managed transactions
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How does an SCA runtime guarantee atomicity across DataSources and components when a global managed transaction is in effect In other words, how does it ensure that persistence operations are handled as a single unit An SCA runtime enforces atomicity by associating a database connection object with the transaction context through the use of a transaction manager If a global transaction is in effect, the SCA runtime will associate the JDBC Connection object returned from the rst call to DataSourcegetConnection() with the active transaction Subsequent calls to DataSourcegetConnection() whether from the same component instance or other components will return the same Connection object as long as the transaction is active In addition, when the Connection object is returned, the runtime will enlist it with a transaction manager If multiple DataSources are used during a transaction, their Connection
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objects will be enlisted with the transaction manager Upon completion of the transaction, the transaction manager will coordinate commits across all enlisted connections, known more generally as resources If an exception is encountered, the transaction manager will coordinate rollbacks across the enlisted resources Fortunately, this work is handled transparently by the runtime Application code does not need to worry about connection management, resource enlistment, or performing coordination
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No Managed Transaction Component implementations may also be con gured to run without a managed transaction In this case, the implementation is responsible for managing its own transaction demarcations boundaries This involves either setting ConnectionsetAutoCommit (boolean) to true, which results in all SQL statements being executed and committed as individual transactions Otherwise, application code must explicitly call ConnectionsetAutoCommit (false) followed by Connectioncommit() or Connection rollback(), as shown in Listing 115
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Listing 115 Manually Managing Transaction Boundaries with a DataSource
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import javaxannotationResource; import javaxsqlDataSource; import orgosoaannotationsNoManagedTransaction; @NoManagedTransaction public class LoanApplicationJDBCDao implements LoanApplicationDao { private DataSource loanDS; @Resource (name = "loanDB") public void setLoanDataSource(DataSource dataSource) { loanDS = dataSource; } public void update(LoanApplication application){ Connection conn = null; try { connsetAutoCommit(false); // update the application conncommit(); } catch (SQLException e) { try {
Using JDBC connrollback(); } catch (SQLException e2) { // log the exception } } nally { // close connection } } // }
As seen in Listing 115, given the added complexity associated with application managed transaction boundaries, it is generally advisable to use either globally or locally managed transactions
Perspective: When to Use Different Transaction Policies
Global transactions involving two-phase commit (2PC) for example, when multiple DataSources are used in a transaction will entail a performance penalty This is because the runtime transaction manager must coordinate commits across multiple resources As a way to avoid this performance penalty, it is often suggested to use local managed or no managed transactions Using local managed transactions will result in persistence operations being performed independently, thereby saving the overhead of 2PC resource coordination The drawback of this approach is that atomicity is lost as the persistence operations may fail independently Using no managed transactions places the burden of committing or rolling back work for a particular database Connection on the application As with local managed transactions, this approach avoids the overhead of 2PC However, it is generally not recommended, as managing connections directly greatly complicates application code Fortunately, most modern transaction managers perform optimizations that make using global transactions generally the best option A common optimization is to avoid 2PC coordination if only one resource is enlisted in a transaction For example, if a component (or set of components) uses a single DataSource for a given transaction, the runtime transaction manager can dispense with 2PC coordination Some transaction managers also implement more sophisticated optimizations (such as the last