Tuesday, May 8, 2018

OSGi R7 Highlights: Transaction Control

The OSGi R7 specification contains a wide variety of exciting new features covering a wide variety of different use cases. The OSGi Transaction Control service is one of these new specifications providing modularity for transactional resource management.

A New Take on an Old Problem


Transactions have been used in software for decades, and over time they have become simpler to use. The Java Transaction API provides a common API for transaction management in Java, but this is still considered too hard to use directly. Java EE and the Spring framework, therefore, created a variety of declarative models. Before we talk about the new OSGi Transaction Control service we should understand how to work (or not) with these technologies.

Working with Declarative Transactions


The recommended approach for using transactions in Java EE and Spring is to apply the @Transactional annotation to your transactional methods.
public class TransactionalBean {

  DataSource ds;

  @Transactional
  public void addUser(String user) throws SQLException {
    // Add the user
    try (Connection conn = ds.getConnection();
         Statement s = conn.createStatement(
                 "insert into users values(?)")) {
      s.setString(1, user);
      s.executeUpdate();
    }
  }
}

This solution is incredibly simple, unfortunately, it is also deceptively so. Rather than the explicit complexity of managing your own transactions all of the complexity is hidden behind the @Transactional annotation. Unfortunately, hiding complexity doesn't make it go away, it just leads to other questions.
  • How did the transaction actually start and stop?
  • How did the database connection know to participate in the transaction?
  • What will cause the transaction to roll back?

Answering the Questions 


It turns out that there are a lot of moving parts behind the curtain!

Firstly, the transaction is started and stopped by container code running immediately before and after your method runs. This is almost always achieved by creating a proxy for a managed instance and ensuring that all access to the instance is through the proxy. Unfortunately, it's pretty easy to violate that restriction:
public class TransactionalBean {

  ...

  @Transactional(SUPPORTED)
  public void addIfNecessary(String user) {
    if(!userExists(user) {
      addUser(user);
    }
  }
}

In this case, we have another method on our object which can run without a transaction. When this method makes a call to the addUser method it does not touch the proxy. As a result, we can end up running the addUser method outside a transaction.

Secondly, the data source is enlisted because it is also proxied - when the getConnection call is made it locates the current transaction and enlists the connection with that transaction. Importantly this only works if the same person is doing all the proxying and uses the correct transaction manager.

Thirdly, a Java EE (or Spring) transaction will roll back if the method completes with an unexpected exception. Checked exceptions are part of the method signature and therefore not considered to be unexpected. This means that the SQLException in our example does not trigger a rollback

The biggest problem with proxying is that you need to have an all-knowing container ready, running and responsible for managing the objects and resources in the system. In Java EE this is the Application Server, in Spring it is the Application Context, but in OSGi? One thing that we learn over and over is that for a system to be modular you cannot have a single global container. The provider of the resources and the provider of the business objects must be free to use whatever frameworks they choose. We also learn that for a system to be robust we cannot rely on other modules to start before we do. Proxying to provide transactions is, therefore, a fundamentally flawed approach in OSGi.

Transaction Control - A Modular Approach


The OSGi Transaction Control service is a new specification which is designed to address the issues with the Java EE/Spring transaction management model.

One of the biggest differences when using Transaction Control is that transaction management is programmatic, not declarative, and uses a functional decorator pattern. This means that there is no need for a proxy to introduce transaction management instructions into your code, and the transaction is guaranteed to start however your method gets called.
@Component
public class TransactionalComponent {

  @Reference
  TransactionControl txControl

  public void addUser(String user) throws SQLException {
    // Add the user
    txControl.required(() -> {
        // This scoped work runs in a transaction
        return 42;
    });
  }
}

The Transaction Control service offers convenient methods for:

  • Requiring a transaction
  • Requiring a new transaction
  • Suspending a transaction
  • Checking to see whether a transaction is active or not

Completing the Transaction


The transaction and all other state associated with the scope is completed when the scoped work returns. Normally the work will return a value, and this value will be returned by the required method (the integer 42 in the example above). Returning a normal value will cause the transaction to be committed. If the commit fails then the Transaction Control service will throw a TransactionException from the required method.

Rolling Back 


One other important difference between Java EE/Spring transactions and the Transaction Control Service is that in Transaction Control every Exception triggers rollback by default. This is much more likely to give the correct behavior when things go wrong.

The easiest way to get a rollback is, therefore, to throw an exception from your scoped work! If your scoped work does complete with an exception then this will be wrapped in a ScopedWorkException and re-thrown by the required method. Sometimes, however, you don't want to commit your work, but it isn't an exceptional circumstance. In this case throwing an exception is the wrong thing to do, instead, you should simply mark the transaction for rollback.

@Component
public class TransactionalComponent {

  @Reference
  TransactionControl txControl

  public void addUser(String user) throws SQLException {
    // Add the user
    txControl.required(() -> {
        // This transaction must roll back
        txControl.setRollbackOnly();
        return 42;
    });
  }
}


A full description of the scope lifecycle is available in the Transaction Control specification.

Scoped Resources


In order for a transaction to be useful, it must have one or more resources participating in it. In Java EE and Spring these resources must be managed by the container so that they can be enlisted. The Transaction Control specification changes this model to let the OSGi bundle take control of enlistment.

Scoped Resources are created from a ResourceProvider, an object which is either found in the service registry or created by a factory service. The ResourceProvider interface is generic and usually subclassed to provide a specific resource type. The Transaction Control specification includes standard interfaces for creating scoped JDBC Connection objects and JPA EntityManager instances.

The easiest way to get a resource provider instance is to configure the resource provider implementation. For example, the Reference Implementation from Apache Aries can be configured as follows (using the configurator JSON format)
{
    // Global Settings
    ":configurator:resource-version" : 1,
    ":configurator:symbolic-name" : "org.osgi.blog.tx.config",
    ":configurator:version" : "0.0.1.SNAPSHOT",
    
    
    // Configure a JDBC resource provider
    "org.apache.aries.tx.control.jdbc.xa~resource": {
           "osgi.jdbc.driver.class": "org.h2.Driver",
           "url": "jdbc:h2:./data/database" }
}

The configured JDBCConnectionProvider can then be combined with a Transaction Control Service to create a scoped resource and used as follows:

@Component
public class TransactionalComponent {

  private final TransactionControl txControl;

  private final Connection txConnection;

  @Activate
  public TransactionalComponent(
      @Reference TransactionControl txControl,
      @Reference JDBCConnectionProvider provider) {
    this.txControl = txControl;
    this.txConnection = provider.getResource(txControl);
  }

  public void addUser(String user) throws SQLException {
    // Add the user
    txControl.required(() -> {
      Statement s = txConnection.createStatement(
             "insert into users values(?)");
      s.setString(1, user);
      return s.executeUpdate();
    });
  }
}

Resource Lifecycle


You may have noticed that the example using Transaction Control never closes its connection or the Statement it creates. In fact, the same connection instance is used for every call to addUser! This isn't a mistake but in fact, the recommended way to make use of a scoped resource. When you call getResource on a resource provider you aren't being given a physical resource, but a transaction-aware resource object.

The first time that you call the scoped resource object inside a piece of scoped work it does several things:

  • It obtains a real physical resource from an underlying pool
  • It enlists the physical resource in the transaction (if there is one)
  • It registers the physical resource with the ongoing transaction context so the same physical resource is used for the rest of the scoped work
  • It registers a completion callback so that the physical resource can be automatically returned to the pool
All of this means that you never need to worry about getting a different connection instance, or about closing it when you're done. All resource access is implicitly and automatically bounded by the scoped work, this includes the objects created by the scoped work such as Statements and ResultSets.

In Summary


The Transaction Control Service provides a simple, reliable, modular solution for transaction lifecycle management and resource access. If you're interested in seeing more usage of Transaction Control then you should check out the data access services from the OSGi enRoute for R7 microservice example.



Want to find out more about OSGi R7?

This is one post in a series of 12 focused on OSGi R7 Highlights.  Previous posts are:
  1. Proposed Final Draft Now Available
  2. Java 9 Support
  3. Declarative Services
  4. The JAX-RS Whiteboard
  5. The Converter
  6. Cluster Information
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