The standard classes and interfaces that a third party vendor has to use in its implementation of a synchronization provider. These classes and interfaces are referred to as the Service Provider Interface (SPI). A vendor may have its implementation included on the JDBC web page that lists available `SyncProvider` implementations by sending email to `jdbc@sun.com`. Doing this helps make developers aware of the implementation. To make it possible for a `RowSet` object to use an implementation, the vendor must register it with the `SyncFactory` singleton. (See the class comment for `SyncProvider` for a full explanation of the registration process and the naming convention to be used.)

## Table of Contents

- 1.0 Package Specification
- 2.0 Service Provider Architecture
- 3.0 Implementer's Guide
- 4.0 Resolving Synchronization Conflicts
- 5.0 Related Specifications
- 6.0 Related Documentation

### 1.0 Package Specification

The following classes and interfaces make up the `javax.sql.rowset.spi` package:

- `SyncFactory`
- `SyncProvider`
- `SyncFactoryException`
- `SyncProviderException`
- `SyncResolver`
- `XmlReader`
- `XmlWriter`
- `TransactionalWriter`

The following interfaces, in the `javax.sql` package, are also part of the SPI:

- `RowSetReader`
- `RowSetWriter`

A `SyncProvider` implementation provides a disconnected `RowSet` object with the mechanisms for reading data into it and for writing data that has been modified in it back to the underlying data source. A reader, a `RowSetReader` or `XMLReader` object, reads data into a `RowSet` object when the `CachedRowSet` methods `execute` or `populate` are called. A writer, a `RowSetWriter` or `XMLWriter` object, writes changes back to the underlying data source when the `CachedRowSet` method `acceptChanges` is called.

The process of writing changes in a `RowSet` object to its data source is known as synchronization. The `SyncProvider` implementation that a `RowSet` object is using determines the level of synchronization that the `RowSet` object's writer uses. The various levels of synchronization are referred to as grades.

The lower grades of synchronization are known as optimistic concurrency levels because they optimistically assume that there will be no conflicts or very few conflicts. A conflict exists when the same data modified in the `RowSet` object has also been modified in the data source. Using the optimistic concurrency model means that if there is a conflict, modifications to either the data source or the `RowSet` object will be lost.

Higher grades of synchronization are called pessimistic because they assume that others will be accessing the data source and making modifications. These grades set varying levels of locks to increase the chances that no conflicts occur.

The lowest level of synchronization is simply writing any changes made to the `RowSet` object to its underlying data source. The writer does nothing to check for conflicts. If there is a conflict and the data source values are overwritten, the changes other parties have made by to the data source are lost.

The `RIXMLProvider` implementation uses the lowest level of synchronization and just writes `RowSet` changes to the data source. This is true because typically XML data sources do not enable transaction techniques for maintaining the integrity of data. However, specific standards groups have considered offering XML-based synchronization. For details, see

         http://www.syncml.org

For the the next level up, the writer checks to see if there are any conflicts, and if there are, it does not write anything to the data source. The problem with this concurrency level is that if another party has modified the corresponding data in the data source since the `RowSet` object got its data, the changes made to the `RowSet` object are lost. The `RIOptimisticProvider` implementation uses this level of synchronization.

At higher levels of synchronization, referred to as pessimistic concurrency, the writer take steps to avoid conflicts by setting locks. Setting locks can vary from setting a lock on a single row to setting a lock on a table or the entire data source. The level of synchronization is therefore a tradeoff between the ability of users to access the data source concurrently and the ability of the writer to keep the data in the `RowSet` object and its data source synchronized.

It is a requirement that all disconnected `RowSet` objects (`CachedRowSet`, `FilteredRowSet`, `JoinRowSet`, and `WebRowSet` objects) obtain their `SyncProvider` objects from the `SyncFactory` mechanism.

The reference implementation (RI) provides two synchronization providers.

- `RIOptimisticProvider`
  The default provider that the `SyncFactory` instance will supply to a disconnected `RowSet` object when no provider implementation is specified.
  This synchronization provider uses an optimistic concurrency model, assuming that there will be few conflicts among users who are accessing the same data in a database. It avoids using locks; rather, it checks to see if there is a conflict before trying to synchronize the `RowSet` object and the data source. If there is a conflict, it does nothing, meaning that changes to the `RowSet` object are not persisted to the data source.
- `RIXMLProvider`
  A synchronization provider that can be used with a `WebRowSet` object, which is a rowset that can be written in XML format or read from XML format. The `RIXMLProvider` implementation does no checking at all for conflicts and simply writes any updated data in the `WebRowSet` object to the underlying data source. `WebRowSet` objects use this provider when they are dealing with XML data.

These `SyncProvider` implementations are bundled with the reference implementation, which makes them always available to `RowSet` implementations. `SyncProvider` implementations make themselves available by being registered with the `SyncFactory` singleton. When a `RowSet` object requests a provider, by specifying it in the constructor or as an argument to the `CachedRowSet` method `setSyncProvider`, the `SyncFactory` singleton checks to see if the requested provider has been registered with it. If it has, the `SyncFactory` creates an instance of it and passes it to the requesting `RowSet` object. If the `SyncProvider` implementation that is specified has not been registered, the `SyncFactory` singleton causes a `SyncFactoryException` object to be thrown. If no provider is specified, the `SyncFactory` singleton will create an instance of the default provider implementation, `RIOptimisticProvider`, and pass it to the requesting `RowSet` object.

If a `WebRowSet` object does not specify a provider in its constructor, the `SyncFactory` will give it an instance of `RIOptimisticProvider`. However, the constructor for `WebRowSet` is implemented to set the provider to the `RIXMLProvider`, which reads and writes a `RowSet` object in XML format.

See the SyncProvider class specification for further details.

Vendors may develop a `SyncProvider` implementation with any one of the possible levels of synchronization, thus giving `RowSet` objects a choice of synchronization mechanisms. A vendor can make its implementation available by registering the fully qualified class name with Sun Microsystems at `jdbc@sun.com`. This process is discussed in further detail below.

### 2.0 Service Provider Interface Architecture

2.1 Overview

The Service Provider Interface provides a pluggable mechanism by which `SyncProvider` implementations can be registered and then generated when required. The lazy reference mechanism employed by the `SyncFactory` limits unnecessary resource consumption by not creating an instance until it is required by a disconnected `RowSet` object. The `SyncFactory` class also provides a standard API to configure logging options and streams that may be provided by a particular `SyncProvider` implementation.

2.2 Registering with the `SyncFactory`

A third party `SyncProvider` implementation must be registered with the `SyncFactory` in order for a disconnected `RowSet` object to obtain it and thereby use its `javax.sql.RowSetReader` and `javax.sql.RowSetWriter` implementations. The following registration mechanisms are available to all `SyncProvider` implementations:

- System properties - Properties set at the command line. These properties are set at run time and apply system-wide per invocation of the Java application. See the section "Related Documentation" further related information.

- Property Files - Properties specified in a standard property file. This can be specified using a System Property or by modifying a standard property file located in the platform run-time. The reference implementation of this technology includes a standard property file than can be edited to add additional `SyncProvider` objects.

- JNDI Context - Available providers can be registered on a JNDI context. The `SyncFactory` will attempt to load `SyncProvider` objects bound to the context and register them with the factory. This context must be supplied to the `SyncFactory` for the mechanism to function correctly.

Details on how to specify the system properties or properties in a property file and how to configure the JNDI Context are explained in detail in the `SyncFactory` class description.

2.3 SyncFactory Provider Instance Generation Policies

The `SyncFactory` generates a requested `SyncProvider` object if the provider has been correctly registered. The following policies are adhered to when either a disconnected `RowSet` object is instantiated with a specified `SyncProvider` implementation or is reconfigured at runtime with an alternative `SyncProvider` object.

- If a `SyncProvider` object is specified and the `SyncFactory` contains no reference to the provider, a `SyncFactoryException` is thrown.

- If a `SyncProvider` object is specified and the `SyncFactory` contains a reference to the provider, the requested provider is supplied.

- If no `SyncProvider` object is specified, the reference implementation provider `RIOptimisticProvider` is supplied.

These policies are explored in more detail in the `SyncFactory` class.

- ### 3.0 SyncProvider Implementer's Guide

  3.1 Requirements

  A compliant `SyncProvider` implementation that is fully pluggable into the `SyncFactory` must extend and implement all abstract methods in the `SyncProvider` class. In addition, an implementation must determine the grade, locking and updatable view capabilities defined in the `SyncProvider` class definition. One or more of the `SyncProvider` description criteria must be supported. It is expected that vendor implementations will offer a range of grade, locking, and updatable view capabilities.

  Furthermore, the `SyncProvider` naming convention must be followed as detailed in the `SyncProvider` class description.

  3.2 Grades

  JSR 114 defines a set of grades to describe the quality of synchronization a `SyncProvider` object can offer a disconnected `RowSet` object. These grades are listed from the lowest quality of service to the highest.

  - GRADE_NONE - No synchronization with the originating data source is provided. A `SyncProvider` implementation returning this grade will simply attempt to write any data that has changed in the `RowSet` object to the underlying data source, overwriting whatever is there. No attempt is made to compare original values with current values to see if there is a conflict. The `RIXMLProvider` is implemented with this grade.

  - GRADE_CHECK_MODIFIED_AT_COMMIT - A low grade of optimistic synchronization. A `SyncProvider` implementation returning this grade will check for conflicts in rows that have changed between the last synchronization and the current synchronization under way. Any changes in the originating data source that have been modified will not be reflected in the disconnected `RowSet` object. If there are no conflicts, changes in the `RowSet` object will be written to the data source. If there are conflicts, no changes are written. The `RIOptimisticProvider` implementation uses this grade.

  - GRADE_CHECK_ALL_AT_COMMIT - A high grade of optimistic synchronization. A `SyncProvider` implementation returning this grade will check all rows, including rows that have not changed in the disconnected `RowSet` object. In this way, any changes to rows in the underlying data source will be reflected in the disconnected `RowSet` object when the synchronization finishes successfully.

  - GRADE_LOCK_WHEN_MODIFIED - A pessimistic grade of synchronization. `SyncProvider` implementations returning this grade will lock the row in the originating data source that corresponds to the row being changed in the `RowSet` object to reduce the possibility of other processes modifying the same data in the data source.

  - GRADE_LOCK_WHEN_LOADED - A higher pessimistic synchronization grade. A `SyncProvider` implementation returning this grade will lock the entire view and/or table affected by the original query used to populate a `RowSet` object.

  3.3 Locks

  JSR 114 defines a set of constants that specify whether any locks have been placed on a `RowSet` object's underlying data source and, if so, on which constructs the locks are placed. These locks will remain on the data source while the `RowSet` object is disconnected from the data source.

  These constants should be considered complementary to the grade constants. The default setting for the majority of grade settings requires that no data source locks remain when a `RowSet` object is disconnected from its data source. The grades `GRADE_LOCK_WHEN_MODIFIED` and `GRADE_LOCK_WHEN_LOADED` allow a disconnected `RowSet` object to have a fine-grained control over the degree of locking.

  - DATASOURCE_NO_LOCK - No locks remain on the originating data source. This is the default lock setting for all `SyncProvider` implementations unless otherwise directed by a `RowSet` object.

  - DATASOURCE_ROW_LOCK - A lock is placed on the rows that are touched by the original SQL query used to populate the `RowSet` object.

  - DATASOURCE_TABLE_LOCK - A lock is placed on all tables that are touched by the query that was used to populate the `RowSet` object.

  - DATASOURCE_DB_LOCK A lock is placed on the entire data source that is used by the `RowSet` object.

  3.4 Updatable Views

  A `RowSet` object may be populated with data from an SQL `VIEW`. The following constants indicate whether a `SyncProvider` object can update data in the table or tables from which the `VIEW` was derived.

  - UPDATABLE_VIEW_SYNC Indicates that a `SyncProvider` implementation supports synchronization to the table or tables from which the SQL `VIEW` used to populate a a `RowSet` object is derived.

  - NONUPDATABLE_VIEW_SYNC Indicates that a `SyncProvider` implementation does not support synchronization to the table or tables from which the SQL `VIEW` used to populate a `RowSet` object is derived.

  3.5 Usage of `SyncProvider` Grading and Locking

  In the example below, the reference `CachedRowSetImpl` implementation reconfigures its current `SyncProvider` object by calling the `setSyncProvider` method.

          CachedRowSetImpl crs = new CachedRowSetImpl();
          crs.setSyncProvider("com.foo.bar.HASyncProvider");

  An application can retrieve the `SyncProvider` object currently in use by a disconnected `RowSet` object. It can also retrieve the grade of synchronization with which the provider was implemented and the degree of locking currently in use. In addition, an application has the flexibility to set the degree of locking to be used, which can increase the possibilities for successful synchronization. These operation are shown in the following code fragment.

          SyncProvider sync = crs.getSyncProvider();

          switch (sync.getProviderGrade()) {
          case: SyncProvider.GRADE_CHECK_ALL_AT_COMMIT
               //A high grade of optimistic synchronization
          break;
          case: SyncProvider.GRADE_CHECK_MODIFIED_AT_COMMIT
               //A low grade of optimistic synchronization
          break;
          case: SyncProvider.GRADE_LOCK_WHEN_LOADED
               // A pessimistic synchronization grade
          break;
          case: SyncProvider.GRADE_LOCK_WHEN_MODIFIED
               // A pessimistic synchronization grade
          break;
          case: SyncProvider.GRADE_NONE
            // No synchronization with the originating data source provided
          break;
          }

          switch (sync.getDataSourcLock() {
            case: SyncProvider.DATASOURCE_DB_LOCK
             // A lock is placed on the entire datasource that is used by the
             // RowSet object
             break;

            case: SyncProvider.DATASOURCE_NO_LOCK
             // No locks remain on the  originating data source.
            break;

            case: SyncProvider.DATASOURCE_ROW_LOCK
             // A lock is placed on the rows that are  touched by the original
             // SQL statement used to populate
             // the RowSet object that is using the SyncProvider
             break;

            case: DATASOURCE_TABLE_LOCK
             // A lock is placed on  all tables that are touched by the original
             // SQL statement used to populated
             // the RowSet object that is using the SyncProvider
             break;

  It is also possible using the static utility method in the `SyncFactory` class to determine the list of `SyncProvider` implementations currently registered with the `SyncFactory`.

      	Enumeration e = SyncFactory.getRegisteredProviders();

  ### 4.0 Resolving Synchronization Conflicts

  The interface `SyncResolver` provides a way for an application to decide manually what to do when a conflict occurs. When the `CachedRowSet` method `acceptChanges` finishes and has detected one or more conflicts, it throws a `SyncProviderException` object. An application can catch the exception and have it retrieve a `SyncResolver` object by calling the method `SyncProviderException.getSyncResolver()`.

  A `SyncResolver` object, which is a special kind of `CachedRowSet` object or a `JdbcRowSet` object that has implemented the `SyncResolver` interface, examines the conflicts row by row. It is a duplicate of the `RowSet` object being synchronized except that it contains only the data from the data source this is causing a conflict. All of the other column values are set to `null`. To navigate from one conflict value to another, a `SyncResolver` object provides the methods `nextConflict` and `previousConflict`.

  The `SyncResolver` interface also provides methods for doing the following:

  - finding out whether the conflict involved an update, a delete, or an insert
  - getting the value in the data source that caused the conflict
  - setting the value that should be in the data source if it needs to be changed or setting the value that should be in the `RowSet` object if it needs to be changed

  When the `CachedRowSet` method `acceptChanges` is called, it delegates to the `RowSet` object's `SyncProvider` object. How the writer provided by that `SyncProvider` object is implemented determines what level (grade) of checking for conflicts will be done. After all checking for conflicts is completed and one or more conflicts has been found, the method `acceptChanges` throws a `SyncProviderException` object. The application can catch the exception and use it to obtain a `SyncResolver` object.

  The application can then use `SyncResolver` methods to get information about each conflict and decide what to do. If the application logic or the user decides that a value in the `RowSet` object should be the one to persist, the application or user can overwrite the data source value with it.

  The comment for the `SyncResolver` interface has more detail.

  ### 5.0 Related Specifications

  - JNDI 1.3
  - Java Logging APIs

  ### 6.0 Related Documentation

  - System properties
  - Resource Files
  - DataSource for JDBC Connections