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Using Adaptive Buffering

Adaptive buffering is designed to retrieve any kind of large-value data without the overhead of server cursors. Applications can use the adaptive buffering feature with all versions of SQL Server that are supported by the driver.

Normally, when the Microsoft JDBC Driver for SQL Server executes a query, the driver retrieves all of the results from the server into application memory. Although this approach minimizes resource consumption on the SQL Server, it can throw an OutOfMemoryError in the JDBC application for the queries that produce very large results.

In order to allow applications to handle very large results, the Microsoft JDBC Driver for SQL Server provides adaptive buffering. With adaptive buffering, the driver retrieves statement execution results from the SQL Server as the application needs them, rather than all at once. The driver also discards the results as soon as the application can no longer access them. The following are some examples where the adaptive buffering can be useful:

  • The query produces a very large result set: The application can execute a SELECT statement that produces more rows than the application can store in memory. In previous releases, the application had to use a server cursor to avoid an OutOfMemoryError. Adaptive buffering provides the ability to do a forward-only read-only pass of an arbitrarily large result set without requiring a server cursor.

  • The query produces very large SQLServerResultSet columns or SQLServerCallableStatement OUT parameter values: The application can retrieve a single value (column or OUT parameter) that is too large to fit entirely in application memory. Adaptive buffering allows the client application to retrieve such a value as a stream, by using the getAsciiStream, the getBinaryStream, or the getCharacterStream methods. The application retrieves the value from the SQL Server as it reads from the stream.

Note Note

With adaptive buffering, the JDBC driver buffers only the amount of data that it has to. The driver does not provide any public method to control or limit the size of the buffer.

Starting with the JDBC driver version 2.0, the default behavior of the driver is "adaptive". In other words, in order to get the adaptive buffering behavior, your application does not have to request the adaptive behavior explicitly. In the version 1.2 release, however, the buffering mode was "full" by default and the application had to request the adaptive buffering mode explicitly.

There are three ways that an application can request that statement execution should use adaptive buffering:

When using the JDBC Driver version 1.2, applications needed to cast the statement object to a SQLServerStatement class to use the setResponseBuffering method. The code examples in the Reading Large Data Sample and Reading Large Data with Stored Procedures Sample demonstrate this old usage.

However, with the JDBC driver version 2.0, applications can use the isWrapperFor method and the unwrap method to access the vendor-specific functionality without any assumption about the implementation class hierarchy. For example code, see the Updating Large Data Sample topic.

When large values are read once by using the get<Type>Stream methods, and the ResultSet columns and the CallableStatement OUT parameters are accessed in the order returned by the SQL Server, adaptive buffering minimizes the application memory usage when processing the results. When using adaptive buffering:

  • The get<Type>Stream methods defined in the SQLServerResultSet and SQLServerCallableStatement classes return read-once streams by default, although the streams can be reset if marked by the application. If the application wants to reset the stream, it has to call the mark method on that stream first.

  • The get<Type>Stream methods defined in the SQLServerClob and SQLServerBlob classes return streams that can always be repositioned to the start position of the stream without calling the mark method.

When the application uses adaptive buffering, the values retrieved by the get<Type>Stream methods can only be retrieved once. If you try to call any get<Type> method on the same column or parameter after calling the get<Type>Stream method of the same object, an exception is thrown with the message, "The data has been accessed and is not available for this column or parameter".

Developers should follow these important guidelines to minimize memory usage by the application:

  • Avoid using the connection string property selectMethod=cursor to allow the application to process a very large result set. The adaptive buffering feature allows applications to process very large forward-only, read-only result sets without using a server cursor. Note that when you set selectMethod=cursor, all forward-only, read-only result sets produced by that connection are impacted. In other words, if your application routinely processes short result sets with a few rows, creating, reading, and closing a server cursor for each result set will use more resources on both client-side and server-side than is the case where the selectMethod is not set to cursor.

  • Read large text or binary values as streams by using the getAsciiStream, the getBinaryStream, or the getCharacterStream methods instead of the getBlob or the getClob methods. Starting with the version 1.2 release, the SQLServerCallableStatement class provides new get<Type>Stream methods for this purpose.

  • Ensure that columns with potentially large values are placed last in the list of columns in a SELECT statement and that the get<Type>Stream methods of the SQLServerResultSet are used to access the columns in the order they are selected.

  • Ensure that OUT parameters with potentially large values are declared last in the list of parameters in the SQL used to create the SQLServerCallableStatement. In addition, ensure that the get<Type>Stream methods of the SQLServerCallableStatement are used to access the OUT parameters in the order they are declared.

  • Avoid executing more than one statement on the same connection simultaneously. Executing another statement before processing the results of the previous statement may cause the unprocessed results to be buffered into the application memory.

  • There are some cases where using selectMethod=cursor instead of responseBuffering=adaptive would be more beneficial, such as:

    • If your application processes a forward-only, read-only result set slowly, such as reading each row after some user input, using selectMethod=cursor instead of responseBuffering=adaptive might help reduce resource usage by SQL Server.

    • If your application processes two or more forward-only, read-only result sets at the same time on the same connection, using selectMethod=cursor instead of responseBuffering=adaptive might help reduce the memory required by the driver while processing these result sets.

    In both cases, you need to consider the overhead of creating, reading, and closing the server cursors.

In addition, the following list provides some recommendations for scrollable and forward-only updatable result sets:

  • For scrollable result sets, when fetching a block of rows the driver always reads into memory the number of rows indicated by the getFetchSize method of the SQLServerResultSet object, even when the adaptive buffering is enabled. If scrolling causes an OutOfMemoryError, you can reduce the number of rows fetched by calling the setFetchSize method of the SQLServerResultSet object to set the fetch size to a smaller number of rows, even down to 1 row, if necessary. If this does not prevent an OutOfMemoryError, avoid including very large columns in scrollable result sets.

  • For forward-only updatable result sets, when fetching a block of rows the driver normally reads into memory the number of rows indicated by the getFetchSize method of the SQLServerResultSet object, even when the adaptive buffering is enabled on the connection. If calling the next method of the SQLServerResultSet object results in an OutOfMemoryError, you can reduce the number of rows fetched by calling the setFetchSize method of the SQLServerResultSet object to set the fetch size to a smaller number of rows, even down to 1 row, if necessary. You can also force the driver not to buffer any rows by calling the setResponseBuffering method of the SQLServerStatement object with "adaptive" parameter before executing the statement. Because the result set is not scrollable, if the application accesses a large column value by using one of the get<Type>Stream methods, the driver discards the value as soon as the application reads it just as it does for the forward-only read-only result sets.

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