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Oracle® C++ Call Interface Programmer's Guide,
11g Release 1 (11.1)

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12 Optimizing Performance of OCCI Applications

This chapter describes a few suggestions that will lead to better performance for your OCCI custom applications.

This chapter contains these topics:

Transparent Application Failover

OCCI Transparent Application Failover enables OCCI to be more robust in handling database instance failures in distributed applications at run time. If a server node becomes unavailable, applications will automatically reconnect to another surviving node.

Some design options should be considered when including Transparent Application Failover in an application:

Note:

It is the user's responsibility to track changes to the SESSION parameters.

To address these problems, the application can register a failover callback function. In the event of failover, the callback function is invoked at different times during the course of reestablishing the user's session.

Using Transparent Application Failover

To enable TAF, the connect string has to be configured for failover and registered on Connection (created from Environment, ConnectionPool and StatelessConnectionPool). To register the callback function, use the Connection Class interface setTAFNotify().

void Connection::setTAFNotify(
   int (*notifyFn)(
      Environment *env,
      Connection *conn,
      void *ctx,
      FailOverType foType,
      FailOverEventType foEvent),
   void *ctxTAF);

Note that TAF support for ConnectionPools does not include BACKUP and PRECONNECT clauses; these should not be used in the connect string.

Objects and Transparent Application Failover

Transparent application failover works with the OCCI navigational and associative access models and the object cache. In a non Oracle RAC setup, you must ensure that the object type definitions and object OIDs in primary and backup instances are identical.

If the application receives ORA-25402: transaction must roll back error after the failover, then it must initiate a rollback to correctly reset the object cache on the client. If a transaction has not started before the failover, the application should still initiate a rollback after the failover to refresh the objects on the client object cache from the new instance.

Connection Pooling and Transparent Application Failover

If the transparent application failover feature is activated, connections created in a connection pool are also failed over. The application failover callback needs to be specified for each connection obtained from the connection pool; these connections will be failed over when used after the primary instance failure.

Connection Sharing

This section covers the following topics:

Introduction to Thread Safety

Threads are lightweight processes that exist within a larger process. Threads each share the same code and data segments, but have their own program counters, machine registers, and stack. Global and static variables are common to all threads, and a mutual exclusivity mechanism may be required to manage access to these variables from multiple threads within an application.

Once spawned, threads run asynchronously to one another. They can access common data elements and make OCCI calls in any order. Because of this shared access to data elements, a mechanism is required to maintain the integrity of data being accessed by multiple threads. The mechanism to manage data access takes the form of mutexes (mutual exclusivity locks), which ensure that no conflicts arise between multiple threads that are accessing shared resources within an application. In OCCI, mutexes are granted on an OCCI environment basis.

This thread safety feature of the Oracle database server and OCCI library enables developers to use OCCI in a multithreaded application with these added benefits:

  • Multiple threads of execution can make OCCI calls with the same result as successive calls made by a single thread.

  • When multiple threads make OCCI calls, there are no side effects between threads.

  • Even if you do not write a multithreaded program, you do not pay any performance penalty for including thread-safe OCCI calls.

  • Use of multiple threads can improve program performance. You can discern gains on multiprocessor systems where threads run concurrently on separate processors, and on single processor systems where overlap can occur between slower operations and faster operations.

In addition to client/server applications, where the client can be a multithreaded program, thread safety is typically used in three-tier or client/agent/server architectures. In this architecture, the client is concerned only with presentation services. The agent (or application server) processes the application logic for the client application. Typically, this relationship is a many-to-one relationship, with multiple clients sharing the same application server.

The server tier in the three-tier architecture is an Oracle database server. The applications server (agent) supports multithreading, with each thread serving a separate client application. In an Oracle environment, this middle-tier application server is an OCCI or precompiler program.

Implementing Thread Safety

In order to take advantage of thread safety by using OCCI, an application must be running in a thread-safe operating system. Then the application must inform OCCI that the application is running in multithreaded mode by specifying THREADED_MUTEXED or THREADED_UNMUTEXED for the mode parameter of the createEnvironment() method. For example, to turn on mutual exclusivity locking, issue the following statement:

Environment *env = Environment::createEnvironment( 
          Environment::THREADED_MUTEXED);

Note that once createEnvironment is called with THREADED_MUTEXED or THREADED_UNMUTEXED, all subsequent calls to the createEnvironment method must also be made with THREADED_MUTEXED or THREADED_UNMUTEXED modes.

If a multithreaded application is running in a thread-safe operating system, then the OCCI library will manage mutexes for the application on a for each-OCCI-environment basis. However, you can override this feature and have your application maintain its own mutex scheme. This is done by specifying a mode value of THREADED_UNMUTEXED to the createEnvironment() method.

Note:

  • Applications running on non-thread-safe platforms should not pass a value of THREADED_MUTEXED or THREADED_UNMUTEXED to the createEnvironment() method.

  • If an application is single threaded, whether or not the platform is thread safe, the application should pass a value of Environment::DEFAULT to the createEnvironment method. This is also the default value for the mode parameter. Single threaded applications which run in THREADED_MUTEXED mode may incur performance degradation.

  • OCCI does not support non-blocking mode

Serialization

As an application programmer, you have two basic options regarding concurrency in a multithreaded application:

  • Automatic serialization, in which you utilize OTIS's transparent mechanisms

  • Application-provided serialization, in which you manage the contingencies involved in maintaining multiple threads

Automatic Serialization

In cases where there are multiple threads operating on objects (connections and connection pools) derived from an OCCI environment, you can elect to let OCCI serialize access to those objects. The first step is to pass a value of THREADED_MUTEXED to the createEnvironment method. At this point, the OCCI library automatically acquires a mutex on thread-safe objects in the environment.

When the OCCI environment is created with THREADED_MUTEXED mode, then only the Environment, Map, ConnectionPool, StatelessConnectionPool and Connection objects are thread-safe. That is, if two threads make simultaneous calls on one of these objects, then OCCI serializes them internally. However, note that all other OCCI objects, such as Statement, ResultSet, SQLException, Stream, and so on, are not thread-safe as, applications should not operate on these objects simultaneously from multiple threads.

Note that the bulk of processing for an OCCI call happens on the server, so if two threads that use OCCI calls go to the same connection, then one of them could be blocked while the other finishes processing at the server.

Application-Provided Serialization

In cases where there are multiple threads operating on objects derived from an OCCI environment, you can chose to manage serialization. The first step is to pass a value of THREADED_UNMUTEXED for the createEnvironment mode. In this case the application must mutual exclusively lock OCCI calls made on objects derived from the same OCCI environment. This has the advantage that the mutex scheme can be optimized based on the application design to gain greater concurrency.

When an OCCI environment is created in this mode, OCCI recognizes that the application is running in a multithreaded application, but that OCCI need not acquire its internal mutexes. OCCI assumes that all calls to methods of objects derived from that OCCI environment are serialized by the application. You can achieve this two different ways:

  • Each thread has its own environment. That is, the environment and all objects derived from it (connections, connection pools, statements, result sets, and so on) are not shared across threads. In this case your application need not apply any mutexes.

  • If the application shares an OCCI environment or any object derived from the environment across threads, then it must serialize access to those objects (by using a mutex, and so on) such that only one thread is calling an OCCI method on any of those objects.

Basically, in both cases, no mutexes are acquired by OCCI. You must ensure that only one OCCI call is in process on any object derived from the OCCI environment at any given time when THREADED_UNMUTEXED is used.

Note:

  • OCCI is optimized to reuse objects as much as possible. Since each environment has its own heap, multiple environments result in increased consumption of memory. Having multiple environments may imply duplicating work with regard to connections, connection pools, statements, and result set objects. This will result in further memory consumption.

  • Having multiple connections to the server results in more resource consumptions on the server and network. Having multiple environments would normally entail more connections.

Application Managed Data Buffering

When you provide data for bind parameters by the setxxx methods in parameterized statements, the values are copied into an internal data buffer, and the copied values are then provided to the database server for insertion. To reduce overhead of copying string type data that is available in user buffers, use the setDataBuffer() and next() methods of the ResultSet Class and the execute() method of the Statement Class.

setDataBuffer() Method

For high performance applications, OCCI provides the setDataBuffer method whereby the data buffer is managed by the application. The following example shows the setDataBuffer() method:

void setDataBuffer(int paramIndex,
   void *buffer,
   Type type,
   sb4 size,
   ub2 *length,
   sb2 *ind = NULL, 
   ub2 *rc = NULL); 

The following parameters are used in the previous method example:

  • paramIndex: Parameter number

  • buffer: Data buffer containing data

  • type: Type of the data in the data buffer

  • size: Size of the data buffer

  • length: Current length of data in the data buffer

  • ind: Indicator information. This indicates whether the data is NULL or not. For parameterized statements, a value of -1 means a NULL value is to be inserted. For data returned from callable statements, a value of -1 means NULL data is retrieved.

  • rc: Return code. This variable is not applicable to data provided to the Statement method. However, for data returned from callable statements, the return code specifies parameter-specific error numbers.

Not all data types can be provided and retrieved by means of the setDataBuffer() method. For instance, C++ Standard Library strings cannot be provided with the setDataBuffer() interface.

There is an important difference between the data provided by the setxxx() methods and setDataBuffer() method. When data is copied in the setxxx() methods, the original can change once the data is copied. For example, you can use a setString(str1) method, then change the value of str1 prior to execute. The value of str1 that is used is the value at the time setString(str1) is called. However, for data provided by means of the setDataBuffer() method, the buffer must remain valid until the execution is completed.

If iterative executes or the executeArrayUpdate() method is used, then data for multiple rows and iterations can be provided in a single buffer. In this case, the data for the ith iteration is at buffer + (i-1) *size address and the length, indicator, and return codes are at *(length + i), *(ind + i), and *(rc + i) respectively.

This interface is also meant for use with array executions and callable statements that have array or OUT bind parameters.

The same method is available in the ResultSet class to retrieve data without re-allocating the buffer for each fetch.

executeArrayUpdate() Method

If all data is provided with the setDataBuffer() methods or output streams (that is, no setxxx() methods besides setDataBuffer() or getStream() are called), then there is a simplified way of doing iterative execution.

In this case, you should not call setMaxIterations() and setMaxParamSize(). Instead, call the setDataBuffer() or getStream() method for each parameter with the appropriate size arrays to provide data for each iteration, followed by the executeArrayUpdate(int arrayLength) method. The arrayLength parameter specifies the number of elements provided in each buffer. Essentially, this is same as setting the number of iterations to arrayLength and executing the statement.

Since the stream parameters are specified only once, they can be used with array executes as well. However, if any setxxx() methods are used, then the addIteration() method is called to provide data for multiple rows. To compare the two approaches, consider Example 12-1 that inserts two employees in the emp table:

Example 12-1 How to Insert Records Using the addIteration() method

Statement *stmt = conn->createStatement(
  "insert into departments (department_id, department_name) values(:1, :2)"); 
char dnames[][100] = {"Community Outreach", "University Recruiting"}; 
ub2 dnameLen[2]; 

for (int i = 0; i < 2; i++) 
  dnameLen[i] = strlen(dnames[i] + 1);

stmt->setMaxIterations(2);    // set maximum number of iterations 

stmt->setInt(1, 7369);        // specify data for the first row 
stmt->setDataBuffer(2, dnames, OCCI_SQLT_STR, sizeof(dnames[0]), dnameLen); 
stmt->addIteration(); 

stmt->setInt(1, 7654);        // specify data for the second row 
                              // a setDatBuffer is unnecessary for the second 
                              // bind parameter as data provided through
                              // setDataBuffer is specified only once. 
stmt->executeUpdate(); 

However, if the first parameter could also be provided through the setDataBuffer() interface, then, instead of the addIteration() method, you would use the executeArrayUpdate() method, as shown in Example 12-2:

Example 12-2 How to Insert Records Using the executeArrayUpdate() Method

Statement *stmt = conn->createStatement(
  "insert into departments (department_id, department_name) values(:1, :2)"); 
char dnames[][100] = {"Community Outreach", "University Recruiting"}; 
ub2 dnameLen[2]; 

for (int i = 0; i < 2; i++) 
  dnameLen[i] = strlen(dnames[i] + 1); 

int ids[2] = {7369, 7654}; 
ub2 idLen[2] = {sizeof(ids[0]), sizeof(ids[1])}; 
stmt->setDataBuffer(1, ids, OCCIINT, sizeof(ids[0]), idLen); 
stmt->setDataBuffer(2, dnames, OCCI_SQLT_STR, sizeof(dnames[0]), dnameLen); 

stmt->executeArrayUpdate(2);      // data for two rows is inserted. 

Array Fetch Using next() Method

If the application is fetching data with only the setDataBuffer() interface or the stream interface, then an array fetch can be executed. The array fetch is implemented through the next() method of the ResultSet class. You must process the results obtained through next() before calling it again.

Example 12-3 How to use Array Fetch with a ResultSet

ResultSet *resultSet = stmt->executeQuery(...);
resultSet->setDataBuffer(...);
while (resultSet->next(numRows) == DATA_AVAILABLE)
   process(resultSet->getNumArrayRows() );

This causes up to numRows amount of data to be fetched for each column. The buffers specified with the setDataBuffer() interface should large enough to hold at least numRows of data.

Modifying Rows Iteratively

To process batch errors, specify that the Statement object is in a batchMode of execution using the setBatchErrorMode() method. Once the batchMode is set and a batch update runs, any resulting errors are reported through the BatchSQLException Class.

The BatchSQLException class provides methods that handle batch errors. Example 12-4 illustrates how batch handling can be implemented within any OCCI application.

Example 12-4 How to Modify Rows Iteratively and Handle Errors

  1. Create the Statement object and set its batch error mode to TRUE.

    Statement *stmt = conn->createStatement ("...");
    stmt->setBatchErrorMode (true);
    
  2. Perform programmatic changes necessary by the application.

  3. Update the statement.

    try {
      updateCount = stmt->executeUpdate ();
    }
    
  4. Catch and handle any errors generated during the batch insert or update.

    catch (BatchSQLException &batchEx)
    {
      cout<<"Batch Exception: "<<batchEx.what()<<endl;
      int errCount = batchEx.getFailedRowCount();
      cout << "Number of rows failed " << errCount <endl;
      for (int i = 0; i < errCount; i++ )
      {
        SQLException err = batchEx.getException(i);
        unsigned int rowIndex = batchEx.getRowNum(i);
        cout<<"Row " << rowIndex << " failed because of "
           << err.getErrorCode() << endl; 
      }
      // take recovery action on the failed rows
    }
    
  5. Catch and handle other errors generated during the statement update. Note that statement-level errors are still thrown as instances of a SQLException.

    catch( SQLException &ex) // to catch other SQLExceptions.
    {
       cout << "SQLException: " << e.what() << endl;
    }
    

Runtime Load Balancing of the Stateless Connection Pool

Runtime load balancing in a stateless connection pool dynamically routs connection request to the least loaded instance of the database. This is achieved through the use of service metrics distributed by the Oracle RAC load-balancing advisory.

The feature modifies the stateless connection pool in the following ways:

Runtime load balancing is turned on by default when the OCCI environment is created in THREADED_MUTEXED and EVENTS modes, and when the server is configured to issue event notifications.

API Support

New NO_RLB option for the PoolType attribute of the StatelessConnectionPool Class disables runtime load balancing.

Fault Diagnosability

Fault diagnosability captures diagnostic data, such as dump files or core dump files, on the OCCI client when a problem incident occurrs. For each incident, the fault diagnosibility feature creates an Automatic Diagnostic Repository (ADR) subdirectory for storing this diagnostic data. For example, if either a Linux or a UNIX application fails with a null pointer reference, then the core file appears in the ADR home directory (if it exists), not in the operating system directory. This section discusses the ADR subdirectory structure and the utility to manage its output, the ADR Command Interpreter (ADRCI).

An ADR home is the root directory for all diagnostic data for an instance of a product, such as OCCI, and a particular operating system user. All ADR homes appear under the same root directory, the ADR base.

See Also:

Oracle Database Administrator's Guide, "Managing Diagnostic Data"

ADR Base Location

The location of the ADR base is determined in the following order:

  1. In the sqlnet.ora file (on Windows, in the %TNS_ADMIN% directory, or on Linux or Unix, in the $TNS_ADMIN directory).

    If there is no TNS_ADMIN directory, then sqlnet.ora is stored in the current directory.

    If the ADR base is listed in the sqlnet.ora file, it is a statement of the type:

    ADR_BASE=/directory/adr
    

    where:

    • adr is a directory that must already exist and be writable by all operating system users who execute OCCI applications and want to share the same ADR base.

    • directory is the path name

    If ADR_BASE is set, and if all users share a single sqlnet.ora file, then OCCI stops searching when directory adr does not exist or if it is not writable by the user. If ADR_BASE is not set, then OCCI continues the search, testing for the existence of othr specific directories.

    For example, if sqlnet.ora contains the entry ADR_BASE=/home/chuck/test then:

    • ADR base is:

      /home/chuck/test/oradiag_chuck
      
    • ADR home may be:

      /home/chuck/test/oradiag_chuck/diag/clients/user_chuck/host_4144260688_11
      
  2. If the Oracle base exists (on Windows: %ORACLE_BASE% , or on Linux and Unix: $ORACLE_BASE), the client subdirectory also exists because it is created by the Oracle Universal Installer when the database is installed.

    For example, if $ORACLE_BASE is /home/chuck/obase , then:

    • ADR base is:

      /home/chuck/obase
      
    • ADR home may be:

      /home/chuck/obase/diag/clients/user_chuck/host_4144260688_11
      
  3. If the Oracle home exists (on Windows: %ORACLE_HOME% , or on Linux and Unix: $ORACLE_HOME), the client subdirectory also exists because it is created by the Oracle Universal Installer when the database is installed.

    For example, if $ORACLE_HOME is /ade/chuck_l1/oracle , then:

    • ADR base is:

      /ade/chuck_l1/oracle/log
      
    • ADR home may be:

      /ade/chuck_l1/oracle/log/diag/clients/user_chuck/host_4144260688_11
      
  4. On the operating system home directory.

    • On Windows, the operating system home directory is %USERPROFILE%.

      The location of folder Oracle is at:

      C:\Documents and Settings\chuck
      

      If the application runs as a service, the home directory option is skipped.

    • On Linux and Unix, the operating system home directory is $HOME.

      The location may be:

      /home/chuck/oradiag_chuck
      

    For example, in an Instant Client, if $HOME is /home/chuck, then:

    • ADR base is:

      /home/chuck/oradiag_chuck
      
    • ADR home may be:

      /home/chuck/oradiag_chuck/diag/clients/user_chuck/host_4144260688_11
      

    See Also:

    "Instant Client"
  5. In the temporary directory.

    • On Windows, the temporary directories are searched in the following order:

      • %TMP%

      • %TEMP%

      • %USERPROFILE%

      • Windows system directory

    • On Linux and Unix, the temporary directory is in /var/tmp.

    For example, in an Instant Client, if $HOME is not writable, then:

    • ADR base is:

      /var/tmp/oradiag_chuck
      
    • ADR home may be:

      /var/tmp/oradiag_chuck/diag/clients/user_chuck/host_4144260688_11
      

If none of these directory choices are available and writable, ADR will not be created and diagnostics will not be stored.

Using ADRCI

ADRCI is a command-line tool that enables you to view diagnostic data within the ADR, and to package incident and problem information into a zip file that can be shared with Oracle Support. ADRCI can be used either interactively and through a script.

A problem is a critical error in OCI or the client. Each problem has a problem key. An incident is a single occurrence of a problem, and it is identified by a unique numeric incident ID. Each incident has a problem key which has a set of attributes: the ORA error number, error parameter values, and similar information. Two incidents have the same root cause if their problem keys match.

The following examples demonstrate how to use ADRCI on a Linux operating system. Note that ARDCI commands are case-insensitive. All user input is in bold typeface.

Example 12-5 How to Use ADRCI for OCCI Application Incidents

To launch ADRCI in a Linux system, use the adrci command. Once ADRCI starts, find out the particulars of the show base command with help, and then determine the base of a particular client using the -product client option (necessary for OCCI applications). To set the ADRCI base, use the set base command. Once ADRCI starts, then the default ADR base is for the rdbms server. The $ORACLE_HOME is set to /ade/chuck_l3/oracle. To view the incidents, use the show incidents command. to exit ADRCI, use the quit command.

% adrci 

ADRCI: Release 11.1.0.6.0 - on Wed July 25 16:16:55 2007
 
Copyright (c) 1982, 2007, Oracle.  All rights reserved.

adrci> help show base
 
  Usage: SHOW BASE [-product <product_name>]
 
  Purpose: Show the current ADR base setting.
 
  Options:
    [-product <product_name>]: This option allows users to show the
    given product's ADR Base location. The current registered products are
    "CLIENT" and "ADRCI".
 
  Examples: 
    show base -product client
    show base
 
adrci> show base -product client
ADR base is "/ade/chuck_l3/oracle/log"

adrci> help set base
 
  Usage:  SET BASE <base_str>
 
  Purpose: Set the ADR base to use in the current ADRCI session.
           If there are valid ADR homes under the base, all homes will
           will be added to the current ADRCI session.
 
  Arguments:
    <base_str>: It is the ADR base directory, which is a system-dependent
    directory path string.
 
  Notes:
    On platforms that use "." to signify current working directory,
    it can be used as base_str.
 
  Example: 
    set base /net/sttttd1/scratch/someone/view_storage/someone_v1/log
    set base .

adrci> set base /ade/chuck_l3/oracle/log

adrci> show incidents
...
adrci> quit

Example 12-6 How to Use ADRCI for Instant Client

Because Instant Client does not use $ORACLE_HOME, the default ADR base is the user's home directory.

adrci> show base -product client
ADR base is "/home/chuck/oradiag_chuck"
adrci> set base /home/chuck/oradiag_chuck
adrci> show incidents
 
ADR Home = /home/chuck/oradiag_chuck/diag/clients/user_chuck/host_4144260688_11:
*************************************************************************
INCIDENT_ID    PROBLEM_KEY           CREATE_TIME
-------------------------------------------------------------------------
1                     oci 24550 [6]              2007-05-01 17:20:02.803697 -07:00                      
1 rows fetched
 
adrci> quit

See Also:

Controlling ADR Creation and Disabling Fault Diagnosability

To disable the fault diagnosability feature, you must turn off the capture of diagnositics. Edit the sqlnet.ora file by changing the values of the DIAG_ADR_ENABLED and DIAG_DDE_ENABLED parameters to FALSE; the default values are TRUE.

To turn off the OCCI signal handler and to re-enable standard operating system failure processing, edit the sqlnet.ora file by adding the corresponding parameter: DIAG_SIGHANDLER_ENABLED=FALSE.