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<title>Database considerations for multithreaded programming</title>
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<body id="rzahwdbc-dbcco"><a name="rzahwdbc-dbcco"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Database considerations for multithreaded programming</h1>
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<div><p>You should consider these items when using databases in multithreaded
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programs.</p>
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<div class="section"><ul><li>Data definition language (DDL):<p>Many of the database configuration,
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administration, and setup type interfaces are threadsafe. Database operations
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that are threadsafe include: <span class="cmdname">Create File</span>, <span class="cmdname">Add
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Member</span>, <span class="cmdname">Delete File</span>, and <span class="cmdname">Remove Member</span>.
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Refer to CL Reference or use the <span class="cmdname">Display Command (DSPCMD)</span> command
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to determine if a command is threadsafe. The online help information for that
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command lists any necessary conditions for threadsafety that apply to a command.</p>
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</li>
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<li>Database record I/O:<p>The database protects the I/O operations
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for the duration of the operation (read, update, insert, or delete). When
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you share an open instance of a file among threads, you must serialize access
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to the I/O feedback areas and I/O buffers to see valid information in these
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areas. These areas are under application control, and the database cannot
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protect them after the database operation has been completed.</p>
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<p>An
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example is a read operation. If thread 1 is in the process of a read operation
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and thread 2 performs any I/O operation against the same open instance, thread
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2 waits until thread 1 has completed the reading. The result of the read operation
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in thread 1 is placed in the I/O buffer. When control returns to thread 1,
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thread 2 begins its I/O operation. Without serialization, thread 2 can change
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the information in the I/O buffer before thread 1 can view the result.</p>
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<p>If
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threads do not share an open instance of a file, no serialization is required.</p>
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</li>
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<li>Distributed files:<p>Access to distributed database files
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used with the DB2<sup>®</sup> Multisystem
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and access to distributed data management (DDM) files of type *SNA are not
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threadsafe. Multithreaded jobs deny access to database files of these types.
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These file types cannot be made threadsafe because intersystem communications
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function (ICF) files and the entire Systems Network Architecture (SNA) layer
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are not threadsafe. If an attempt is made to open one of these file types,
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a CPF4380 message (<tt class="msgph">Open attributes not valid in a multithreaded process</tt>)
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is sent to the function attempting to open the file. </p>
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</li>
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<li>Trigger programs:<p>You can fire trigger programs in a multithreaded job.
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The same threadsafety restrictions apply to trigger programs as to any other
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code that runs in a multithreaded job. Parameters on the <span class="cmdname">Add Physical
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File Trigger (ADDPFTRG)</span> command allow for specification of the trigger's
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threadsafety status and the action to take if the trigger is fired in a multithreaded
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job. </p>
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</li>
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<li>Format selector programs:<p>For logical files with multiple formats, the
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use of format selector programs is not threadsafe. You should not use format
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selector programs in a multithreaded job.</p>
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</li>
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<li>Stored procedures:<p>DB2 Structured Query Language (SQL) stored-procedure
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support provides a way for an SQL application to define and call an external
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program though SQL statements. Stored procedures can be called in a multithreaded
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job. The same thread safety restrictions apply to stored procedures as to
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any other code that runs in a multithreaded job. Unlike trigger programs,
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there is no way to specify the stored procedure's thread safety status and
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the action to take if the stored procedure is called in a multithreaded job.</p>
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</li>
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<li>SQL statements:<p>The use of DDL SQL statements might not be threadsafe.
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Data manipulation language (DML) statements are threadsafe. </p>
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</li>
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<li>Server mode for SQL:<p>Using the server mode for SQL is the preferred
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method for accessing databases with multithreaded applications. A job can
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use the server mode for SQL in order to manage multiple database connections
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and transactions. When the application is using server mode for SQL, <span class="keyword">i5/OS™</span> uses the connections in the
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job as a more encapsulated representation of the current database context
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than previously permitted in <span class="keyword">i5/OS</span>.
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It allows for connections to a database by multiple users, multiple connections
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to a database by the same or different users, and the existence of multiple,
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independent transactions by connections to a database. </p>
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<p>Use one of the
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following mechanisms to activate server mode for SQL before data access occurs
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in the application:</p>
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<ul><li>Use the Open Database Connectivity (ODBC) API, <span class="apiname">SQLSetEnvAttr()</span> and
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set the SQL_ATTR_SERVER_MODE attribute to SQL_TRUE before doing any data access. </li>
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<li>Use the <span class="cmdname">Change Job API, QWTCHGJB()</span> and set the 'Server
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mode for Structured Query Language' key before doing any data access. </li>
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<li>Use Java™ Database Connectivity (JDBC) to access
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the database. JDBC automatically uses server mode to preserve required semantics
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of JDBC. </li>
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</ul>
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<div class="p">Server mode for SQL behavior:<ul><li>For embedded SQL, each thread in a job is a separate transaction
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that can be committed or rolled back, even if there are multiple connections
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within that thread. </li>
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<li>For ODBC, call level interface (CLI), and JDBC, each connection handle
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represents a stand-alone connection to the database and can be committed and
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used as a separate entity.</li>
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</ul>
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</div>
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<p>The CLI for SQL is threadsafe. </p>
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</li>
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<li>Commitable transactions:<p>The introduction of threads does not change
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the scope of committable transactions. You can scope committable units of
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work to either a job-level commitment definition or an activation group-level
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commitment definition. A thread commit or rollback operation commits or rolls-back
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all operations done under the commitment definition. If you want an application
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where each thread (or group of threads) has a separate committable transaction,
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you must either use server mode for SQL or manage these transactions with
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separate activation groups. </p>
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</li>
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</ul>
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</div>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="rzahwrzahwe22e22rx.htm">Example: Work with local SQL databases in multithreaded Pthread programs</a></strong><br />
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This example shows how to work with local SQL databases in multithreaded Pthread programs.</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzahwas4co.htm" title="All programs have at least one thread, referred to as the initial thread. In a program with multiple threads, each thread runs its code independently of the other threads in the program.">Threads on i5/OS</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="rzahwdioco.htm" title="You can use the I/O feedback area of a database file to communicate results of I/O operations to the record I/O user. A typical application might have problems with thread safety because of the nature of the feedback area.">Database record I/O and thread safety</a></div>
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<div><a href="rzahwrzahwe22e22rx.htm" title="This example shows how to work with local SQL databases in multithreaded Pthread programs.">Example: Work with local SQL databases in multithreaded Pthread programs</a></div>
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<div><a href="rzahwexico.htm" title="The i5/OS registration facility allows you to define exit points for functions in an application and to register programs that run at those exit points.">Exit points</a></div>
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<div><a href="rzahwmitco.htm" title="Database transactions in i5/OS are scoped to the job or the activation group.">Commit operations in multithreaded programs</a></div>
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</div>
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<div class="relinfo"><strong>Related information</strong><br />
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<div><a href="../rbam6/rbam6clmain.htm">CL Reference</a></div>
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<div><a href="../db2/rbafzmstthreads.htm">Threads topic in SQL Reference</a></div>
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<div><a href="../cli/rzadpkickoff.htm">SQL Call Level Interface (ODBC)</a></div>
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</div>
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</div>
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</body>
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