IBM Informix Developer's Handbook - IBM Redbooks

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13.1 Concurrency and locking 13.1.1 Types of locks Concurrency involves two or more independent uses of the same data at the same time. In a database system with many users, each user needs to be able to access and modify data. Unless the developer and database system impose controls, there can be negative consequences. Programs might access old data that is in the process of being changed by another user, and changes might seem to disappear even though it seems like the change was performed successfully. To take advantage of database server controls, tables in a multiple user environment should be logging tables. At a minimum, if you must use a nonlogging table within a transaction, either set Repeatable Read isolation level or lock the table in exclusive mode. To avoid concurrency problems, the database server imposes a system of locks. A lock is a claim, or reservation, that a program can place on a piece of data. The database server assures that no other program can modify it, as long as the lock is in place. When another application requests the data, the database server either makes the program wait or turns it back with an error. The application developer can control the effect of lock access using a combination of SQL statements and with the buffering mode selected for the database. The most used SQL statements are SET LOCK MODE, SET ISOLATION, and SET TRANSACTION. We discuss these in more detail later. For now, we need a better understanding of the types of locks that we can encounter. IBM Informix offers several server edition. IBM Informix Extended Parallel Server, Informix Online, and Standard Engine will have different syntax for concurrency related commands. You can find more information about the command syntax for the discussion in this chapter in IBM Informix Guide to SQL: Syntax, v11.50, SC27-3611. A lock is implemented as a variable associated with a data item. It can be explicitly placed by an application or, more frequently, is implicitly handled by the database management system. The lock is used to mark a data item as reserved; the type of lock designation determines what actions are permitted by users in regard to the data item. 438 IBM Informix Developer’s Handbook
An IBM Informix instance can have several types of locks: ► A shared lock reserves its object for reading only. It prevents the object from changing while the lock remains. More than one program can place a shared lock on the same object. More than one object can read the record while it is locked in shared mode. In the lock list output visible from onstat -k, a thread with a shared lock is be designated with an S. If an object is currently locked in exclusive mode and the user thread wants to acquire a shared lock, the designation is “IS” (intent-shared). When a session first connects to a database, IBM Informix Servers place a shared lock on the database, to prevent another session from acquiring an exclusive lock on the same database. SELECT queries place a shared lock at the table level, because it is faster for the engine to find a table level lock than it is to search through potentially thousands of row locks. ► Intent-exclusive locks are set automatically by Informix. If a row in a table is updated, an exclusive lock is placed on the row and an intent-exclusive lock is placed on the table. This assures that no other session can place a shared or exclusive lock on the table as long as an individual row is locked exclusively. In the lock list output visible from onstat -k, a thread with an intent exclusive lock is designated with an IX. A related designation that is sometimes visible is SIX. This designation indicates the object is currently shared, with Intent-exclusive when the chance arrives. ► An exclusive lock reserves its object for the use of a single application. This lock type is used when the application needs to change the object. You cannot place an exclusive lock where any other kind of lock exists. After you place an exclusive lock, you cannot place another lock on the same object. In the lock list output visible from onstat -k, a thread with an exclusive lock is designated with an X. ► A promotable (or update) lock establishes an intent to update. You can only place it where no other updatable or exclusive lock exists. You can place an updatable lock on records that already have shared locks. When the application is about to change the locked object, you can promote the update lock to an exclusive lock, but only if no other locks, including shared locks, are on the record at the time the lock would change from update to exclusive. If a shared lock was on the record when the update lock was set, you must drop the shared lock before the update lock can be promoted to an exclusive lock. In the lock list output visible from onstat -k, a thread with an update lock is designated with an U. Chapter 13. Application development considerations 439
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IBM Informix Developer’s Handbook
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Note: Before using this information
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3.2 Setup and configuration . . . .
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8.3.1 Connecting to the database .
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12.5.2 Compiling the wrapper code .
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Trademarks IBM, the IBM logo, and i
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Krishna Doddi (also known as Prasad
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Comments welcome Your comments are
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1.1 Server options There are a numb
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This edition offers the following a
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either an HDR secondary or RSS seco
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There are three edition options for
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functions. An application calls the
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The IBM Data Server Driver for JDBC
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OLEDB Microsoft OLE DB is a specifi
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The DataBlade Developers Kit provid
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1.3.1 Architecture overview An unde
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Physical and logical logging As eac
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Before you select an API or library
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Hidden, behind-the- scenes tasking
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2.1 Server setup In this section, w
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A typical sqlhosts file contains th
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Figure 2-1 shows the Server Informa
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with the existing Informix instance
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2.2 Client setup ► Edit the Infor
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Figure 2-4 illustrates the options
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2. Prepare the environment. You nee
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5. If you selected a Custom install
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At this stage, the Client SDK insta
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You can download some of these comp
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46 IBM Informix Developer’s Handb
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2. On the Software License Agreemen
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5. Verify the installation selectio
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3. Extract the IBM Data Server Driv
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For a silent installation on both W
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Configuring the JDBC driver To use
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3.1 ODBC and Informix ODBC is a sof
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► IBM Informix ODBC Driver (64-bi
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You can set optional configuration
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Parameter Description Insert Cursor
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Table 3-5 shows the ODBC libraries
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Keyword Description DriverODBCVer S
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Keyword Description DESCRIBEDECIMAL
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If there is no database alias defin
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Example 3-5 shows a simple db2cli.i
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The application sends SQL request t
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} 78 IBM Informix Developer’s Han
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SQLSMALLINT DecimalDigits, /* ibSca
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If your application requires only S
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} SQLCHAR sname[15+1]; SQLLEN lcode
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fprintf (stdout, "Connected\n"); 88
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SQLAllocHandle (SQL_HANDLE_ENV, SQL
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SQLAllocHandle (SQL_HANDLE_ENV, SQL
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contains the data or special attrib
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Function Description ifx_lo_specset
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SQLSMALLINT lospec_size; SQLINTEGER
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4. Call ifx_lo_open() to open the s
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SQLBindParameter (hstmt, 1, SQL_PAR
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} SQLFreeHandle (SQL_HANDLE_ENV, he
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SQLCHAR*insertStmt = (SQLCHAR *) "I
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Accessing a complex data type Examp
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C:\work> 3.3.5 Error handling Ludwi
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C:\work>connect_error demo_on;UID=w
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We create a function called CheckDi
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Figure 3-14 shows the Tracing tab.
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Example 3-36 demonstrates how to se
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DRDADEBUG All IBM Data Server drive
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124 IBM Informix Developer’s Hand
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4.1 Informix ESQL/C Informix ESQL/C
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Example 4-1 shows a simple sqlhosts
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Compiling ESQL/C programs The esql
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Platform Environment variable Linux
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Example 4-3 shows a statement that
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:fname[i], :lname[i], :company[i],
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The SELECT statement shown in Examp
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EXEC SQL fetch ex_cursor into :n1,:
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Table 4-3 lists a few ESQL/C data t
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the sbspace in the server before yo
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Example 4-10 shows the ESQL/C comma
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Example 4-12 shows the output of Ex
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} EXEC SQL END DECLARE SECTION; 150
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C->S (16) Time: 2010-07-06 20:21:58
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5.1 JDBC drivers for an Informix da
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5.1.2 IBM Data Server Driver for JD
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Example 5-4 shows the CLASSPATH env
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Example 5-6 shows the compile line
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programmer uses Java data types to
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Example 5-9 shows the syntax of the
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" Connected to Informix Dynamic Ser
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5.4.2 Manipulating data In this sec
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conn.close(); } catch ( Exception e
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Example 5-15 demonstrates how to di
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conn = DriverManager.getConnection(
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con.setAutoCommit(false); Statement
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System.out.format("Last row: \t%d,"
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PreparedStatement pstmt=conn.prepar
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System.err.println(e); } } } Exampl
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Issuer: CN=bedfont.uk.ibm.com, O=ib
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5.6.1 Class not found errors The fo
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5.7.2 IBM Data Server Driver for JD
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6.1 Hibernate for Java This section
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A Hibernate application includes th
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6.2.2 Configuration File Descriptio
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Example 6-3 shows a hibernate.cfg.x
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name="State" table="States">
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Example 6-6 shows the CLASSPATH var
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informix password org.hibernate
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Example 6-11 shows basic Java code
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The only difference between the loa
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1, AZ, Arizona 2, CA, California C:
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Example 6-19 shows the output of th
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private Long id; private String cod
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7.1 IBM Informix OLE DB Provider In
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through COM. You can use it to test
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7.3.1 Supported interfaces An OLE D
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7.3.3 Type mapping Keyword Descript
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7.3.4 Cursors Informix OLE DB Provi
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Example 7-3 demonstrates how to con
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Connect to the Database Server CHEC
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#define CHECK(hr) if (((HRESULT)(hr
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} 232 IBM Informix Developer’s Ha
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if(lDataLength > 0) { // Call GetCh
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_bstr_t bstrPassWord = "password";
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Provider with some of the available
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cx.connectionstring = "Data Source=
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California 0.08250 c:\work> 7.4.3 S
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The user ID for the Informix databa
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happen that the 32-bit provider is
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7.5.2 Tracing Failure when handling
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A44: 53C Exit Object Address: 0x032
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8.1 Informix and .NET data provider
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Both providers have the same assemb
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Example 8-3 shows how to compile an
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Attribute Description Default Exclu
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The majority of the connection stri
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Data Server Provider for .NET type
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Example 8-7 shows the output of Exa
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Table 8-9 lists the methods that th
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Method Description IfxMonthSpan Get
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} } int drows=0; 272 IBM Informix D
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Example 8-16 shows the output of Ex
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Method Description Int64 Seek(offse
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... 280 IBM Informix Developer’s
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IfxConnection conn; IfxClob vclob;
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Property Type Description Year Int3
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Hour : 8 Minute: 20 C:\work>c Arith
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Method Description Compare(IfxDec1,
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C:\work>decimal Value Sales Tax : 0
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Ceiling Sales Tax : 1.0 Negative Sa
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Visual Studio Add-In provides tools
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9.1 Informix and PHP extensions The
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- Console mode Enter one of the fol
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9.2.4 Verifying connectivity You ca
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► Secure Although PHP is open sou
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*--- persistent connect trusted use
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In a web application, the input dat
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Dynamic SQL statements Dynamic SQL
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$stmt->bindValue(':p9', $_POST["pho
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*--- Verification ---*/ $stmt1=$dbc
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$row=$stmt1->fetch(PDO::FETCH_ASSOC
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} /*--- Get the BLOB from the datab
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In Example 9-17 on page 316, the da
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Function Description db2_bind_param
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The IBM_DB2 extension This PHP exte
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9.3.5 Exception handling Error hand
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The error information that is gener
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9.3.6 Troubleshooting printf("NonCr
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10.1 An overview of UDRs and databa
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10.1.2 About UDRs specific examples
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Invoking a UDR You can invoke a UDR
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10.2 Developing UDRs To develop a U
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One variable in, one result out For
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10.2.2 UDRs in Java 5 1005 6 1006 7
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Note: The paths for .jar files that
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We use the following procedure to c
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For our example, JVPCLASSPATH is se
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10.2.3 UDRs in C and break somethin
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3. Set the bld file permissions as
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4. Create the function in dbaccess.
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Installing the DataBlade Any DataBl
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To connect to a database, run one o
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IBM Informix DataBlade module name
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interface, called BladeSmith, allow
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11.1 A brief overview of Ruby on Ra
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The following drivers allow Ruby to
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Data types The Ruby Informix driver
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03/07/2010 17:30 1,088 LICENSE 03/0
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The Rails Adapter can be installed
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server: demo_on username: informix
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Using Ruby Informix driver In this
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If the SQL statement returns only o
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Example 11-17 shows how to select a
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Example 11-19 shows a simple Ruby s
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# create a prepare object stmt = IB
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11.4 Using the Rails Adapter with R
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For more information, refer to: htt
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Page 406 and 407: Figure 11-1 shows the Listing items
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Page 410 and 411: 11.5.4 Migrating the model Ruby on
Page 412 and 413: 11.5.5 Starting the Rails web serve
Page 416 and 417: 12.1 Basic concepts In this section
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Page 428 and 429: Example 12-8 shows the APP SERVER f
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Page 432 and 433: Source file information Finally, yo
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Page 450 and 451: 12.6.2 Tracing A developer can use
Page 456 and 457: 13.1.2 Lock duration The length of
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Page 462 and 463: 13.2.1 Deadlocks reader can read a
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Page 474 and 475: 13.5 Working with your database adm
Page 476 and 477: ► Metadata updates affect logging
Page 478 and 479: Guidelines for setting VPCLASS: ►
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Page 490 and 491: } { printf("XA_PREPARE failed with
Page 492 and 493: When using distributed transactions
Page 496 and 497: Online resources ► IBM Informix D
Page 498 and 499: cast function 332 cast support func
Page 500 and 501: IfxDecimal public method 287 ifxdot
Page 502 and 503: jvpjavavm 341 jvplog 347 jvplogfile
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sql_longvarbinary 80, 96 sql_longva
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IBM Informix Developer’s Handbook
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