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Transactions

Firebird supports three transaction isolation levels:

  • READ COMMITTED

  • SNAPSHOT (“concurrency” or “repeatable read”)

  • SNAPSHOT TABLE STABILITY “consistency”)

The default isolation level of the ODBC/JDBC driver is READ COMMITTED, which maps with read committed in other database systems.Firebird’s other isolation levels do not map so easily.In the ODBC/JDBC driver, SNAPSHOT maps to REPEATABLE READ and SNAPSHOT TABLE STABILITY maps to SERIALIZABLE, with some tweaks.

Since version 2.0, the driver has been able to support every transaction configuration that Firebird can support, including table reservation (“table blocking”).That was achieved by incorporating the so-called “EmbeddedSQL” syntax that is native to the old pre-compiler, gpre, to prepare calls to the ODBC API by the function SQLExecDirect.

Locking

Firebird implements optimistic row-level locking under all conditions.A transaction does not attempt to lock a record until it is ready to post an update operation affecting that record.It can happen, though rarely, for an update to fail because another client has a lock on the record, even if the transaction that fails started before the one which secured the lock.

Firebird’s record versioning engine is able to achieve a granularity finer than that provided by traditional row-level locking.Versioning allows any number of transactions to read a consistent copy of any given record, even if other transactions are updating the same row simultaneously.Readers and writers never block one another and Firebird’s maintenance of record versions is totally transparent to the user.

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Transaction Request Syntax

The syntax for an ODBC-friendly transaction request follows.

SET | DECLARE TRANSACTION [LOCAL] [NAME transaction-name [USING namedUniqueWorkspace]]
[READ WRITE | READ ONLY]
[WAIT | NO WAIT]
[AUTOCOMMIT]
[NO_AUTO_UNDO]
| REPEATABLE READ
| SERIALIZABLE
| READ COMMITTED [[NO] RECORD_VERSION]}]
[RESERVING table-name-1 [, table-name-2[, ...table-name-n] ]
[FOR [SHARED | PROTECTED] {READ | WRITE}] [, ]

What the Options Mean

DECLARE TRANSACTION…​ declares the described transaction, without activating it.SET TRANSACTION…​, on the other hand, activates the transaction, temporarily switching the SQL_ATTR_AUTOCOMMIT global attribute of the ODBC API to SQL_AUTOCOMMIT_OFF.The transaction will have to be finished explicitly;when it ends, the abiding rule of the API resumes.

LOCAL limits a transaction to acting only within the context of the current connection.

NAME transaction-name is a uniquely-named transaction, prepared for use by any connections in the global environment.

USING namedUniqueWorkspace is a uniquely-named transaction workspace in which NAME transaction-name can be set to run by any connections in the global environment.Identically named transactions with differing parameters can run in the same named workspace.

Named Transactions and Transaction Workspaces

The construct DECLARE TRANSACTION …​ NAME transaction-name [USING namedUniqueWorkspace] allows explicit transactions to be configured and saved into the global environment in preparation for repeated use for any connection request or by any active connection.An instance of the saved transaction can be called into action by a specific form of the SET TRANSACTION command:

For a connection request:

SET TRANSACTION NAME MyReadTransaction

or

SET TRANSACTION NAME MyReadTransaction USING MyDsnDb1

for separate requests within a single active connection:

SET TRANSACTION LOCAL NAME MyReadTransaction

or

SET TRANSACTION LOCAL NAME MyReadTransaction USING MyDsnDb1

and, in this connection, for another request:

SET TRANSACTION LOCAL NAME MyWriteTransaction

or

SET TRANSACTION LOCAL NAME MyWriteTransaction USING MyDsnDb1

The form SET TRANSACTION …​ NAME transaction-name [USING namedUniqueWorkspace] differs from earlier implementations whereby the configuration set by the SET command would be repeated for the next transaction.The inclusion of the NAME and/or USING clauses makes the configuration repeatable on demand by use of the name.

Important

A return to the usual mode of operation requires a detach/connect cycle.
Ending Explicit Transactions

In SQL, a transaction is completed by a COMMIT or ROLLBACK request.ODBC has methods that do one or the other, such as SQLEndTran.Some programs are able to invoke SQLExecDirect but cannot call SQLEndTran.For those programs it is necessary to call an explicit

SQLExecDirect( hStmt, "COMMIT" )

to ensure that the interface will call

SQLEndTran( SQL_HANDLE_DBC, hConnection, SQL_COMMIT );
Note

If a transaction is initiated locally, the driver will execute SQLEndTran for the local hStmt.

Two Phase Commit Transactions

The ODBC/JDBC driver supports two-phase commit transactions, that is, a single transaction across different Firebird databases.Up to 16 databases can be accessed simultaneously in one such transaction — that is an absolute limit.

The call to start a two-phase commit transaction is:

SQLSetConnectAttr (connection, 4000, (void*) TRUE, 0);

To cancel the common connection:

SQLSetConnectAttr (connection, 4000, (void*) FALSE, 0);

More Transactions

Firebird ODBC by default uses one transaction per connection.Programmatically you can use a more flexible transaction structure.For example, you can use multiple transactions within one connection, whereby a single connection can be using a number of read/write transactions simultaneously.

An Example
HSTMT stmtRd;
HSTMT stmtWr;
SQLAllocHandle( SQL_HANDLE_STMT, connection, &stmtRd );
SQLAllocHandle( SQL_HANDLE_STMT, connection, &stmtWr );
SQLExecDirect( stmtRd, (UCHAR*)
  "SET TRANSACTION LOCAL\n"
  "READ ONLY\n"
  "ISOLATION LEVEL\n"
  "READ COMMITTED NO RECORD_VERSION WAIT\n",
  SQL_NTS );
SQLExecDirect( stmtWr, (UCHAR*)
  "SET TRANSACTION LOCAL\n"
  "READ WRITE\n"
  "ISOLATION LEVEL\n"
  "READ COMMITTED NO RECORD_VERSION WAIT\n",
  SQL_NTS );
SQLExecDirect( stmtRd,(UCHAR*)
  "SELECT CURRENCY FROM COUNTRY"
  "   WHERE country = 'Canada'"
  "   FOR UPDATE OF CURRENCY",
  SQL_NTS );
SQLFetch( stmtRd );
SQLPrepare( stmtWr, (UCHAR*)
  "update COUNTRY\n"
  "set    CURRENCY = 'CndDlr'\n"
  "where  COUNTRY = 'Canada'\n",
  SQL_NTS );
SQLExecute( stmtWr );
SQLExecDirect( stmtWr, (UCHAR*)"COMMIT", SQL_NTS );

MS DTC Transactions

The Microsoft Distributed Transaction Coordinator (MS DTC) service is a Windows component that is responsible for coordinating transactions that span multiple resource managers, such as database systems, message queues, and file systems.It can perform global, single-phase or two-phase commit transactions involving Microsoft SQL Server, Sybase and other servers that are able to work with it.Our ODBC/JDBC driver provides that capability for Firebird servers.

An Example Using MS DTC
// Include MS DTC specific header files.
//------------------------------------------------------------------------------
#define INITGUID
#include "txdtc.h"
#include "xolehlp.h"

ITransactionDispenser *pTransactionDispenser;
ITransaction *pTransaction;
// Obtain the ITransactionDispenser Interface pointer
// by calling DtcGetTransactionManager()
DtcGetTransactionManager( NULL,// [in] LPTSTR pszHost,
     NULL,// [in] LPTSTR pszTmName,
     IID_ITransactionDispenser,// [in] REFIID rid,
     0,// [in] DWORDdwReserved1,
     0, // [in] WORDwcbReserved2,
     NULL,// [in] void FAR * pvReserved2,
     (void **)&pTransactionDispenser // [out] void** ppvObject
     );
// Establish connection to database on server#1
LogonToDB( &gSrv1 );
// Establish connection to database on server#2
LogonToDB( &gSrv2 );
// Initiate an MS DTC transaction
pTransactionDispenser->BeginTransaction(
     NULL,// [in] IUnknown __RPC_FAR *punkOuter,
     ISOLATIONLEVEL_ISOLATED,// [in] ISOLEVEL isoLevel,
     ISOFLAG_RETAIN_DONTCARE,// [in] ULONG isoFlags,
     NULL,// [in] ITransactionOptions *pOptions
     &pTransaction// [out] ITransaction **ppTransaction
     );
// Enlist each of the data sources in the transaction
SQLSetConnectOption( gSrv1->hdbc, SQL_COPT_SS_ENLIST_IN_DTC, (UDWORD)pTransaction );
SQLSetConnectOption( gSrv2->hdbc, SQL_COPT_SS_ENLIST_IN_DTC, (UDWORD)pTransaction );
// Generate the SQL statement to execute on each of the databases
sprintf( SqlStatement,
  "update authors set address = '%s_%d' where au_id = '%s'",
   gNewAddress, i, gAuthorID );
// Perform updates on both of the DBs participating in the transaction
ExecuteStatement( &gSrv1, SqlStatement );
ExecuteStatement( &gSrv2, SqlStatement );
// Commit the transaction
hr = pTransaction->Commit( 0, 0, 0 );
// or roll back the transaction
//hr = pTransaction->Abort( 0, 0, 0 );

Password Security

When a DSN is created with the username and password in place, the database password is encrypted and is saved in odbc.ini.Alternatively, the login credentials can be entered during the database connection phase or can be passed using the connection string.

Cursors

In the current Firebird ODBC/JDBC driver, the Dynamic and Keyset cursors are modified to use the Static cursor, through which it is not possible to update sets.

For best performance, use the cursor ForwardOnly.

The read operators SQLFetch, SQLExtendedFetch and SQLScrollFetch use SQL_ROWSET_SIZE and SQL_ATTR_ROW_ARRAY_SIZE.

For best performance using BLOB fields, use the operator SQLBindParameter, regardless of the size of the BLOB field, as this will work much faster than using SQLPutData/SQLGetData.

To use the Firebird driver’s cursors, call the following statements:

// Specify that the Firebird ODBC Cursor is always used, then connect.
SQLSetConnectAttr( hdbc, SQL_ATTR_ODBC_CURSORS, (SQLPOINTER)SQL_CUR_USE_DRIVER, 0 );
SQLConnect( hdbc, (UCHAR*)connectString, SQL_NTS, NULL, 0, NULL, 0 );

ODBC Cursor Library

This topic is well documented in MSDN.However, we must stress the absolute requirement to use these statements before connecting:

// Specify that the ODBC Cursor Library is always used, then connect.
SQLSetConnectAttr( hdbc, SQL_ATTR_ODBC_CURSORS, (SQLPOINTER)SQL_CUR_USE_ODBC, 0 );
SQLConnect( hdbc, (UCHAR*)connectString, SQL_NTS, NULL, 0, NULL, 0 );

That data sets keys (?) in the rowset buffers.Updating the buffers requires this statement:

SQLFetchScroll( hstmtSel, SQL_FETCH_RELATIVE, 0 );

Stored Procedures

In Firebird, we can have two types of stored procedures, known as executable and selectable.Both types can take input parameters and return output, but they differ both in the way they are written and in the mechanism for calling them.

  • Output from an executable procedure is optional and any output returned is a set of not more than one “row” of values.If output is defined and none is produced, the output is null.

    Returning data is not the primary goal of an executable procedure.Its purpose is to perform data operations that are invisible to the user.

    The mechanism for calling an executable procedure is the SQL statement EXECUTE PROCEDURE.For example,

    execute procedure MyProc(?,?)

  • A selectable procedure is written with the objective of returning a set of zero, one or many rows of data.It can be used to change data, but it should not be written to do that.The PSQL statement SUSPEND is used in this style of procedure to pass a row of output that has been collected inside an iteration of a FOR SELECT.. loop out to a buffer.

    The mechanism for calling a selectable procedure is the SQL statement SELECT.

    In this example we have a selectable procedure from which we expect to receive a set of zero or more rows based on the input parameters:

    select * from MyProc(?,?)

Microsoft Excel and some other applications use this statement to call a stored procedure:

{[? =] Call MyProc (?,?)}.

The Firebird ODBC/JDBC driver determines what call to use when executing a stored procedure, from the metadata obtained from the Firebird engine.Firebird flags a procedure as ‘executable’ or ‘selectable’ according to count of SUSPEND statements in the assembled (BLR) code of its definition.For a trivial example:

create procedure TEST
  as
    begin
    end

Because the procedure has no SUSPEND statements, the ODBC driver knows to pass the call as execute procedure TEST.

For this procedure:

create procedure "ALL_LANGS"
   returns ("CODE" varchar(5),
         "GRADE" varchar(5),
         "COUNTRY" varchar(15),
         "LANG" varchar(15))
   as
   BEGIN
     "LANG" = null;
     FOR SELECT job_code, job_grade, job_country FROM job
     INTO :code, :grade, :country
     DO
       BEGIN
         FOR SELECT languages FROM show_langs(:code, :grade, :country)
         INTO :lang
           DO
             SUSPEND;
             /* Put nice separators between rows */
             code = '=====';
             grade = '=====';
             country = '===============';
             lang = '==============';
             SUSPEND;
       END
     END

the BLR code for the stored procedure contains more than zero SUSPEND statements, so the ODBC Driver will use select * from "ALL_LANGS".

ARRAY Data Type

To modify single dimension array data type fields, you need to conform to the following rules:

  • Specify simple types (INTEGER, etc.) as {1, 2, 3}

  • Specify string types (CHAR, etc.) as {'1', '2', '3'}

Warning
TRAPS!

If you edit an element of the array e.g. element 1, 2 and 5, and do not specify the other elements of the array, e.g. 3 and 4, then the other elements of the array will be zeroed (integer), or blank (string).

With some programs where columns are dependent on array data, it is possible to enter array data into a currently NULL array column without a validity check being made on the various array elements.Under these circumstances it is essential to enter the array elements before entering the column data.
fb odbc ModArrayField
Figure 1. Data loss when updating an ARRAY field (1)
fb odbc ruleModArrayField
Figure 2. Data loss when updating an ARRAY field (2)

Usage with Clarion

Jorge Brugger; Vernon Godwin; Vladimir Tsvigun

Clarion users can work with mixed-case object names in Firebird.

  1. Create your database in Firebird.You can use table names like "Pending_Invoices" and fields like "Order_Number".

  2. Create the DSN for the Database, making sure to check all options in “Extended Identifier Properties”

  3. Open your dictionary, and import multiple tables as normal from the odbc source.It will work, but do not try to browse or use the files in an application yet.

  4. For every field, type in the “External Name” the name of the field surrounded by quotes (for example, type "Order_Number" in the external name).

That’s it!Now use your dictionary with mixed case identifiers, without problems.But remember — you must use double quotes around object names in all SQL statements from inside Clarion.