Microsoft KB Archive/111888

= FIX: Inserts May Fail and Updates May Not Be Seen =

Article ID: 111888

Article Last Modified on 3/25/2000

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APPLIES TO


 * Microsoft Open Database Connectivity 1.0

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This article was previously published under Q111888



1.01.1928 1.01.2115

WINDOWS

kbusage kbbug1.01.1928 kbbug1.01.2115 kbfix1.02.1403



BUG# ODBCDBASE: 1915 (1.01.1928)



SYMPTOMS
Under certain multi-user situations, applications using the Microsoft ODBC Access driver versions 1.01.1928 and 1.01.2115 may encounter the following two problems:  Read locks are retained longer than necessary. As a result of this, when an application that executes a SELECT statement on a table such that the WHERE clause contains a column with a unique index on it, other applications will not be able to insert, delete, or update that table. Attempts to do so will cause the following error:

Commit Lock failed due to outstanding read lock

Locks will not be released until the application that did the SELECT calls SQLDisconnect. Applications may be unable to "see" updates (row insertions, deletions, and updates) made by other applications. This occurs under the following circumstances:

Application A executes a SELECT statement on a table and fetches all the rows; then, application B updates (insert, delete or update) the same table. Now if A re-executes the same SELECT statement and fetches all the rows, it will see the exact same rows that it saw before; it does not see the updates made by B. For A to be able to see the updates, it needs to call SQLDisconnect on the hdbc on which the original SELECT was performed.

The ODBC Access driver is based on a sub component of the Access database engine. This Access database engine implements a performance enhancing caching scheme, as rows are retrieved from the database or updated by an application they are cached. In most cases, this permits rows to be re-retrieved without having to re-read from the actual database file, thereby, enhancing performance.

Locks held on the actual rows of the database from which they were retrieved are not released until the corresponding cached rows are flushed from the cache. Since locked rows are not updated due to reasons of integrity, it follows that updates are not initiated until the cache is flushed. The database engine caching scheme relies on the application that is using the engine to initiate the process of flushing the cache. This permits the application to "time" the flushing of the cache to correspond with the operations the application is performing at the time.

For example, the Access product initiates the flushing process during user "think time" to maximize the responsiveness of the user interface. In the Access ODBC driver, the driver itself initiates the flushing process, since relying on the application to do this would reduce the interoperability of applications. If the driver relied on the application to initiate this process, it would require applications to write special code to use the Access ODBC driver. This is contrary to the goal of ODBC.

In Access ODBC driver versions 1.01.1928 and 1.01.2115, this flushing process was initiated when SQLDisconnect was called by the application. This proved to be inadequate for some multi-user scenarios and caused the problems described above. A recent "hotfix" version of the driver (version 1.02.1403) initiates the flushing process at the following times:
 * 1) SQLFreeStmt with the SQL_DROP option
 * 2) SQLFreeStmt with the SQL_CLOSE option
 * 3) SQLDisconnect

This causes cached rows to be flushed more often. It also means that the database file will be read more frequently, with the possibility of some performance degradation. However, it solves the problem of locks being held for longer than is necessary. It also solves the problem of visibility of updates in many multi-user scenarios.

In a few scenarios, it is possible that locking conflicts will still be encountered and updates will not be visible across applications, unless it is dropped and re-establish the connection. In particular, an application that executes a SELECT statement on a table such that the WHERE clause contains a column with a unique index on it, will not be able to see updates made to the table by other applications, until it drops and re-establishes the connection.

For the hotfix to work properly, the following entries should be inserted in the [Red ISAM] section of ODBCISAM.INI file: IdleFrequency=1 PageTimeout=1 LockedPageTimeout=1 This minimizes the chance of encountering the "updates-not-visible" problem. The ODBCISAM.INI file is usually located in the \WINDOWS directory.

You will also note that the Access product does not, in general, exhibit this same behavior. The reason is the mechanism used to flush cached rows is somewhat different in the Access product which includes the complete Access database engine, than in the Access ODBC driver which uses a sub component of that engine. The complete Access database engine, enables an application to detect when it has successfully flushed all rows from the cache. Therefore, to ensure concurrency and the visibility of updates, Access flushes the cache until all rows have been forced out. The Access ODBC driver, using the sub component of the Access engine, can flush rows from the cache, but is unable to tell when all rows have been forced out. Therefore, it cannot guarantee that locking conflicts and the visibility of updates is resolved for every scenario.

Version 2.0 of the ODBC Driver Pack will be based on the complete Access database engine and will exhibit the same capability in this area as the Access product. Driver Pack version 2.0 is currently scheduled for release 2H 1994.

It should also be noted that even in the Access product, applications written in Access BASIC, can encounter substantially the same lock conflict scenario as outlined above. The way that locking conflicts are dealt with in this case, for example in Access BASIC applications, is for the application to retry the retrieval when such a conflict is encountered. Applications written to the ODBC Access driver can employ the same strategy to resolve such conflicts.



STATUS
Microsoft has confirmed this to be a problem in the Access Driver versions 1.01.1928 and 1.01.2115. This problem has been fixed in Access driver version 1.02.1403. For more information, please contact your primary support provider.

Additional query words: 1.01.1928 DDD VISUAL C++ 1.5 Word Excel

Keywords: kbbug kbfix KB111888

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