Tuesday, October 11, 2016

When Migrating to FireDAC, You Must Understand UpdateMode

Let's start with a little history lesson. When Delphi first shipped it had one data access framework, the Borland Database Engine (BDE). While the BDE was a breakthrough product in its early years, providing a fast, independent, and easy-to-use data access layer, it was cumbersome to install, used a lot of network bandwidth, and had limited support for remote database servers. Over time, it became increasingly obsolete.

The need for a new data access mechanism for Delphi became even more apparent during the development of Kylix, a Delphi-based compiler and IDE (integrated development environment) for Linux. Porting the BDE to Linux was ruled out, and dbExpress was born. The dbExpress data access framework is high-speed and client/server friendly, largely based on pass-through SQL (structured query language).

The dbExpress framework has one major drawback, however. In most cases converting a BDE project to dbExpress required a major refactoring of the data access logic, and dbExpress did not support the old-style file server databases, such as Paradox, dBase, or MS Access until Delphi XE2. As a result, for most many developers dbExpress was a poor option.

FireDAC changes that. Conversion from the BDE to FireDAC is pretty smooth, and Delphi even ships with a tool, named reFind, that helps with much of the conversion process. It is for this reason that I use FireDAC in all of my new projects (and all of the database projects included in my source code for Delphi Developer Days). In addition, I think that a good argument can be made for migrating legacy applications to use FireDAC when a major revision is scheduled. Yes, it is that good.

One Big Difference: UpdateOptions.UpdateMode

One reason why migration to FireDAC from the BDE is so easy is that FireDAC provides a solid implementation of the TDataSet Interface, and all of the basic operations available in the BDE can be found in FireDAC. Better still, FireDAC implements many of the higher-end features previously only found in ClientDataSets. As a result, if you know the BDE and/or ClientDataSets, using FireDAC is a no brainer.

There is one very significant difference between how FireDAC and the BDE/ClientDataSets work, and this difference is very easy to miss, in part because it is related to the default value of a single property. And because the effects of this property have such a profound impact on how your database applications work, it is important that you know about this difference when you decide to start using FireDAC. The property is UpdateMode.

UpdateMode is a property found in the UpdateOptions property of FireDAC TDataSets. It is also a property of BDE TDataSets as well as the TDataSetProvider when used to apply updates to a caching ClientDataSet. In all of these mentioned classes the UpdateMode property performs the same task, it influences how the corresponding component generates the SQL used to write changes to the underlying database.

The UpdateMode property is of the type TUpdateMode. The following is the declaration of the TUpdateMode type:

TUpdateMode = (upWhereAll, upWhereChanged, upWhereKeyOnly);

Here is how it works. When UpdateMode is set to upWhereAll, all non-BLOB fields are included in the WHERE clause of UPDATE and DELETE queries. This results in update failures if any of the non-BLOB fields of the underlying record were modified since the time that the table was opened, the query or stored procedure was executed, or the FDMemTable was loaded. This approach is known as optimistic locking, and when two or more users are trying to apply changes at about the same time to the same record, only the first to apply changes will succeed. All others will fail.

When UpdateMode is set to upWhereChanged, only the primary field keys and the fields that have been modified are included in the WHERE clause of UPDATE queries. (Again, INSERTs are not affected. DELETE queries continue to use an exact match criteria since there are no changed fields in a deleted record.) As long as none of the primary key fields of an updated record are affected, and all non-primary key fields that have been modified have also not been updated in the underlying table since the time the data was loaded into the DataSet, these queries should succeed.

Using upWhereChanged permits two or more users to apply their changes to the underlying database so long as none of them have made changes to common fields. For example, if one user changed a field called Address, and another changed a field called PhoneNumber, and neither of these fields are part of the primary key, both users will successfully apply their changes. This type of update permits merges.

The final UpdateMode value is upWhereKeyOnly. With UpdateMode set to upWhereKeyOnly, the WHERE clause of UPDATE queries only includes the values of the primary key fields. (INSERT and DELETE queries continue to act as they do with upWhereChanged.) Using this mode, so long as the key fields of the underlying record have not been changed, the updates are applied, replacing any updates that other users may have applied.

Assuming that key fields are not touched (this is a pretty safe assumption in most database architectures), the use of upWhereKeyOnly permits everyone to succeed in posting their changes. As a result, the last user to post is the user whose data appears in the underlying database.

The Difference and Why It Is Important

The default value of FireDAC's UpdateOptions.UpdateMode is upWhereKeyOnly, and this is a really big deal. It means that if you use FireDAC datasets without changing this default value, there is a good chance that some users may report that their changes have disappeared. Or, even worse, never know that their edits have been overwritten.

Here is how this can happen. If two or more users read the same record (by way of a query, stored procedure call, or by opening an FDTable), and two or more post a change to that record, the last user to post their record will replace those changes posted before them. What's problematic about this is that the users who posted before the last user will have no idea that their changes have been overwritten.

By comparison, most developers prefer to use either pessimistic locking (the first user to start editing the record prevents any other user from editing the record until changes have been posted), or optimistic locking (once the first user posts a change to a record, subsequent attempts to post to that same record will fail, since the original record no longer can be found, based on the WHERE clause predicates). In these scenarios, the first user to post wins, and other users must first re-read the record, after which they can decide whether or not to update the newly posted contents.

FireDAC defaults to an UpdateMode of upWhereKeyOnly, since the queries required to update the database tend to execute faster. It is up to you, however, to decide whether or not the performance improvement is more important that the possible loss of data.

The DataSetProvider class, the class which ClientDataSets use to resolve changes back to the underlying database, and BDE DataSets (TTable, TQuery, and TStoredProc) also have an UpdateMode property. For these objects, the default value of UpdateMode is upWhereAll, the conservative setting that prevents a user from overwriting another user's edits.

So, the bottom line is this. You need to understand how FireDAC's UpdateOptions.UpdateMode affects how records are updated in the underlying database, and set this property to the value that meets the needs of your application.

An Easy Solution, If You Want to Change UpdateMode

Fortunately, another feature of FireDAC makes it easy to change the default value of UpdateOptions.UpdateMode property. The UpdateOptions property, like a number of other properties in FireDAC, is found in a number of classes in the FireDAC framework, including FDManager, FDConnection, FDQuery, FDMemTable, and so on.

These classes are related in a hierarchical sense, in that the settings found in the UpdateOptions property of the FDManager (a singleton object) are inherited by all FDConnections configured to use the FDManager. Similarly, any FireDAC dataset, such as an FDQuery or FDMemTable, will employ the settings found in the FDConnection to which they are connected.

So, if your application is using one FDConnection, changing the UpdateOptions.UpdateMode property of that FDConnection will automatically apply that setting to any FDTable, FDQuery, FDStoredProc using that connection (so long as you have not already touched the UpdateOptions.UpdateMode property on any of these TDataSets). If you are using more than one FDConnection, add an FDManager and change the value of the UpdateOptions.UpdateMode property there, and that change will likewise cascade down to any FDConnections, and then on down to the individual TDataSets, again, so long as that property has not also been overridden in any of the individual FDConnections or TDataSets.

Note that if your FDQuery, FDStoredProc, or FDTable is already connected to an FDConnection, and that FDConnection currently uses upWhereKeyOnly, changing the FDConnection UpdateOptions.UpdateMode property alone might not change the FireDAC DataSet. You might have to disconnected, and then re-connect the DataSet to the FDConnection before the DataSet's UpdateMode property updates.

Copyright © 2016 Cary Jensen. All Right Reserved

3 comments:

  1. Thanks, Cary. Excellent post! Very much appreciated.

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  2. Thanks, Your post is excellent. I'm coming back to Delphi after several projects in Java and its great to see how maturity is Delphi.

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