Using a Client Dataset to Cache Updates
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By default, when you edit data in most datasets, every time you delete or post a record, the dataset generates a transaction, deletes or writes that record to the database server, and commits the transaction. If there is a problem writing changes to the database, your application is notified immediately: the dataset raises an exception when you post the record.
If your dataset uses a remote database server, this approach can degrade performance due to network traffic between your application and the server every time you move to a new record after editing the current record. To minimize the network traffic, you may want to cache updates locally. When you cache updates, you application retrieves data from the database, caches and edits it locally, and then applies the cached updates to the database in a single transaction. When you cache updates, changes to a dataset (such as posting changes or deleting records) are stored locally instead of being written directly to the dataset's underlying table. When changes are complete, your application calls a method that writes the cached changes to the database and clears the cache.
Caching updates can minimize transaction times and reduce network traffic. However, cached data is local to your application and is not under transaction control. This means that while you are working on your local, in-memory, copy of the data, other applications can be changing the data in the underlying database table. They also can't see any changes you make until you apply the cached updates. Because of this, cached updates may not be appropriate for applications that work with volatile data, as you may create or encounter too many conflicts when trying to merge your changes into the database.
Although the BDE and ADO provide alternate mechanisms for caching updates, using a client dataset for caching updates has several advantages:
- Applying updates when datasets are linked in master/detail relationships is handled for you. This ensures that updates to multiple linked datasets are applied in the correct order.
- Client datasets give you the maximum of control over the update process. You can set properties to influence the SQL that is generated for updating records, specify the table to use when updating records from a multi-table join, or even apply updates manually from a BeforeUpdateRecord event handler.
- When errors occur applying cached updates to the database server, only client datasets (and dataset providers) provide you with information about the current record value on the database server in addition to the original (unedited) value from your dataset and the new (edited) value of the update that failed.
- Client datasets let you specify the number of update errors you want to tolerate before the entire update is rolled back.
The following topics describe in more detail on how to use a client dataset to cache updates:
- Overview of using cached updates.
- Choosing the type of dataset for caching updates.
- Indicating what records are modified.
- Updating records.