To prevent the data in a cache from becoming out of date, the driver must periodically refresh the cache data with data from the remote data source. To minimize the amount of data that needs to be moved when a cache is refreshed, and therefore the time required to refresh it, the driver checks to see which records in the remote table have been added, modified, or deleted since the last time the cache was refreshed. The driver retrieves only data for added or modified records and removes only deleted records from the cache. You or the application can refresh the cache manually or the driver can refresh the cache automatically.

You can refresh a cache manually at any time by using the Refresh Cache statement (refer to "Refresh Cache (EXT)" in the DataDirect Connect Series for ODBC Reference). The Refresh Cache statement can also be used to perform a Clean (complete) refresh in addition to the standard optimized refresh. A Clean refresh discards all of the data from the cache and repopulates it with data from the remote data source.

The driver can refresh a cache automatically in one of two ways. When you create a cache, one of the attributes that you set is the refresh interval for the cache. During each cache query, the driver checks to see whether the time elapsed since the last refresh exceeds the refresh interval for the cache. If it has, the driver refreshes the cache before satisfying the query.

Update operations to a table that is cached can trigger the driver to refresh the cache automatically. The caches maintained by the Salesforce driver are write-through caches. For any operation that modifies data in a table that is cached, the driver performs the operation on the remote data first and then updates the cache as much as possible. The driver may not be able to update the cache with all of the modifications because some of the modified data may have been generated by the remote data source. For example, if a row is inserted but a value for all columns in the row is not required, any default values generated by the remote data source for columns not specified in the Insert statement would not be set in the cache. Because the driver cannot reflect all of the changes made when a cached table is modified, it sets the cache state to dirty. When a cache state is dirty, the next query that attempts to fetch data from that cache causes the driver to refresh the cache before the fetch operation is performed. This allows the fetch to see the values populated by the remote data source.

Automatically refreshing a dirty cache is not always desirable. For example, if an application alternates fetches and inserts on a table, and the insert does not depend on any remote data source generated values, then the refresh between fetches is unnecessary. The RefreshDirtyCache (RDC) connection option (see Connection Option Descriptions) controls whether the driver automatically refreshes a cache with a dirty state. The state of a cache can be viewed by selecting the STATUS column of the SYSTEM_CACHES catalog table. See SYSTEM_CACHES Catalog Table for more information.