Configuring the Cache of a View¶
Before enabling the cache in a specific view, you have to enable the cache module in the Server (see section Configuring the Cache).
To enable the cache of a view, open the view, click Options and select one of the following cache modes:
Partial. When a query is executed against the view, the Server checks if the cache contains the data required to answer the query. If not, it queries the data source directly.
Full. The Server assumes that the cache contains all the tuples of the view. Therefore, queries on this view will always use the cached data and will never “hit” the data source. The data source will be only accessed for refreshing cache data. The cache is always loaded explicitly, meaning that you need to execute query to load the cache.
Full incremental. With this mode, the queries to this view will be “incremental queries”. That is, the queries to this view merge the results obtained from the cache with the most recent data retrieved from the source. The main benefit of this mode is that the queries will always return fully up to date results without needing to retrieve the full result set from the data source, just the rows that are added/changed since the last cache refresh.
To be able to use this mode, the view has to have a field that contains when a row was last inserted/updated. For example, if the view has a field
last_modified_date, you have to enter a condition like
last_modified_date > '@LASTCACHEREFRESH'.
When executing the query, the variable
@LASTCACHEREFRESHis replaced with the timestamp of the last time a query to load the cache was executed and finished successfully.
As it is a variant of the full mode, unlike with partial mode, the cache of the view still needs to be loaded explicitly (i.e. you have to execute a query with the appropriate parameters to load the cache).
The incremental mode has several requirements and limitations that you need to be aware of. The section Incremental Mode lists them.
The section Cache Modes explains in detail how these cache modes work.
Time to Live (seconds) is the Time to live in seconds of the entries stored in the view’s cache. Once this limit is reached, these entries expire and the next query will retrieve the data from the data source, instead of from the cache. If you select Never expire, the Server will never invalidate the entries stored in the cache. However, you still can invalidate the cache manually by clicking on Invalidate cache.
The Default time to live is the Time to live defined for the database in the “Cache configuration” dialog of the database (Administration > Database Management dialog). If the database does not set the “Time to live”, the cache module uses the global value of this property, which is defined in the “Cache” tab of the Server Configuration dialog.
The rows obtained from the source are inserted in cache in batches. The Batch insert size (rows) determines the number of rows per batch. The Default value is the Batch size defined for the database in the “Cache configuration” dialog of the database (Administration > Database Management dialog). If the database does not set the “Batch size”, the cache module uses the global value of this property, which is defined in the “Cache” tab of the Server Configuration dialog.
The entries of the cache of a view expire when they reach their Time to live. However, you can invalidate these entries manually. To do it, click on Invalidate cache and select the following options:
Invalidate all. Remove all the cache content for the view.
Invalidate partially. Remove only the entries that match the condition in the Edit condition dialog.
Invalidate on cascade. If selected, the cache module also invalidates the cache of the subviews of this view. This option is disabled for views with “Full” cache mode. In addition, if you invalidate on cascade a view, the cache of the subviews whose cache mode is “Full” will not be invalidated.
See the section Invalidate Cache for information about invalidating the cache content of more than one view at a time.
When enabling the cache for a view, the Cache Module creates a table in the cache’s database that will store the cached data of this view. You can define “Cache indexes” for this view to speed up the retrieval of data from the view’s cache.
The difference between the cache indexes and the indexes defined in the “Indexes” tab is that the latter ones represent the indexes defined in the source. The cache module actually creates the “Cache indexes” in the cache’s database, when they are defined in this tab.
When the cache is enabled for a view and its cache mode is “Full”, the indexes propagated to its derived views and to the ODBC and JDBC clients, are the “Cache indexes” and not the indexes defined in the “Indexes” tab. That is because the queries of the view will always retrieve the data from the cache and not from the source.
To create a cache index, follow these steps:
If the view has indexes (see the “Indexes” tab), you can replicate them in the table that will store the view’s cached data, by clicking on Import indexes from view.
Note that the indexes will not be created until you click Ok to close the dialog.
To define more indexes in the cache table, click Create index and follow these steps:
Enter the name of the index.
Depending on the database you are using to store the cache, the name of the index may need to be unique across the entire Virtual DataPort database and not just the view you are editing. That is because some databases do not allow defining two indexes with the same name, even if they are defined over different tables.
Select Is unique if the index represents a unicity index.
In the left side of the dialog, there is a list of the fields of the view. Select the ones that belong to the index and click Add >>. To select several fields at once, hold Ctrl or Shift and click on the fields you want to select.
To change the order of the fields in the index, select the check box of the field you want to move and click on and .
To remove a field from the index, select its check box in the list of the right side and click << Remove.
Click Ok to create the index.
To edit a cache index, select its check box and click Edit selected index.
To remove a cache index, select its check box and click Remove selected index.