WebSphere eXtreme Scale Product Overview > Cache concepts > Database integration > Side cache and in-line cache > In-line caching scenarios
Loaders
A Loader is a plug-in that works as a link between a BackingMap and a back-end such as a database.
The Loader is invoked when the cache is unable to satisfy a request for a key, providing read-through capability and lazy-population of the cache. A loader also allows updates to the database when cache values change. All changes within a transaction are grouped together to allow the numbe rof database interactions to be minimized. A TransactionCallback plug-in is used in conjunction with the loader to trigger the demarcation of the backend transaction. Using this plug-in is important when multiple maps are included in a single transaction or when transaction data is flushed to the cache without committing.
The loader can also use overqualified updates to avoid keeping database locks. By storing a version attribute in the cache value, the loader can see the before and after image of the value as it is updated in the cache. This value can then be used when updating the database or back end to verify that the data has not been updated. A Loader can also be configured to preload the grid when it is started. When partitioned, a Loader instance is associated with each partition. If the "Company" Map has ten partitions, there are ten Loader instances, one per primary partition. When the primary shard for the Map is activated, the preloadMap method for the loader is invoked synchronously or asynchronously which allows loading the map partition with data from the back-end to occur automatically. When invoked synchronously, all client transactions are blocked, preventing inconsistent access to the grid. Alternatively, a client preloader can be used to load the entire grid.
For more information about loaders, see Loaders.
- Data pre-loading and warm-up
In many scenarios that incorporate the use of a loader, you can prepare the grid by pre-loading it with data.
Parent topic
In-line caching scenarios