Important Terms

The following is a list of terms that you need to understand. These terms describe basic concepts important to the understanding of the cache structure and cache services.

Table 1. Terms for Caching
Term	Definition
cast out/casting out	Process of writing changed data that is in the cache structure to permanent storage. Casting out is implemented through the association of data items with cast-out classes.
cast-out class	Class assigned to a data item used with cast-out processing. Users of the store-in method of caching must assign data items in the cache structure to cast-out classes. Cast-out class assignments simplify the cast-out process by grouping data items together with similar characteristics. Users must also develop their own cast-out algorithms that make use of these cast-out classes when they cast out data.
cast-out lock	Lock used with a data item for cast-out processing. The user must obtain the data item's cast-out lock to serialize the update to permanent storage. When the cast-out lock is held for a data item, the data item is said to be locked for cast-out. When a data item is locked for cast-out, the cast-out lock (composed of the connection identifier of the holder of the cast-out lock and, optionally, the process identifier of the task or process that holds the lock) is part of the directory entry for the data item. Any user can still make updates to the data item even when the data item is locked for cast out.
changed data (changed data item)	A data item in the cache structure that is an updated version of the same data item on permanent storage. When a user updates the copy of a data item in the local cache buffer and then writes the updated data to the cache structure, the data item is considered changed data. If a user has written to the cache structure but has not yet cast out the data to permanent storage, the data in the cache structure is said to be changed. A data item that is locked for cast-out processing is also considered changed until the update is made to permanent storage and the cast-out lock is released. An unchanged data item is a data item in the cache structure that is the same as the version on permanent storage.
data item	A single unit of information that is referred to by a single name in local cache buffers, the cache structure, and on permanent storage. If a data item is in the cache structure, it is contained in a data entry. A user will keep a copy of a data item in a local cache buffer. Wherever copies of the same data item exist, that data item is referred to by a single name.
deregistration/deregistering interest	A way to indicate to users information about the validity of a data item. Users with registered interest in a data item can have their interest deregistered if the data item has changed and the local copy of the data is no longer valid. When a shared data item is updated, the system indicates to interested users, through the users' associated local cache vector entry, that the data item has been changed. The copy of the data item in users' local cache buffer is then considered not valid. This process is also referred to as invalidation of local cache copies of data items.
directory-only cache	A cache structure that contains directory entries but not data items. Contrast with store-in and store-through cache. See Directory-only Cache.
invalidation	See deregistration/deregistering interest.
reclaim	The management of resources in the cache structure. When a user writes a data item to the cache structure and a resource like a directory entry or data entry is unavailable, the system attempts to reclaim an existing directory entry or data entry to satisfy the request. Not all resources are available for reclaim. For example, a data entry containing changed data cannot be reclaimed. Reclaim is implemented through the association of data items with storage classes. Users can define a reclaim vector and use IXLCACHE to control reclaim processing. Otherwise, a system default for reclaim is in effect.
registration/registering interest	A way to indicate to users information about the validity of a data item. Users that use the cache structure can register interest in a data item. When a user registers interest in a data item, an association is formed between the local cache vector entry associated with the user's local copy of the data item and the directory entry for the data item in the cache structure. When interest has been registered, the system uses the local cache vector entry to indicate the validity or invalidity of the data in the user's local cache buffer. If a user has registered interest in a data item, the copy of that data item in the user's local cache buffer is considered valid.
storage class	Class assigned to a data item in the cache structure used in the reclaim process. Each data item that is defined to the cache structure (either through a directory-only cache structure or a cache structure that contains both directory entries and data entries) must be assigned to a storage class. Storage class assignments simplify the reclamation of resources by grouping together data items with similar characteristics.
store-in cache	A cache structure in which data items are stored in data entries. Users of a store-in cache write changed data to the cache structure but not to permanent storage at the same time. Users perform an independent cast-out process after the updates have been made and then make the changes to permanent storage. Contrast with store-through cache and directory-only cache. See Store-in Cache.
store-through cache	A cache structure in which data items are stored in data entries. Users of a store-through cache write changed data to the cache structure and to permanent storage at the same time, that is, under the same serialization. Contrast with store-in cache and directory-only cache. See Store-through Cache.
valid data	The state of data in a user's local cache buffer. If a user's copy of a data item is valid, the copy contains the latest changes. If a data item copy is not valid, it does not reflect the latest changes. See also registration/registering interest.
validation	See registration/registering interest.