Store-through Cache System

In a store-through cache system, changed and unchanged data items might be handled as follows:

When writing a new data item that is identical to the copy on permanent storage, code CHANGED=NO. You can also code CROSSINVAL=NO and GETCOLOCK=NO, or omit these keywords and use the system defaults.
When writing a data item that you have read from permanent storage or the cache structure and updated, or when writing an updated data item back to the cache structure:
- Code CHANGED=NO to mark the data item as unchanged. Remember, in a store-through cache system you intend to immediately write the data item to permanent storage. You can mark the data item as unchanged to indicate to the system that the data item resources are eligible for reclaim.
- Code CROSSINVAL=YES to cause the system to invalidate copies of the data item that might be in local cache buffers of other users.

When you write unchanged data items to the cache structure in the store-through cache system, you can also code GETCOLOCK=YES to obtain the cast-out lock for the data item. Obtaining the cast-out lock serializes the update to permanent storage. If another user attempts to obtain the same cast-out lock, that user's request fails. Whether or not you serialize your permanent updates to storage by obtaining the cast-out lock depends on your protocol.