Interviewer - Server > Architecture > Interview Service tier > Value Cache > Using the Value Cache and the Transfer Service
 
Using the Value Cache and the Transfer Service
There are three things to consider when deciding when to transfer data from the Value Cache:
Real time reporting. When up-to-the-minute results are a priority, it is best to transfer data to the database as each question is answered.
Fault tolerance. If never losing data is a priority, it is best to transfer data to the database as each question is answered.
SQL load. If reducing the load on the database server is a priority, it is best to transfer data to the database on a scheduled basis (not yet possible) or when an interview completes.
The default set-up transfers data from the Value Cache to the case data database whenever an interview completes. Each project also defaults to having the AutoUpdateDataModel property set to 1 so that data is copied into the database for each submitted page. You must have AutoUpdateDataModel set to 1 if you want to allow stopped or timed out interviews to be restarted from a URL (that is, using sample management). This set-up provides the greatest fault tolerance, as follows:
If data cannot be written to the case data database:
Data continues to be written to the shared cache.
Once the interview completes, the data is automatically transferred when the database becomes available again.
Manual recovery is required for partially completed interviews.
If the shared cache fails:
Data, but not temporary variables, continues to be written to database.
Interviews that fail over to a different interview server can still be restarted directly from the database.
Restarts will be slower as the interview is replayed.
If both the database and the shared cache fail
Data is written to the local cache.
Interviews that fail over to a different interview server cannot be restarted.
Once the interview completes, the data is automatically transferred when the database becomes available again.
Manual recovery required for partially completed interviews.
If the local cache fails:
It is likely that local disk subsystem has also failed.
The session engine for that computer is then made out of service.
See also
Value Cache