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