If the network connection is broken, the user will lose the transaction, and the application will halt. For applications in retail or banking, this can mean an expensive break in trading:
Resilience is the mechanism used to ensure non-stop working of your application. To accomplish this, two or more copies of your database are maintained at the same time. If the network connection to one of the databases is broken, operation continues with only one database.
A resilience scheme requires either the presence of at least two servers, or keeping a local database on each workstation on your system. DP4 offers you the flexibility to decide on a scheme which best meets your requirements:
Your scheme can make use of the standalone capabilities of the workstations, or you can use a dedicated server machine
The workstations may be standard PCs or POS terminals and you can mix workstations with standalone capacity with terminals that have no standalone capacity
There are various possible schemes for automatically maintaining "duplex" databases, that are intended to contain exactly the same data. However, in practice, it is most effective to use collector programs to retrieve transaction data from workstations' local databases. These allow a meaure of load balancing, and help to eliminate the possibility of contention errors from the database, by delegating tasks such as maintaining stock totals (which is especially likely to cause contention), to a single process.
You can combine resilience with the distributed database feature, so that the more commonly accessed tables are read from the local database on the workstation to ensure better performance
You can decide on your own scheme, or you can use one of the several pre-defined resilience schemes offered by DP4.