Let’s examine an example, entering an invoice into an accounting system. When we create an invoice, several data files may be updated. The customer master file may have a field for each customer that keeps the current balance. This balance must be increased if we issue an invoice. There is an invoice file that has a record added to it. There may be a separate invoice detail file, keeping a record of each line on the invoice. In addition, each invoice detail will affect the balance of an inventory item. All of these files must be in sync - the customer balance must match the total of the open invoices, the detail lines must all be present, and the inventory balances must be accurate as affected by the invoices and other activities.
What happens if the computer crashes while these files are being updated? You could have some of the files already updated, but others not. For instance, the customer and invoice files could be updated, but the inventory records may not. Depending on the way the operating environment works, those records that already exist (the customer and inventory records) could be updated, but the records that are to be added to the file (the invoice and invoice details) may not be saved yet. In any case, you now have files that are not synchronous. Adjusting the files to bring them back into balance can be difficult, as you may not be sure which files have been updated and which have not. Often your only choice is to go to a backup copy of the files, where you know that all of the files are synchronous. Unfortunately, these backups could be days out of date (or more!), and it can take time to make them current.
It doesn’t have to be a disaster situation to cause problems, either. The file updating in our example requires a number of files and records, and the user has to have the proper file and record locks in order to be able to complete the processing. What if you find that you are part way through the updating process and you cannot get one of the records to update? For instance, if you have a large number of details in the invoice, you may find that part way through the process one of the inventory records is locked by another user. Do you just skip this item? If you have done a large number of updates already, for the other details in the invoice, how do you go back?
The answer to these problems is transaction processing with c-tree Plus single-user mode or the c-tree Server.