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Example Situation

To set the scene, here's a typical situation in our web applications:

• David pulls up data in his browser. Goes to lunch.
• Felicia pulls up same data, makes a change and saves. Changes Felicia's changes are committed to the DB.
• David comes back from lunch, changes the data on his screen and saves. Changes David's changes are committed, overwriting Felicia's changes.

Obviously this isn't good; when David finally submits changes, we need the app to detect that someone else has changed the data, and to handle the situation appropriately. Two aspects to doing this are:

1. Detection - how to detect a concurrent update
2. Handling - what to do if we detect a concurrent update

We'll take these in reverse order: