Background: There is an old system, which is used a lot, but has many defects, so a new system was developed, and now a small amount of traffic is diverted from the old system to the new system by some means.
Goal: After the new system is online for a period of time, the data records of switching from the old system to the new system should be counted.
Premise: Try to let new users use the new system from the beginning (premise is very important)
Assumption: the old system uses the A database, and the new system uses the B database.
Option One:
1. Before the new system goes online, make a backup of database A, which is the basic data.
2. Then the new system is launched, and the new system runs for a period of time.
3. At this time, make another backup of the A database (in fact, you don't need to do it again).
4. Take the difference set of the two data, and use the data reduced by the old system, that is, the data diverted from the old system to the new system.