You shouldn't be downvoted; people should help you get up to speed.
In the scenario you describe, the database commit works, you send the email but... you get an error back from the email send function. Now what?
This is a basic distributed computing problem. Choosing how to solve it can have a drastic effect on your code and your infrastructure. If the secondary function is "charge a credit card" obviously you can't do that twice. But if it's merely "send an email" then maybe it's okay if people get a duplicate.
Google lease/lock/deadlock/race condition and read up on why databases tend to make for bad implementations.
In the scenario you describe, the database commit works, you send the email but... you get an error back from the email send function. Now what?
This is a basic distributed computing problem. Choosing how to solve it can have a drastic effect on your code and your infrastructure. If the secondary function is "charge a credit card" obviously you can't do that twice. But if it's merely "send an email" then maybe it's okay if people get a duplicate.
Google lease/lock/deadlock/race condition and read up on why databases tend to make for bad implementations.