Android SDCard I / O管理挂载和卸载

Question

I know there are various version of this question asked, but I was having trouble getting a clear answer on what the best approach is for my problem.

What I am doing is I am creating a SQLite database on the SD card. I want to be able to query from it and write to it.

The question I have is what is the best way to manage when the SD card is unmounted. I am totally fine with my application closing like the stock MP3 player does. However, I want to make sure any write action to the db do not get partially done.

One thought I had is the use beginTransaction, mark it as successful, and then call end transaction. The question I have is what happens if end of transaction does not get called. Can that potentially lead to data corruption? Also, I need a little help understanding what to listen to or hook into to get notification of the sd card being unmounted.

Thanks

Answer 1

That is the great thing about transactions in databases - you almost never need to worry:

"All changes within a single transaction in SQLite either occur completely or not at all, even if the act of writing the change out to the disk is interrupted by

a program crash, an operating system crash, or a power failure."

Taken from http://www.sqlite.org/transactional.html

The disk being removed on which the database resides should (in the worst case) behave like a power failure while writing the data to the disk. The database will discard that data on next startup.

Thus, as soon as your transaction is committed using commit or end transaction and the method call executing your statement returns all data has been stored. Otherwise NO data from your transaction will have been stored - both cases leave you in a consistent state.

Beware of the only catch: You will need to make sure that all statements you need to execute together (ie atomically) must be within one transaction!