If the block chain gets changed, is there any way to notify about it? Reason being, I have a database being written to based off of every new block that arrives, so if one of the older blocks changes, I'd like to know about it.
The "older blocks" won't actually change. When a block chain forks, and you happen to be on the false part of it when one of the forks becomes the "longest", all your blocks from the fork up are simply no longer valid (and all transactions contained in it).
You don't get a direct notification of it, but you can "easily" know when there is/was a fork, because the new block you receive will NOT reference the latest block you have in your database.
In fact, there are 2 possibilities (current block height in your database is N):
- The new block has a previous hash that you already have in your database (most probably references block N-1, the start of a fork). You can either choose to ignore this new block (as you already have a block chain that is longer or same length), OR you can drop all blocks up to the previous hash of the new block and append this new block as usual.
- The new block has a previous hash "PH" that doesn't correspond to any of your current block hashes. This means there is/was a fork and your part of the bitcoin network did not get one or more blocks of the other branch. You will have to ask with a "getblock" message the block X with this hash "PH" and check if the previous hash in this block X is a hash you know of. If so, you can reconstruct this branch (and like in the first possibility decide what branch to keep), if not you'll have to do a "getblock" again (and loop until you get a known hash).