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Let's assume I create and send a series of txs:

|------block A--------|------block B----------|----block C-----|----block D-----|
|Tx1......Tx2.........|...Tx3.................|................|......Tx4.......|

but then a fork happens and a block reorganization takes place:

|------block A--------|------block B----------|----block E-----|----block F-----|--block G--|
|Tx1......Tx2.........|...Tx3.................|......???.......|......???.......|.....???...|

Whatever happens to Tx4? Will it be included in one of blocks E, F, G or another block? Is it gone? Conflicted/invalidated? Do I need to resend it? If yes, can I spend the same inputs I did when creating Tx4?

Doug Peters
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1 Answers1

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Will it be included in one of blocks E, F, G or another block?

It's likely already included in one of those blocks. If not, it can be included in a block after that.

Do I need to resend it?

Probably not. If block D or Tx4 was widely seen by the network, it should get into a block soon. (It might already be in blocks E, F, or G.)

If yes, can I spend the same inputs I did when creating Tx4?

Yes. When doing a reorg, the Bitcoin client first rolls the chainstate back to the first common ancestor.

The standard Bitcoin client actually includes a database for the sole purpose of rolling the blockchain back. (It's called an undofile.)

Alin Tomescu
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Nick ODell
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  • Thanks Nick! How is it determined which block (E/F/G/other) Tx4 is likely to be included into? If Tx4 is not included in any of the "new" blocks, what is the number of blocks after that point at which we can safely assume that Tx4 will not be included in any of the future blocks? – Doug Peters Jan 08 '15 at 17:17
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    @DougPeters How is it determined which block (E/F/G/other) Tx4 is likely to be included into? It's decided by the miner before they mine those blocks. Miners will include most transactions, provided that they are legal, pay fees, and the miner knows about them. – Nick ODell Jan 08 '15 at 17:21
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    @DougPeters what is the number of blocks after that point at which we can safely assume that Tx4 will not be included Never. You could wait 10 years, put Tx4 into a block, and it would still be legal. However, if it's been 4 days, the transaction has probably been forgotten. – Nick ODell Jan 08 '15 at 17:22
  • 4 days is an arbitary number I assume? – Doug Peters Jan 08 '15 at 17:28
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    @DougPeters Pretty much. This answer explains it better than I can. – Nick ODell Jan 08 '15 at 17:32