what happens to transactions when a block is orphaned.

[Sniper44]

lets assume block 455300 is orphaned and it contained tx1 and tx2.

block 455300 is mined by another miner and that is the longest chain.
it contains tx1 and tx3

- is that possible?

- what will happen to tx2? will it exist in memory pool or disappears?

[achow101]

If tx1, tx2, and tx3 are unrelated to each other, then scenario is entirely possible and basically happens every time there is an orphaned block.

tx2 will go back to the mempool if it is not included in the longest chain.

[odolvlobo]

lets assume block 455300 is orphaned and it contained tx1 and tx2.

block 455300 is mined by another miner and that is the longest chain.
it contains tx1 and tx3

- is that possible?

- what will happen to tx2? will it exist in memory pool or disappears?

When a node dumps an orphaned block, its transactions are returned to the list of unconfirmed transactions, unless it has already been included in a block in the new chain.

So, in your example, tx1 is still confirmed, but tx2 is no longer confirmed and is available to be included in the next block.

[MegaDice]

I case of orphaned block, your transaction might not automatically go into the mempool after it no longer exists on the longer chain. Some bitcoin clients have support for re-broadcasting transactions and you can even do it online. If your transaction disappears and is unseen on block explorer sites like blockchain.info.... then you need to rebroadcast. Google if and you will find some ways to do it.