I sent bitcoins and found that transaction is too large - what to do ?

[DannyHamilton]

I have a question concerning this. I read that e.g. blockchain.info will "forget" (a.k.a. remove from mempool) a transaction after 48 hours, iirc.

That sounds about right.  I'm not sure exactly what duration blockchain.info has chosen for their database, but it seems to be more than a day and less than a week.

Is that a special implementation at blockchain.info or is that something that applies to any wallet, no matter if it's on a mobile phone, a web wallet or a locally installed one on your PC/Notebook/Mac/whatever?

Each wallet handles unconfirmed transactions in its own way as determined by the person (or people) that created the wallet.  As an example, Bitcoin Core never forgets a transaction that it has sent (unless the user makes extraordinary effort to remove the transaction from the wallet.  Instead, it just keeps on re-broadcasting the transaction every now and then in case any peers have forgotten about it.  Most full nodes will hold unconfirmed transactions that they have received for a while, but eventually they will remove them to make room for new transactions in the memory pool.  The amount of time that the received transactions are held in the memory pool depends on the implementation, there are no protocol rules forcing any node to keep a transaction in their memory pool for any specific amount of time.

And, do such transactions indeed appear in the block explorers (as they were propagated to the network, already), stay in mempool and as soon as the period of 48 hours has passed, they disappear from the block explorers (if they were not included into a block)? Are they sort of "unpropagated" then, or do they have a flag set which "undoes" those transactions,

That's up to the people running the block explorer.  Each block explorer can decide for themselves how they want to handle transactions that aren't confirmed after a significant amount of time.  You'd have to list which explorers specifically you want to know about for anyone to be able to tell you what each one does.

or do they disappear forever from the blockchain then?

If they were unconfirmed, then they were never in the blockchain in the first place.  Therefore, they can't "disappear forever from the blockchain".

[Lauda]

Nobody can undo a transaction.

Wrong. I can.

Bitcoin transactions are irreversible. What you are saying is different than reversing it but I can't understand it. Can you elaborate?

You can't undo a transaction that has been fully confirmed (possible but very hard, and it gets harder with each confirmation). Unconfirmed transactions are not in the blockchain. You can however (shall the need arise), delete the old (still unconfirmed) transaction from your wallet and re-send it with a higher fee. This is what you a double spend. Your second transaction will confirm much faster and the previous one will be invalidated.

OP if you really feel like you need to do this instead of waiting then look at this link.

If they were unconfirmed, then they were never in the blockchain in the first place.  Therefore, they can't "disappear forever from the blockchain".

Bolded, because most will chose to ignore it and reply to the thread. Interestingly only a small amount of users are aware of double spending a unconfirmed transaction.

[crairezx20]

bro whats is  the minimunm fee of elctrum per kb? some times i can send without fee.... but this time my sending btc still pending....

[amaclin]

You can't undo a transaction that has been fully confirmed

Wrong. I can not undo bitcoin transaction today.
1) Can I undo confirmed transaction in some dead coin? AuroraCoin or one other ShitCoin? Yes.
2) Can I undo confirmed transaction in bitcoin some day in future? Yes.

[DannyHamilton]

2) Can I undo confirmed transaction in bitcoin some day in future? Yes.

That is very unlikely.

Perhaps, if you are very VERY lucky, you *might* be able to undo a transaction that has only 1 or 2 confirmations.  As the number of confirmations increases, it becomes exponentially more unlikely until for all intents and purposes it is no longer possible at all.

If there is a day in the future where you can reliably undo a bitcoin transaction that has more than a few confirmations, then bitcoin will have become completely broken and worthless.  As such, it will no longer carry any value and there will be nobody interested in performing any value based transactions any longer.

[amaclin]

That is very unlikely.

But this will be possible.
Take any "dead" altcoin where no one is mining.
Are you able to create alternate blockchain not from the top block? Yes.
The future of bitcoin will be the same when the difficulty drop down.

Yes. This will be not today and even not tomorrow.

[allyouracid]

@ranochigo and DannyHamilton: thank you both for your answers. Very enlightening, helped me a lot to understand how transactions work in general. Much appreciated.

[crairezx20]

No one can undo the transaction unlike paypal transaction... may you can wait 5 days to get back your bitcoin to your wallet,....

[amaclin]

No one can undo the transaction unlike paypal transaction...

Wrong. The miner with 51% power can.

[ranochigo]

No one can undo the transaction unlike paypal transaction...

Wrong. The miner with 51% power can.

A miner with 51% of the hashrate can do it with 100% success rate while one with 40% can do it with 50% success rate when it is confirmed. That aside, it is possible for it to be reversed by double spending if majority of the network drops your transaction from mempool, those with reference client would relay it as there is no conflicting inputs. Alternatively, a miner can mine a block and include your new transaction in and the rest of the network would consider the other transaction as invalid and reject it.

[DannyHamilton]

No one can undo the transaction unlike paypal transaction...

Wrong. The miner with 51% power can.

A miner with 51% of the hashrate can do it with 100% success rate while one with 40% can do it with 50% success rate
- snip -

That depends on how long they can maintain that hash rate.  If the network surpasses their hash rate before they complete enough blocks, they won't be able to.