I have to correct myself here, Tx2 cannot be pruned before Tx1 is pruned.

Thanks a lot for the very detailed explanation. I think I completely get it.

Storage size magnitude will not be in the order of total past transactions, but in the order of total (non-empty) accounts (or addresses).

One final question, a bit off-topic: Would it be necessary to treat coinbase transactions differently? Couldn't they just be pruned like normal transactions?

 

My point is to try and find out how efficiently previous transactions can be pruned.

Transaction 0 can be pruned, but it's information (and stored data) is effectively moved to transaction 1.

Transaction 1 also contains the data of A transferring a coin to B. I expected this data to be prunable after transaction 2. But transaction 1 can only be pruned when A has spent its remaining 49 bitcoins, which might take a long time.

I agree that when C would send 1 bitcoin to D in transaction 3, transaction 2 could be pruned.

Or, if A would send 1 bitcoin to D in transaction 3, transaction 3 would take transaction 1 as referenced transaction, send 1 bitcoin to D and 48 back to A. After this, transaction 1 could then also be pruned. This need not take so long.

Could you confirm I am thinking correctly here?

Let me restate my example to be more in accordance with the specification.

Say at some point A, B and C own 0 bitcoins.

In transaction 0, 50 bitcoins are sent to A (A just found a block).

A sends 1 bitcoin to B in transaction 1. I read in the forum (http://forum.bitcoin.org/index.php?topic=8689.msg125994#msg125994) that the current client will 'use up' all of the money from transaction 0 and send the change to a new address (or back to A I presume). I think that is why you say the previous transaction doesn't exist?

Then B sends 1 bitcoin to C in transaction 2. Now transaction 0 can be safely forgotten, but the amount of data to be stored hasn't really decreased, since the complete transaction 1 (containing 2 tx_outs) is to be stored. Am I right here?

You're saying that the option that the tx won't be completely deleted is just one of the possibilities. What I am very interested in, is whether there is a possibility that previous transactions can be completely deleted, as is claimed in Nakamoto's paper.

After studying the protocol, for now my conclusion is that this is not the case. Referencing a former transaction instead of an account unfortunately doesn't make a difference. Could anyone please invalidate my conclusion?


Whether pruning is necessary or not is another question, but I'm glad you brought it up. Assuming only the transaction hash sticks around like you say and 2000 transactions per second as mentioned on the scalability page, this means a storage need of 32 * 2000 * 3600 =5529600000  bytes = 5.5 Gb per day. Not to mention that this must be stored in such a way that it is searchable quickly, which will likely increase storage needs.

All and all, quite a lot more than the 'best cast' of 80 bytes per block (or 1.16 MB / day) that is mentioned on the scalability page. I hope you understand why I'm so damned curious about this.

Thank you, in yours and Mike's answer I read the same, transaction 1 can be forgotten. But, what happens then if B keeps a copy of the transaction and after a while resends it to the network? My guess is that, if the original transaction 1 was pruned, another 1 bitcoin is transferred from A to B?

Having studied Bitcoin in quite some depth (the 'scalibility page', Nakamoto's paper and some forum questions), I cannot find an answer to the question:

How is it that old transactions can be pruned from the ledger that the blockchain is.

Simple example:

Say at some point A owns 10 bitcoins, B and C own 0 bitcoins.

A transfers 1 bitcoin to B (transaction 1), after which B transfers 1 bitcoin to C (transaction 2).

Forget for a while about transaction fees, but can one of these transactions be pruned (after being confirmed by enough blocks in the longest chain)?

If not, can someone please give me a typical example in which a transaction can be pruned?