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Chapter 2
Lecture 11& 12
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Why must I sometimes wait for my
transaction to clear?
Because your transaction must be
verified by miners, you are sometimes
forced to wait until they have finished
mining. The bitcoin protocol is set so that
each block takes roughly 10 minutes to
mine.
Some merchants may make you wait
until this block has been confirmed,
meaning that you may have to make a
cup of coffee and come back again in a
short while before you can download the
digital goods or take advantage of the
paid service.
On the other hand, some merchants
won’t make you wait until the
transaction has been confirmed. They
effectively take a chance on you,
assuming that you won’t try and spend
the same bitcoins somewhere else before
the transaction confirms. This often
happens for low value transactions,
where the risk of fraud isn’t as great.
What if the input and output amounts
don’t match?
Because bitcoins exist only as records
of transactions, you can end up with
many different transactions tied to a
particular bitcoin address. Perhaps “X”
sent “Y” two bitcoins,
“Z”sent her three
bitcoins and “A” sent her a single bitcoin,
all as separate transactions at separate
times.
These are not automatically combined
in “Y”
,s wallet to make one file
containing six bitcoins. They simply sit
there as different transaction records.
When “Y” wants to send bitcoins to
john,her wallet will try to use transaction
records with different amounts that add
up to the number of bitcoins that she
wants to send john.
The chances are that when “Y” wants to
send bitcoins to john, she won’t have
exactly the right number of bitcoins from
other transactions. Perhaps she only
wants to send 1.5 BTC to john.
None of the transactions that she has
in her bitcoin address are for that
amount, and none of them add up to that
amount when combined. “Y” can’t just
split a transaction into smaller amounts.
You can only spend the whole output of a
transaction, rather than breaking it up
into smaller amounts.
Instead, she will have to send one of
the incoming transactions, and then the
rest of the bitcoins will be returned to
her as change.
“Y” sends the two bitcoins that she got
from “X” to john. “X” is the input, and
john is the output. But the amount is only
1.5 BTC, because that is all she wants to
send. So, her wallet automatically creates
two outputs for her transaction: 1.5 BTC
to john, and 0.5 BTC to a new address,
which it created for “Y”to hold her
change from john.
Next Topic
Are There Any Transaction Fees.