<blockquote>
  <p>Let's say the hash of the previous block was "11b83f1dbb421b7539f755". If I make a transaction (Transaction A) for about 1 BTC and 1 second after me someone other makes new a transaction (Transaction B), which transaction becomes the previous hash "11b83f1dbb421b7539f755", because it needs 15 seconds till every node knows the new chain?</p>
</blockquote>

<p>Not quite. </p>

<p>When you submit a transaction to the network, the network's nodes will check to see if it is valid. If it is, then the nodes will each store the transaction in their local 'mempool' of unconfirmed transactions. Note that a transaction does not reference a previous block, but it will reference some valid unspent transaction outputs (UTXOs) to consume as inputs (which means, it will specify which coins it is spending). It is possible for different nodes to have different mempools, and a node can even elect to not keep a mempool at all. </p>

<p>Miners will select transactions from their local mempool, and attempt to mine blocks which include those transactions. Once a miner finds a valid block with a certain transaction included in it, that transaction is said to be 'confirmed'. Note that a valid block must reference the previous block's hash. </p>

<p>If two transactions consume the same UTXOs, then only one of them can possible be validly included in the blockchain record. </p>

<p>Likewise, if two blocks reference the same previous block hash, then only one of them will eventually become a part of the blockchain record (whichever one is extended first).</p>

CC BY-SA 4.0
2019-12-11T22:46:41.867Z
2019-12-11T22:46:41.867Z
chytrik
<p>I read this article <a href="https://medium.com/@brendanrius/where-is-the-blockchain-stored-b7e9e00bc2c2" rel="nofollow noreferrer">blockchaing stored</a> and now I am bit confused. In this article it is described, that the transaction needs 15 seconds to propagate to the whole blockchain network. I know that the new transaction block contains the hash of the previous block. So my question is, if anyone in this 15 seconds create another transaction, how the systems doesn't crash?</p>

<p>To make my question a little bit more clear, I make one example. Let's say the hash of the previous block was "11b83f1dbb421b7539f755". If I make a transaction (Transaction A) for about 1 BTC and 1 second after me someone other makes new a transaction (Transaction B), which transaction becomes the previous hash "11b83f1dbb421b7539f755", because it needs 15 seconds till every node knows the new chain? So after one second not every node, knows about the Transaction A.</p>

<p>Thank you very much in advance!</p>

<p>PS: English is not my mother language :)</p>

<blockchain.info>
How is Bitcoin/Blockchain programmed to do multi transaction at the same time?
BitcoinBeginner_Germany
# How is Bitcoin/Blockchain programmed to do multi transaction at the same time?

(Asked by: username_0 on 2019-12-11)

I read this article [blockchaing stored](https://medium.com/@brendanrius/where-is-the-blockchain-stored-b7e9e00bc2c2) and now I am bit confused. In this article it is described, that the transaction needs 15 seconds to propagate to the whole blockchain network. I know that the new transaction block contains the hash of the previous block. So my question is, if anyone in this 15 seconds create another transaction, how the systems doesn't crash?

To make my question a little bit more clear, I make one example. Let's say the hash of the previous block was "11b83f1dbb421b7539f755". If I make a transaction (Transaction A) for about 1 BTC and 1 second after me someone other makes new a transaction (Transaction B), which transaction becomes the previous hash "11b83f1dbb421b7539f755", because it needs 15 seconds till every node knows the new chain? So after one second not every node, knows about the Transaction A.

Thank you very much in advance!

PS: English is not my mother language :)

**Tags:** blockchain.info


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## 1 answers

### Answer by username_1 on 2019-12-11. Score: 1

> Let's say the hash of the previous block was "11b83f1dbb421b7539f755". If I make a transaction (Transaction A) for about 1 BTC and 1 second after me someone other makes new a transaction (Transaction B), which transaction becomes the previous hash "11b83f1dbb421b7539f755", because it needs 15 seconds till every node knows the new chain?

Not quite.

When you submit a transaction to the network, the network's nodes will check to see if it is valid. If it is, then the nodes will each store the transaction in their local 'mempool' of unconfirmed transactions. Note that a transaction does not reference a previous block, but it will reference some valid unspent transaction outputs (UTXOs) to consume as inputs (which means, it will specify which coins it is spending). It is possible for different nodes to have different mempools, and a node can even elect to not keep a mempool at all.

Miners will select transactions from their local mempool, and attempt to mine blocks which include those transactions. Once a miner finds a valid block with a certain transaction included in it, that transaction is said to be 'confirmed'. Note that a valid block must reference the previous block's hash.

If two transactions consume the same UTXOs, then only one of them can possible be validly included in the blockchain record.

Likewise, if two blocks reference the same previous block hash, then only one of them will eventually become a part of the blockchain record (whichever one is extended first).


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