<p>Each node can choose to build and maintain its own UTXO set, which it can then verify incoming transactions and blocks against. </p>

<blockquote>
  <p>If there are servers which keep track of UTXOs, is it not leading to some centralization?</p>
</blockquote>

<p>The network remains decentralized as long as a sufficient number of users are running their own nodes around the world. Each node could be considered a 'server' in some sense, but I think you are talking more about industrial server farms, which is not the case for btc nodes. </p>

<blockquote>
  <p>the owner of these servers may tamper with the UTXOs.</p>
</blockquote>

<p>This is not possible, the other nodes would recognize that the transactions involving tampered UTXOs are invalid, and reject them. </p>

<blockquote>
  <p>If there are servers which keep track of UTXOs, how can one doublespend?</p>
</blockquote>

<p>Double spends are not possible, once a transaction is confirmed in a block, all of the nodes will update their UTXO set appropriately, and reject any transactions / blocks attempting to spend already-spent or otherwise invalid transactions. </p>

<blockquote>
  <p>If there is a double spent transaction in a block then the whole block gets orphaned right?</p>
</blockquote>

<p>No, the block would simply be invalid. Orphaned blocks are valid, but not included in the longest blockchain (likely because another valid block with the same blockheight was found and relayed through the network first). </p>

CC BY-SA 3.0
2018-02-13T10:17:06.020Z
why will a miner want to verify the previous block when his sole aim is to generate blocks and get rewards? He may build the next block over a block which he didn't verify right?
Vasista
CC BY-SA 3.0
2018-02-13T11:17:31.273Z
If a miner does not verify a block, then they are at risk of mining on top of an invalid block. Doing that would mean they would not possibly get a reward, so it is in the miner's self-interest to verify that all blocks and transactions are valid.
chytrik
CC BY-SA 3.0
2018-02-13T10:03:12.173Z
2018-02-13T10:03:12.173Z
chytrik
<p>I know that the inputs for a transaction(which is not coinbase) are UTXOs.
Are UTXOs kept track of by some servers and also the owner's wallet?
If there are servers which keep track of UTXOs, is it not leading to some centralization? Because,the owner of these servers may tamper with the UTXOs.</p>

<p>If there are servers which keep track of UTXOs, how can one doublespend?
Is it the case that servers and wallets get updated of the existing UTXOs only after creation of the block by miners in which that transaction exists?
If there is a double spent transaction in a block then the whole block gets orphaned right?
Please correct me if my understanding isn't correct </p>

<doublespend><utxo>
Double spending UTXOs
Vasista
# Double spending UTXOs

(Asked by: username_0 on 2018-02-13)

I know that the inputs for a transaction(which is not coinbase) are UTXOs.
Are UTXOs kept track of by some servers and also the owner's wallet?
If there are servers which keep track of UTXOs, is it not leading to some centralization? Because,the owner of these servers may tamper with the UTXOs.

If there are servers which keep track of UTXOs, how can one doublespend?
Is it the case that servers and wallets get updated of the existing UTXOs only after creation of the block by miners in which that transaction exists?
If there is a double spent transaction in a block then the whole block gets orphaned right?
Please correct me if my understanding isn't correct

**Tags:** doublespend, utxo


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## 1 answers

### Answer by username_1 on 2018-02-13. Score: 1 | Accepted answer

Each node can choose to build and maintain its own UTXO set, which it can then verify incoming transactions and blocks against.

> If there are servers which keep track of UTXOs, is it not leading to some centralization?

The network remains decentralized as long as a sufficient number of users are running their own nodes around the world. Each node could be considered a 'server' in some sense, but I think you are talking more about industrial server farms, which is not the case for btc nodes.

> the owner of these servers may tamper with the UTXOs.

This is not possible, the other nodes would recognize that the transactions involving tampered UTXOs are invalid, and reject them.

> If there are servers which keep track of UTXOs, how can one doublespend?

Double spends are not possible, once a transaction is confirmed in a block, all of the nodes will update their UTXO set appropriately, and reject any transactions / blocks attempting to spend already-spent or otherwise invalid transactions.

> If there is a double spent transaction in a block then the whole block gets orphaned right?

No, the block would simply be invalid. Orphaned blocks are valid, but not included in the longest blockchain (likely because another valid block with the same blockheight was found and relayed through the network first).

> **Comments:**
> **username_0**: why will a miner want to verify the previous block when his sole aim is to generate blocks and get rewards? He may build the next block over a block which he didn't verify right?  
> **username_1**: If a miner does not verify a block, then they are at risk of mining on top of an invalid block. Doing that would mean they would not possibly get a reward, so it is in the miner's self-interest to verify that all blocks and transactions are valid.  

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