Bitcoin Forum

An unconfirmed transaction is basically a promise: it may be confirmed or not.

When you send Bitcoin from your wallet to some other address it needs some time for transaction confirmation by nodes in the blockchain,.........

But the problem about this comes when the transaction confirms really fast, is about luck but it happens sometimes. And once the transaction is confirmed there is no step back.

Just a small correction:
Transactions are not confirmed by nodes. They are confirmed by miners. Nodes only verify blocks.

Transaction Outputs and Inputs (https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch06.asciidoc#transaction-outputs-and-inputs)

The fundamental building block of a bitcoin transaction is a transaction output. Transaction outputs are indivisible chunks of bitcoin currency, recorded on the blockchain, and recognized as valid by the entire network. Bitcoin full nodes track all available and spendable outputs, known as unspent transaction outputs, or UTXO. The collection of all UTXO is known as the UTXO set and currently numbers in the millions of UTXO.

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It’s not just Alice’s wallet that needs to retrieve UTXO referenced in the inputs. Once this transaction is broadcast to the network, every validating node will also need to retrieve the UTXO referenced in the transaction inputs in order to validate the transaction.

Fast transaction confirmation is not really about luck,

Nodes not only verify blocks, they also verify/validate transactions that go into the mempool and at every block.
Transactions are validated by all nodes. The network more decentralized than it looks like at first.

Nodes not only verify blocks, they also verify/validate transactions that go into the mempool and at every block.
Transactions are validated by all nodes. The network more decentralized than it looks like at first.

Not all nodes validate transactions.
Transactions are validated only by full nodes.

A bitcoin wallet application that runs as a full-node client actually contains a copy of every unspent output from every transaction in the blockchain. This allows a wallet to construct transaction inputs as well as quickly verify incoming transactions as having correct inputs.

However, because a full-node client takes up a lot of disk space, most user wallets run "lightweight" clients that track only the user’s own unspent outputs.

https://medium.com/avado-node/what-is-a-blockchain-node-334dc90a0ca0
Andreas Antonopoulos, a well-known Bitcoin evangelist, explains it even better and in more detail, than I could, so perhaps I’ll let him speak instead:
“A node is a computer that is participating in the global peer-to-peer Bitcoin network. They propagate transactions and blocks everywhere. Full nodes act independently as authoritative verifiers. Onion routing. Double-spending protection. Levels of isolation for untrustworthy nodes. Why would you run one? To be in control of your money — your keys, your money. You don’t trust anyone else to tell you what the truth is, you cross-check every claim yourself. Be part of the revolution in money. #RunYourOwnNode”
https://www.youtube.com/watch?v=fNk7nYxTOyQ

Fast transaction confirmation is not really about luck, but it depends on the feerate paid and also the state of the network when the transaction was broadcasted, in such situations where the transaction gets confirmed quickly, it's almost impossible to reverse. (If you're the recipient for larger transactions, it's better to wait for 3 or 6 confirmations).

You're right that a transaction cannot be canceled, but can only be replaced, however, in the context it is used, it essentially means the initial transaction would be overridden by one with a higher fee (if it's not confirmed yet)

Well, and what is a full node?
A full node doesn't have to mine...

Try to check this very well, you are getting something not right, that hosseinimr93 post that I quoted along is absolutely correct, because only full nodes validate blocks and transactions, but not all nodes are full nodes.

Transactions are not confirmed by nodes. They are confirmed by miners. Nodes only verify blocks.

I guess you are not getting something as well... he said that nodes do not verify transactions and only miners verify them. That's not correct. Then I explained that full node verify transactions, and they are not miners.

Nodes not only verify blocks, they also verify/validate transactions that go into the mempool and at every block.
Transactions are validated by all nodes. The network more decentralized than it looks like at first.

All nodes include the routing function to participate in the network and might include other functionality. All nodes validate and propagate transactions and blocks, and discover and maintain connections to peers. In the full-node example in A bitcoin network node with all four functions: wallet, miner, full blockchain database, and network routing, the routing function is indicated by a circle named "Network Routing Node" or with the letter "N."

Some nodes, called full nodes, also maintain a complete and up-to-date copy of the blockchain. Full nodes can autonomously and authoritatively verify any transaction without external reference. Some nodes maintain only a subset of the blockchain and verify transactions using a method called simplified payment verification, or SPV. These nodes are known as SPV nodes or lightweight nodes. In the full-node example in the figure, the full-node blockchain database function is indicated by a circle called "Full Blockchain" or the letter "B." In The extended bitcoin network showing various node types, gateways, and protocols, SPV nodes are drawn without the "B" circle, showing that they do not have a full copy of the blockchain.

Mining nodes compete to create new blocks by running specialized hardware to solve the Proof-of-Work algorithm. Some mining nodes are also full nodes, maintaining a full copy of the blockchain, while others are lightweight nodes participating in pool mining and depending on a pool server to maintain a full node. The mining function is shown in the full node as a circle called "Miner" or the letter "M."

User wallets might be part of a full node, as is usually the case with desktop bitcoin clients. Increasingly, many user wallets, especially those running on resource-constrained devices such as smartphones, are SPV nodes. The wallet function is shown in A bitcoin network node with all four functions: wallet, miner, full blockchain database, and network routing as a circle called "Wallet" or the letter "W."

In addition to the main node types on the bitcoin P2P protocol, there are servers and nodes running other protocols, such as specialized mining pool protocols and lightweight client-access protocols.

Different types of nodes on the extended bitcoin network shows the most common node types on the extended bitcoin network.

More information about RBF can be found on Bitcoin Core website (https://bitcoincore.org/en/faq/optin_rbf/)

My transaction has been unconfirmed for a long time. What can I do?

If you have made a transaction that is unconfirmed, you can:

• Wait for a long time. Eventually, your transaction will either be confirmed or cancelled. This might take several days.
• Increase the transaction fee. This is only possible for “replaceable” transactions. To create this type of transaction, you must have checked “Replaceable” on the send tab before sending the transaction. If you’re not seeing the “Replaceable” option on the send tab go to Tools menu > Preferences > Fees tab and set “Propose Replace-By-Fee” to “Always”. Transactions that are replaceable have the word “Replaceable” in the date column on the history tab. To increase the fee of a replaceable transaction right click on its entry on the history tab and choose “Increase Fee”. Set an appropriate fee and click on “OK”. A window will popup with the unsigned transaction. Click on “Sign” and then “Broadcast”.
• Create a “Child Pays for Parent” transaction. A CPFP is a new transaction that pays a high fee in order to compensate for the small fee of its parent transaction. It can be done by the recipient of the funds, or by the sender, if the transaction has a change output. To create a CPFP transaction right click on the unconfirmed transaction on the history tab and choose “Child pays for parent”. Set an appropriate fee and click on “OK”. A window will popup with the unsigned transaction. Click on “Sign” and then “Broadcast”.

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About child-pays-for -parent (CPFP), bitcoin users needs to really know the reason why a transaction can support CPFP, because not all transactions can support CPFP.

Any transaction can be included in a CPFP transaction. It is not like RBF where the transaction must be marked as opted-in before it being made. Any unconfirmed output can be included as an input in a new transaction with or without additional inputs and used to make a CPFP transaction. As you say, it might be that the person who made the first transaction cannot make a CPFP since all the outputs from the first transaction are to other people and there is no change output, but that does not mean the transaction itself does not support CPFP.

I think it should be like this: Transactions are not confirmed by nodes. They are confirmed by miners. Nodes only verify blocks and transactions.

Well, nodes (full nodes) verify everything. Since the genesis block, they verify block information, headers, each block hash, each transaction and each UTXO and so on.

As you are referring to a full node, you are 100% correct. A full node verify all transactions and blocks.
But I don't think a node (I mean a non-full node) verify all transactions inside a block one by one. A non-full node relies on other full nodes instead of verifying all transactions one by one.

Am I right? Someone correct me if I'm wrong please.

8. Simplified Payment Verification

It is possible to verify payments without running a full network node. A user only needs to keep a copy of the block headers of the longest proof-of-work chain, which he can get by querying network nodes until he's convinced he has the longest chain, and obtain the Merkle branch linking the transaction to the block it's timestamped in. He can't check the transaction for himself, but by linking it to a place in the chain, he can see that a network node has accepted it, and blocks added after it further confirm the network has accepted it.

SPV nodes, which are the nodes which doesn't have the full blockchain, are described in the whitepaper:

The extended bitcoin network includes the network running the bitcoin P2P protocol, described earlier, as well as nodes running specialized protocols. Attached to the main bitcoin P2P network are a number of pool servers and protocol gateways that connect nodes running other protocols. These other protocol nodes are mostly pool mining nodes (see [mining]) and lightweight wallet clients, which do not carry a full copy of the blockchain.

But I don't understand a little about the Cancel (double-spend) feature, will this feature return the bitcoin that we sent (unconfirmed) to the same address or to another address? Honestly, I haven't tried the Cancle (double-spend) feature so far. I need a little explanation on this. However, I do know that this feature can cancel the first transaction for a low fee.

Does this process involves transactions with much coins.

Does it mean once a transaction is made and it is been sent to a wrong address it can be cancelled and resent to the correct wallet.

Because I have read in someother article that bitcoin sent to another address cannot be recovered or traced to the owner.

Yes, that is correct.

SPV nodes, which are the nodes which doesn't have the full blockchain, are described in the whitepaper

How about if you run Pruned Bitcoin node?

They perform almost exactly like a full node, you don't need to have complete copy of the blockchain stored on your hard drive and I believe you can also verify Bitcoin transactions.

You are right, a pruned node can relay blocks and transactions to other nodes, In this regard, a prune node can also still verify blocks and transactions. It is just an exeption, but pruned node is not entirely different from full node, as it is the continuation of a full blockchain (which means prunned node begins from previous full blockchain, and continue as a prunned node) which makes it just act like a full node in blocks and transactions validation.

The pruned nodes, they are a type of node which performs almost all the functions of a full node. In fact, your only exception is that the pruned nodes do not have a complete copy of the blockchain. That is, the pruned nodes only maintain a substantial part of the blockchain, using it to carry out their verification processes like any other full node.
https://academy.bit2me.com/en/what-is-a-pruned-node/

Although you will save on disk space, you will not save bandwidth if you run a pruned node. They use the same bandwidth as a full node.

True that unconfirmed transactions are highly risky in crypto trading. Recently I read an article about a dust attack through bitcoin funding. It’s actually a great post that helps to get an idea on how to avoid such unconfirmed transactions.