Mempool?

[Hienrich]

Hi all,

I am new to Bitcoin. I have checked the terminology thread for mempool, but couldn't find any results. I would appreciate if someone could explain to me as to what the mempool is?

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[TryNinja]

The bitcoin mempool is a large collection of network transactions waiting to be confirmed. All nodes have a different RAM capacity to store those unconfirmed transactions.

Every new bitcoin transaction that is validated by the network will automatically be added to the mempool, where it awaits confirmations from miners. Once a miner picks up the transaction in question for inclusion in the next block, it will automatically receive its first confirmation and the transactions will be removed from the mempool, opening space for other transactions.

[Hienrich]

The bitcoin mempool is a large collection of network transactions waiting to be confirmed. All nodes have a different RAM capacity to store those unconfirmed transactions.

Every new bitcoin transaction that is validated by the network will automatically be added to the mempool, where it awaits confirmations from miners. Once a miner picks up the transaction in question for inclusion in the next block, it will automatically receive its first confirmation and the transactions will be removed from the mempool, opening space for other transactions.

How do transaction accelerators work? Do they work on the concept of the mempool?

[vh]

There is limited space on the block, so not all transactions (tx) waiting in the mempool can be placed into the block.   Accelerators are simply a commitment by the corresponding pool to include the requested tx over all other tx in the next block they find.

[Hienrich]

There is limited space on the block, so not all transactions (tx) waiting in the mempool can be placed into the block.   Accelerators are simply a commitment by the corresponding pool to include the requested tx over all other tx in the next block they find.

So I am guessing they would pay more? And is there more than one mempool?

[pooya87]

How do transaction accelerators work? Do they work on the concept of the mempool?

"transaction accelerator" is not really the best name for the service that is being offered.
what they do is that they offer an option for users with a stuck transaction to "prioritize their transaction" that otherwise would have had a lower priority. for example if they had a tx with 50 satoshi per byte when everyone is paying 330 s/b.
it can be for free or it can be for additional costs.

[vh]

There is limited space on the block, so not all transactions (tx) waiting in the mempool can be placed into the block.   Accelerators are simply a commitment by the corresponding pool to include the requested tx over all other tx in the next block they find.

So I am guessing they would pay more? And is there more than one mempool?

Some yes.  Some offer it for free but with a limited quota per hour.   

There is one mempool per node in the network.   They should be similar, but not always identical.

Each transactions also has a slot for optional fees to be collected by the one constructing the block.   So transactions in the mempool tend to be added to blocks in a predictable way.   Most recent wallets come with a fee estimator to position the transaction in a favoriable place.

The accelerators is like a fast pass for future blocks found by the pool offering the service.

[Hienrich]

There is limited space on the block, so not all transactions (tx) waiting in the mempool can be placed into the block.   Accelerators are simply a commitment by the corresponding pool to include the requested tx over all other tx in the next block they find.

So I am guessing they would pay more? And is there more than one mempool?

Some yes.  Some offer it for free but with a limited quota per hour.  

There is one mempool per node in the network.   They should be similar, but not always identical.

Each transactions also has a slot for optional fees to be collected by the one constructing the block.   So transactions in the mempool tend to be added to blocks in a predictable way.   Most recent wallets come with a fee estimator to position the transaction in a favoriable place.

The accelerators is like a fast pass for future blocks found by the pool offering the service.

Is a node basically a miner? And what stops a miner confirming his/her transaction?

[Lauda]

Is a node basically a miner?

No. There are both mining and non mining nodes.

And what stops a miner confirming his/her transaction?

Nothing. A miner is free to include any transactions of his choosing, or none at all, as long as they do not break any consensus rules.

[bitart]

There is limited space on the block, so not all transactions (tx) waiting in the mempool can be placed into the block.   Accelerators are simply a commitment by the corresponding pool to include the requested tx over all other tx in the next block they find.

So I am guessing they would pay more? And is there more than one mempool?

Some yes.  Some offer it for free but with a limited quota per hour.  

There is one mempool per node in the network.   They should be similar, but not always identical.

Each transactions also has a slot for optional fees to be collected by the one constructing the block.   So transactions in the mempool tend to be added to blocks in a predictable way.   Most recent wallets come with a fee estimator to position the transaction in a favoriable place.

The accelerators is like a fast pass for future blocks found by the pool offering the service.

Is a node basically a miner? And what stops a miner confirming his/her transaction?

Miners are working for profit, so they are cherry picking the transactions with the highest fees (understandable, they have to pay the electricity bill monthly...). So it's not like the miners don't confirm some of the transactions, but the opposite, they are confirming only transactions with decent fees offered for the confirmation, the other transactions are hanging in the mempool, and waiting for a period of time that the mempool shrinks and has only transactions with similar or lower fees. If someone only uses a really low fee when submitting a transaction, the transaction can stuck in the mempool for a really long time, because people are usually raising the fees in order to get their transactions confirmed in time.
You can check number of transactions in the mempool e.g. here:
https://blockchain.info/en/charts/mempool-size?timespan=60days