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The mempool is one of the most intriguing yet least understood topics in Bitcoin. In this blog post, we delve into what the mempool is and its role in Bitcoin.
The mempool is an elusive concept. We could tell you that the name is an abbreviation of “memory pool” and that is also known as the “transaction pool.” We could add that it’s sort of a purgatory in which pending transactions live while waiting for miners to add them to a block. However, did you know that the mempool is related but not part of the blockchain? And what if we told you that each Bitcoin node hosts its own version of the mempool? That complicates everything, we know.
Luckily, Blink is here to break it down. Let’s begin with mempool.space’s definition:
“A mempool (short for "memory pool") is the queue of pending and unconfirmed transactions for a cryptocurrency network node. There is no one global mempool: every node on the network maintains its own mempool, so different nodes may hold different transactions in their mempools.”
When a user sends Bitcoin, the wallet ensembles a transaction using the available UTXOs and signs it. The network doesn’t send the Bitcoin right away. Instead, it broadcast the transaction to every available node and waits for confirmation. The accumulation of all of those forms the mempool. From that ethereal place, miners extract transactions to include in blocks. When those blocks go through, they are added to the blockchain, and the now-confirmed transactions disappear from the mempool. After that, every block the system adds on top of the one we’re following is considered one extra confirmation.
It’s important to point out that only Bitcoin’s main chain uses and requires a mempool. Blink is a Lightning Network wallet and Lightning only interacts with said blockchain while opening and closing channels. Now, Blink offers on-chain transactions for our users to get in and out of Lightning. Learn more about our low fees here.
The Bitcoin network is always shifting and the mempool is no exception. At any given time, it confirms transactions that disappear while new ones take their place in the queue. Also, some transactions get canceled. The network can only store so many unconfirmed ones in the mempool, with the limiting factor being the number of active nodes. According to Johoe's Bitcoin Mempool Statistics, “The default mempool of a Bitcoin node is 300 MB, but the size is computed differently: there is no segwit discount and small transactions take more space in the mempool than they take when they are written into a block. Therefore, the default limit of 300 MB corresponds to 50-120 vMB, depending on the shape of the transactions.”
So, taking the limits into account, transactions that don’t go through after a two-week period get automatically canceled. However, since every node has its individual mempool, sometimes seemingly canceled transactions get added to blocks and confirmed. According to Andreas Antonopoulos, there’s over 99% overlap of transactions in each node’s mempools, but apparently that 1% is enough to cause these instances.
The Bitcoin network is the closest thing to a decentralized system that the world has. The law of supply and demand determines fees, there’s no central authority imposing them. There’s only so much block space, so transactions compete to get added and confirmed. It stands to reason that the miners will pick the transactions that carry the bigger fees. It’s also logical that if the mempool clears, the fees decrease as the fierceness of the competition subsides.
The other mitigating factor is the amount of vBytes a transaction consists of, but we covered that beast in the UTXOs article already.
If a transaction doesn’t offer a competitive fee, sometimes it gets stuck in the mempool. Besides waiting for it to disappear and taking the associated risks, a user has two options. A.- Accelerate the process using a Child Pays For Parent or a Replace By Fee transaction. B.- The nuclear option is to send the same UTXO to yourself through a new transaction with higher fees. If and when that one gets confirmed, the previous one gets canceled.
As we already established, every transaction gets broadcasted to the nodes and each one keeps a mempool of its own. When the transactions arrive, each node does a series of checks to ensure it’s valid. It checks the presence of an input and an output, the value transmitted against what’s available in the UTXO, and if the block contains a coinbase transaction, among other things. If everything is correct, the transaction enters the mempool and the node relays it to other nodes within its range.
According to the already quoted mempool.space:
“A mempool explorer is a tool that enables you to view real-time and historical information about a node's mempool, visualize its transactions, and search and view those transactions”
Besides mempool.space, you could use:
Now, some of these services also serve as block explorers. A similar tool, Bblock explorers allow anyone to search the open ledger that is the blockchain in a visual way. A user can look up specific transactions or get a general feel of the current state of the network. Said user could also get a feel of the state of the network at any given point through its history.
These are some of the most popular block explorers:
The radical transparency of the Bitcoin network is one of Satoshi’s greatest hits. And now you know how to access this wealth of information from the comfort of your own home.
