The mempool is often looked at as a key metric for cryptocurrency network health. Bitcoin has one, as do Ethereum, Litecoin, DigiByte, and so forth. However, there is a lot more to this metric than most people give it credit for.
It’s not a Centralized “Pool”
To onlookers, the name mempool may seem confusing. It is a memory pool of all network transactions waiting to be confirmed. However, these transactions are not bundled into a centralized entity. Nor is there a central server to store the number of unconfirmed transactions. Instead, all network nodes have their own mempool, which can be shared with other nodes on the network. The only centralized aspect is the websites displaying this data in a more convenient format.
All Individual Mempools are Equal-ish
With thousands of nodes keeping records of their own mempool, it is crucial to have as much common data as possible. All of these mempools are, in nearly all cases, virtually identical. As such, there is no such thing as one mempool, but it is rather a combination of different mempools together. This further removes any illusion of dealing with a centralized aspect in a decentralized system.
There are Size Restrictions
Collecting and storing all of this network data pertaining to unconfirmed transactions can take up a lot of hard disk space. By default, individual nodes storing the mempool have a size limit of 300 MB. If this size is insufficient, the lowest fee transactions will be dropped from the node’s pool.
This does not mean they are gone from the network, however. Low-fee transactions receive a much lower priority on the list, which is why they often take much longer to receive network confirmations. In rare cases, the transaction is dropped from public sight, warranting it to be broadcasted again at a higher fee.
Where can I see it in Action?
A text-based explanation of these intricacies is one thing, but seeing is believing. There are many ways to check the mempool – for Bitcoin, in this case. One of the more convenient dashboards can be found here. It shows the number of transactions, fee rates, and so forth.