When a transfer of Bitcoin occurs, it is sent to the network nodes who examine the transactions before storing them in their respective mempools. These mempools are storage areas for valid but unconfirmed transactions.
When miners choose to build a block, they have the ability to pick transactions from the mempools of their choice.
Nevertheless, every node possesses their own mempool. Therefore, the mempool system is a list of transactions not shared among all the network nodes of the Bitcoin network.
Some nodes might have some transactions which other nodes might not possess. In times of limited space for blocks, transactions might stay unconfirmed for a longer period of time.
Although the mempool system serves an important role in tracking unconfirmed transactions and helping miners to look out for transactions to add into a block, being in the mempool does not guarantee that the transaction will be confirmed in the next block.
The confirmation of a transaction is based on whether the miner adds the transaction in the block.
Solana has a unique approach to transaction processing. Unlike in the case of Bitcoin, which employs the network-wide mempool, transactions are usually submitted to the RPC service towards validators who build new blocks. Such validators are called leaders.
Once a transaction reaches a validator, it undergoes verification of its signatures and structure. The validator also verifies the necessary accounts and tries to run the transaction program.
If the execution was successful, the changes can be committed. If not, then other changes are undone by the transaction. However, even in case of failure, there might still be a transaction fee.
It doesn’t mean that there are no transaction queues or that all transactions are handled instantly. There can be transaction delays, rejection, and failures of transaction processing.
The only thing that matters here is that the organization of transactions before their inclusion into blocks is different. In Bitcoin, transactions can remain in mempools of nodes. In Solana, they are sent to leaders.
