Mempool
What is the mempool in blockchain?
The mempool is public by design, and that is both the feature and the exposure. Open propagation is how any node can verify and relay a pending transaction without asking permission. It also means a broadcast transaction is a public statement of intent, complete with its price impact, sitting in the open for the seconds before it settles.
That gap is where extraction happens. A large swap announces exactly what it will do to a pool before it does it, and anyone watching can trade ahead of it, behind it, or both. The transaction is not leaked. It is published, which is a considerably harder problem to design around.
The response has been to route around the public queue entirely. Private endpoints such as Flashbots Protect send transactions straight to block builders instead of broadcasting them, and since the Merge in September 2022 the large majority of Ethereum blocks have been assembled by a small set of specialist builders. A growing share of transaction flow now never appears in the public mempool at all.
Equity markets arrived at the same place decades earlier, when visible order flow pushed institutional size into dark venues. The practical question for a wallet holder is narrow. For any trade large enough to move a price, know whether your wallet broadcasts publicly or privately, and what the private route asks of you in return.