Electrum Server

Electrum Server, or electrs, is the piece that lets ordinary Bitcoin wallets talk to your own node. It builds an index over every transaction input and output in the blockchain, turning the question of what a given address holds and has ever received — something a bare node answers slowly or not at all — into an instant lookup.

This matters because of how most wallets actually work. Point Electrum, Sparrow, BlueWallet, or a hardware wallet's companion app at a public Electrum server and you hand that operator your addresses, your balances, and your IP address, which they can link together and watch indefinitely. Point it at your own server and there is nobody to tell.

It is a Rust reimplementation built specifically for this case: modest hardware, personal scale. The index adds only around ten percent on top of the blockchain itself, because it relies on your local node to retrieve full transactions rather than storing them a second time, and a mempool tracker gives connected wallets better fee estimates.

Several other apps here depend on it — BTC RPC Explorer can use it for full address history, and Mempool relies on it for address lookups. Worth heeding the developers' advice: this build is tuned for personal or family-scale use and is not meant to be exposed publicly, where it would be an easy target.

Once it has finished indexing, it is the quiet piece of infrastructure that makes every wallet you own private by default.