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name: bitcoin-infrastructure-fulcrum description: | Fulcrum: Calin Culianu's Electrum-protocol indexer in C++. Faster than electrs for high-volume queries. Serves Electrum clients + LN nodes. USE WHEN: deploying high-load Electrum server, comparing with electrs. allowed-tools: Read, Grep, Glob

Fulcrum

C++ Electrum-protocol server. Faster than electrs for query throughput, at higher RAM cost.

Repo: github.com/cculianu/Fulcrum.

Setup

Bitcoin Core with txindex=1. Then Fulcrum:

[options]
datadir = /var/lib/fulcrum
bitcoind = 127.0.0.1:8332
rpcuser = ...
rpcpassword = ...
tcp = 0.0.0.0:50001
ssl = 0.0.0.0:50002

Indexes in 4 to 20+ hours on SSD, per the upstream README (a figure carried unchanged there since v1.0, January 2020). Budget a lot of disk: the same README recommends ~133 GB for the BTC mainnet database "as of Aug 2023" - still the figure quoted in September 2026 - on top of the node's own blocks.

Current release 2.1.2 (August 2026). Fulcrum 2.0.0 (September 2025) rewrote the database format: the datadir is crash-safe and platform-neutral, fast-sync / utxo_cache are gone (use db_mem), and a 1.x datadir needs a one-time --db-upgrade run to convert.

When to choose Fulcrum

• High-traffic public Electrum server.
• Power users with multiple wallets connecting.
• BTCPay backend with many invoices.

When to choose electrs

• Personal use, low traffic.
• Resource-constrained devices.
• Simpler config preferred.

Note the node-side prerequisites differ: electrs v0.12.0 (13 September 2026) indexes over bitcoind's REST interface and needs Bitcoin Core 31.0+ with rest=1, while Fulcrum works against Core v0.17.0+ over JSON-RPC with txindex=1.

Compatibility

Same as electrs — Sparrow, Electrum, BTCPay, etc.

Common bugs

• High RAM usage (8+ GB) → tune db_mem, db_max_open_files.
• TLS cert mismatch on SSL endpoint → wallets fail to connect.
• Bitcoin Core RPC connection drops → Fulcrum reconnects but may miss blocks; restart in extreme cases.

See also

• electrs/SKILL.md
• esplora/SKILL.md
• btcpay/SKILL.md