Custom blockchain indexers are one of those projects that look like a week of work and turn into a quarter of work. Not because the core loop is hard, but because every edge case compounds.
When you need Bitcoin data in your application, you have two fundamental options: query a node's JSON-RPC API directly, or use an indexed API built on top of the blockchain data.
"Just run your own indexer" is common advice in blockchain development circles. It's not wrong advice - but it undersells what "just run" really involves.
Every time you deploy a new Bitcoin indexer - or migrate to a new server - you face the same problem: you need to process 880,000+ blocks from genesis to the current tip. How you do it determines whether this takes 6 hours or 6 days.