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I am trying to understand how bitcoin core builds the list of block locator hashes needed for sendheaders and getheaders messages. I have looked on the wiki and I have seen that after the 10 most recent block hashes, it goes back exponentially until the genesis block.

So it seems that I have to get a block hash knowing its index in the blockchain. But how can bitcoin core find a block hash by its index? From my understanding leveldb lets you query data only by a key, and in this case the key is what I want.

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The answer is that it doesn't use LevelDB for this.

It has an in-memory data structure with all known block headers, each pointing back to its parent. It also has a vector with pointers to all block headers in the currently active chain.

To build locator objects, it simply looks in this vector at specific indexes.

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  • So every time bitcoin core receives a new block it has to update both this data and the database, am I right?
    – giacomocaironi
    Oct 22, 2020 at 21:34
  • Indeed, but the in-memory structure is very cheap to update.
    – Pieter Wuille
    Oct 22, 2020 at 21:35

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