Question about getbestblockhash

[BlackHatCoiner]

Command getbestblockhash returns the latest block hash that has been mined. But, in case of two blocks being broadcasted at the same time, there are two chains, with the same height, and the next block will determine which one has the most work (AKA, is the right one).

My question is: How does getbestblockhash determine what is the best block hash in that case? Is best block hash the first that the node receives, and ignores the rest that come later? Shouldn't it return an array of best block hashes since there is potentially more than one?

[ranochigo]

The getbestblockhash by definition should only return the tip of the most work and fully validated chain. In this case for the same height, the block that has arrived earlier would be the best chain for your node. Since nodes can still propagate the headers of competing chain, you can still refer to getchaintips to observe blocks that are on competing chains.

Conventionally, Bitcoin Core has always considered whichever chain that has the highest work and arrived first to be the best.

[BlackHatCoiner]

The getbestblockhash by definition should only return the tip of the most work and fully validated chain.

If both chains have equal work, then there are either two tips, or no tips. Picking one would mean you're introducing subjectivity.

In this case for the same height, the block that has arrived earlier would be the best chain for your node.

But, why? It can very well be the other chain, the correct one, because miners mine on top of the other tip.

Since nodes can still propagate the headers of competing chain, you can still refer to getchaintips to observe blocks that are on competing chains.

I didn't know about getchaintips. Thank you. I think this answers my question.

[ranochigo]

If both chains have equal work, then there are either two tips, or no tips. Picking one would mean you're introducing subjectivity.

Fully validated in that case. If there are any competing blocks at the same height, then Bitcoin Core doesn't immediately validate it until the competing block becomes the new longest chain or isn't validated at all if it becomes orphaned.

But, why? It can very well be the other chain, the correct one, because miners mine on top of the other tip.

Traditionally, the reference implementation is used for mining as well. Miners are thus mining directly ontop of the best tip and it's better to start work on a chaintip that is better propagated, in the view of the individual node. Since each node has no way of knowing what is the block that is seen (and currently worked on) by the majority, it's better to prefer the block that is seen first and not having to swap over when another block at the same height appears.

Note: I don't think this is the actual reason, but the actual reason should be just for the sake of simplicity. This was a conclusion that I made after some discussion and research previously.