Consensus model

The consensus model in the codebase can be thought of as a database of the current state of the blockchain. When a new block is learned about it is processed and the consensus code must determine which block is the current best. Consensus can be thought of as a function of available information — it’s output is simply a deterministic function of its input.

There are a simple set of rules for determining the best block:

1. Only consider valid blocks

2. Where multiple chains exist choose the one with the most cumulative Proof of Work (PoW)

3. If there is a tie-breaker (same height and work), then use first-seen

The result of these rules is a tree-like structure from genesis to the current day, building on only valid blocks.

Whilst this is easy-enough to reason about in theory, the implementation doesn’t work exactly like that. It must consider state, do I go forward or backwards for example.

Validation in Bitcoin Core

Originally consensus and validation were much of the same thing, in the same source file. However splitting of the code into strongly delineated sections was never fully completed, so validation.* files still hold some consensus codepaths.