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61 lines
2.6 KiB
Markdown
61 lines
2.6 KiB
Markdown
# Tendermint
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An implementation of the Tendermint state machine in Rust.
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This is solely the state machine, intended to be mapped to any arbitrary system.
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It supports an arbitrary signature scheme, weighting, and block definition
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accordingly. It is not intended to work with the Cosmos SDK, solely to be an
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implementation of the [academic protocol](https://arxiv.org/pdf/1807.04938.pdf).
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### Caveats
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- Only SCALE serialization is supported currently. Ideally, everything from
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SCALE to borsh to bincode would be supported. SCALE was chosen due to this
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being under Serai, which uses Substrate, which uses SCALE. Accordingly, when
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deciding which of the three (mutually incompatible) options to support...
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- The only supported runtime is tokio due to requiring a `sleep` implementation.
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Ideally, the runtime choice will be moved to a feature in the future.
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- It is possible for `add_block` to be called on a block which failed (or never
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went through in the first place) validation. This is a break from the paper
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which is accepted here. This is for two reasons.
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1) Serai needing this functionality.
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2) If a block is committed which is invalid, either there's a malicious
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majority now defining consensus OR the local node is malicious by virtue of
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being faulty. Considering how either represents a fatal circumstance,
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except with regards to system like Serai which have their own logic for
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pseudo-valid blocks, it is accepted as a possible behavior with the caveat
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any consumers must be aware of it. No machine will vote nor precommit to a
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block it considers invalid, so for a network with an honest majority, this
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is a non-issue.
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### Paper
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The [paper](https://arxiv.org/abs/1807.04938) describes the algorithm with
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pseudocode on page 6. This pseudocode isn't directly implementable, nor does it
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specify faulty behavior. Instead, it's solely a series of conditions which
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trigger events in order to successfully achieve consensus.
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The included pseudocode segments can be minimally described as follows:
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```
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01-09 Init
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10-10 StartRound(0)
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11-21 StartRound
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22-27 Fresh proposal
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28-33 Proposal building off a valid round with prevotes
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34-35 2f+1 prevote -> schedule timeout prevote
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36-43 First proposal with prevotes -> precommit Some
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44-46 2f+1 nil prevote -> precommit nil
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47-48 2f+1 precommit -> schedule timeout precommit
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49-54 First proposal with precommits -> finalize
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55-56 f+1 round > local round, jump
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57-60 on timeout propose
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61-64 on timeout prevote
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65-67 on timeout precommit
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```
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The corresponding Rust code implementing these tasks are marked with their
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related line numbers.
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