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Reaching consensus with Tendermint

https://anto.pt/gomad24

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in this talk

  • a couple known problems in distributed systems
  • an overview of Raft
  • an overview of Tendermint
  • writing a Go app using CometBFT

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not in this talk

  • precise or formal definitions

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A simple state machine

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A single node scenario

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What to do in case of multiple nodes?

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Two generals problem

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Two generals problem

impossible

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Raft

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Raft: leader election

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this is called eventual consistency

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Byzantine generals problem

even harder than the two

generals problem,

when you have bad friends

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BFT algorithms

Byzantine-Fault Tolerant

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Tendermint

Currently, CometBFT is the only production ready implementation

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Tendermint: leader election

  • Only valid for the current round
  • Deterministic, round-robin
  • A simple example:

Round: 123

Validators count: 5

Current leader: 123 % 5 = 3

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Leader sends

proposal

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Prevote phase

Each nodes checks if the proposal is valid or not, without actually writing anything to their state.

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Prevote phase

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Prevote phase

Each nodes wait for

at least ⅔ of OKs.

​

⅔ * 5 ~= 4

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Prevote phase

Each nodes wait for

at least ⅔ of OKs.

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⅔ * 5 ~= 4

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Precommit phase

Works exactly the same as the Prevote phase.

​

Each node acts independently. When it sees ⅔+ prevotes, it moves to the precommit phase.

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Commit phase

Each node updates its local state (i.e. to disk) when it sees ⅔+ Precommit votes.

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The non-validator nodes will commit the new state too.

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Repeat

Round: 124

Validators count: 5

Current leader: 124 % 5 = 4

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Let’s recap

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Building a Go app

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type Application interface {

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CheckTx(ctx, req) (res, error)

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FinalizeBlock(ctx, req) (res, error)

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Commit(ctx, req) (res, error)

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...

​

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type Application interface {

...

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Query(ctx,req) (res, error)

​

}

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Demo

MeteorKV @ https://github.com/Pitasi/x