Tendermint

Tendermint is software that lets a network of computers agree on the order of transactions even if some of them fail or act maliciously, and it was one of the first practical consensus engines designed for proof-of-stake blockchains. It has two main parts: a consensus algorithm based on Byzantine fault tolerance, and an interface that separates the consensus layer from the application logic of a blockchain. Developers can therefore build a chain by writing only their application and plugging it into Tendermint for networking and agreement.

Tendermint was created by Jae Kwon, who described it in a 2014 paper, and developed with Ethan Buchman; the two later co-founded the Cosmos project, whose hub chain launched in 2019 on Tendermint. Many chains in the Cosmos ecosystem and beyond have used it. In 2023 the main implementation was forked and renamed CometBFT, which is maintained as the continuation of Tendermint Core, so the two names are often used interchangeably.

Picture a committee that must sign off on the minutes of every meeting. One member, taking turns, proposes the minutes. The others look them over and say "I'm happy" in a first round, and then, once they see that enough colleagues agree, "I'm committed" in a second round. As soon as more than two-thirds of the committee have committed, the minutes are final and can never be reopened.

Tendermint runs a process like that for each block. A rotating proposer suggests the next block, validators vote twice, and once enough votes are in, the block is final straight away. Unlike some blockchains, where you wait for more blocks to pile up before you feel safe, a block agreed under Tendermint does not get reversed, as long as fewer than a third of the voters are dishonest.

Tendermint's consensus proceeds in rounds for each block height:

1. Propose. A validator chosen in turn, weighted by stake, broadcasts a proposed block.
2. Prevote. Each validator signs a prevote for the block if it is valid, or for nothing if no valid proposal arrives in time.
3. Precommit. If a validator sees prevotes from more than two-thirds of the voting power, it signs a precommit.
4. Commit. When precommits from more than two-thirds of the voting power are collected, the block is committed and final.

If a round fails, for example because the proposer is offline, the network moves to a new round with a new proposer. Voting power is usually tied to staked tokens under proof of stake, and validators who sign conflicting votes can be punished by slashing. The design tolerates up to one-third of voting power being faulty or malicious, the classic limit for solving the Byzantine generals problem in this setting.

A distinctive feature of Tendermint is the Application Blockchain Interface (ABCI), which defines how the consensus engine talks to the application. Tendermint orders and finalizes transactions; the application, written in any programming language, decides whether each transaction is valid and how it changes state. This split made it possible to build very different blockchains, such as exchanges, payment networks or general-purpose platforms, on the same consensus software. The Cosmos SDK, a framework for building appchains, sits on top of ABCI, and chains built this way can connect through the Inter-Blockchain Communication protocol.

Tendermint offers what is often called instant or deterministic finality: once a block is committed, it is final, which suits exchanges and payment applications that need certainty. This contrasts with the probabilistic finality of proof-of-work chains such as Bitcoin, where the chance of reversal shrinks with each new block but never quite reaches zero. Tendermint chains typically produce blocks every few seconds.

The trade-off is in liveness. If more than a third of voting power goes offline, the chain stops producing blocks rather than risk inconsistency, favouring safety over availability. Because each validator must communicate with every other in each round, the number of validators is usually capped, often in the low hundreds, to keep message traffic manageable.

The Cosmos Hub, launched in March 2019, was the flagship Tendermint chain, and many independent networks followed, including exchange chains, privacy networks and DeFi platforms. Tendermint also influenced the design of other proof-of-stake systems and was compared with frameworks such as Substrate. The company originally behind it went through several changes of name and leadership. In early 2023, maintainers from Informal Systems and the Interchain Foundation forked Tendermint Core into CometBFT, which became the actively developed version used across the Cosmos ecosystem.

Critics note that the cap on validator numbers concentrates block production among a relatively small set of operators, many of them large staking businesses. The tendency to halt under heavy faults can cause outages, and several Tendermint-based chains have stopped for hours during bugs or upgrades. As with other proof-of-stake systems, stake tends to concentrate with the largest validators, and the security of each chain depends on the value of its own token. Supporters respond that clear finality and a flexible application interface made Tendermint one of the most widely reused pieces of blockchain infrastructure, alongside other forms of practical Byzantine fault tolerance.

• CometBFT Documentation
• Cosmos