Feature Upgrade

Overview of feature upgrade process and related issues in Ethereum and Ethereum Classic.

Feature upgrades in Ethereum and Ethereum Classic are usually conducted through hard fork.

Hard Fork

Hard fork feature upgrade process requires all participating nodes to be updated. This certainly requires a lot of coordination. As a result, client implementors regularly gathers together and decide on the next hard fork. A regular hard fork process contains the following steps:

  • Writers propose specifications to be included in a hard fork. Developers review those specifications and give feedbacks.

  • Developers vet on specifications to be included. Then, everyone agrees on an accepted list.

  • Developers go on to implement those new features in clients. In the mean time, testing starts.

  • Developers decide on a testnet hard fork block number.

  • If the testnet hard fork goes well, developers decide on a mainnet hard fork block number.

  • Finally, announce and ask miners, exchanges and users to upgrade.

Ethereum is currently adopting a new EIP-centric hard fork process.

Surprisingly, in Ethereum, a hard fork is usually decided on a date, then predice a block number, which then gets written into clients. This has created complications because block time varies, and thus it makes the block number hard to predict. A proposal to fix this is to use block’s timestamp as the factor for hard fork. However, the proposal received push-back from core developers, citing it may be over-engineering, and block timestamp might be gamed by miners.

For controversial features, the hard fork process had used signaling.

Soft Fork

Soft fork is a feature upgrade process that does not require all nodes to be updated. This is done by making upgraded nodes execute on a more restrictive set of state transition function than non-upgraded nodes. As long as the upgraded nodes are the majority, new applied restrictions will be followed.

Soft fork usually must require signaling. We need majority of upgraded nodes before the soft fork features are applied, in order to ensure non-upgraded nodes follow the same chain head. What’s more, due to the turing-complete nature of EVM, many restrictions related to state (for example, blacklisting accounts) cannot be implemented as soft forks. Otherwise, denial of service attacks will be possible.


Signaling allows everyone who participate in the coin to vote on whether a particular feature upgrade should happen.

Carbon Voting Process

Carbon vote allows coin holders to signal their preferences. One coin, one vote. Non-enforced carbon vote had happened in the past. The first notable event was the DAO hack. Coin holders used carbon vote to signal whether they support an irregular state transition from the hackers. The second notable one was the Parity multi-sig hack. Coin holders again used carbon vote to signal whether they support unfreeze the affected contracts.

Specifications for enforced carbon vote proposals have also been writte, such as 28-CARBONVOTE.

Miner Voting Process

Proposals: 27-MINERVOTE

Miner vote allows miners to use their mining power to signal their preferences. One hash, one vote.

A block has an extra data field, which allows miners to insert anything they want, in a limited length. This is usually how most miner vote process happens.

Informal miner vote just requires miners to configure their software so that the extra data is written in a particular pattern, for example, with a prefix or postfix, and then manually count them. This happened in Ethereum with the ProgPoW signaling.

Formal miner vote takes the voting process more seriously, and optionally bind it to an actual upgrade process. Once enough miners signal support, an upgrade will happen automatically. Those are processes such as 27-MINERVOTE.

Generalized Voting Process

Proposals: 36-STATEVOTE

Because Ethereum support turing-complete smart contracts, carbon and miner voting process can be generalized and unified. A proposal, 36-STATEVOTE, is designed for this. With minimal performance and backward compatibility overhead, any type of voting, be it carbon vote, miner vote, or a DAO style vote, can be applied with this framework.