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93 lines
6.4 KiB
93 lines
6.4 KiB
---
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title: The Merge
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sort_key: A2
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---
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The Merge is probably the most significant upgrade to Ethereum since Homestead. This page explains what The Merge is
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and how it will affect Geth users. Instructions on how to configure an Ethereum node in
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anticipation of The Merge are provided on the [Consensus Clients page](/docs/interface/consensus-clients).
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## What is The Merge
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[The Merge](https://ethereum.org/en/upgrades/merge/) is an upcoming upgrade to Ethereum that swaps out its current
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proof-of-work (PoW) consensus mechanism for a proof-of-stake (PoS) mechanism. The term "Merge" refers
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to the fact that the initial phase of the transition involved launching a PoS chain in parallel with
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the existing PoW Ethereum Mainnet. This PoS chain, known as the Beacon Chain, has been executing the PoS
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consensus logic since November 2020 but without integrating real Ethereum transaction data. The Merge refers
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to the moment when Ethereum's existing blockchain and the Beacon Chain join together to form one unified chain.
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At the moment of the merge, execution clients will switch off their proof-of-work and block gossiping functions
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and hand responsibility for all consensus and fork choice logic over to consensus clients. This is a profound
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change to how Ethereum operates and it will require node operators to run a consensus client in addition to
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Geth.
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## How will Geth change?
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Geth is an execution client. Historically, an execution client alone has been enough to run a full Ethereum node.
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However, when The Merge happens, Geth will not be able to track the Ethereum chain on its own. Instead, it will need to
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be coupled to another piece of software called a ["consensus client"][con-client-link]. In this configuration,
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the execution client will be responsible for transaction handling, transaction gossip, state management and
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the Ethereum Virtual Machine (EVM). However, Geth will no longer be responsible for block building, block
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gossiping or handling consensus logic. These will be in the remit of the consensus client.
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For Geth users that intend to continue to run full nodes after The Merge, it is sensible to start running
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a consensus client in advance, so that The Merge can happen smoothly. There are five consensus clients available, all
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of which connect to Geth in the same way. Instructions for this are available on the
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[Consensus Clients page](/docs/interface/consensus-clients).
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## Client architecture
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The relationship between the two Ethereum clients is shown in the schematic below. The two clients each connect
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to their own respective peer-to-peer (P2P) networks. This is because the execution clients gossip transactions over
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their P2P network enabling them to manage their local mempool. The consensus clients gossip blocks over their P2P
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network, enabling consensus and chain growth.
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For this two-client structure to work, consensus clients must be able to pass bundles of transactions to Geth
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to be executed. Executing the transactions locally is how the client validates that the transactions do not
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violate any Ethereum rules and that the proposed update to Ethereum's state is correct. Likewise, when the node
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is selected to be a block producer the consensus client must be able to request bundles of transactions from Geth to
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include in the new block. This inter-client communication is handled by a local RPC connection using the
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[`engine` API][engine-api-link] which is part of the JSON-RPC API exposed by Geth.
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## Transition
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The transition from PoW to PoS will happen when a pre-announced total difficulty is reached by the chain.
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This is different to other forks which usually occur at a certain scheduled block number. The total difficulty threshold
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that will trigger the Merge is also known as the [*Terminal
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Total Difficulty* (TTD)](https://ethereum.org/en/glossary/#terminal-total-difficulty). In
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case of an emergency delay, the TTD can be overriden in Geth using the `--override.terminaltotaldifficulty` command-line
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flag. Once the merge block exists, Geth will no longer be able to follow the head of the chain without a consensus
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client. If Geth is not connected to a consensus client it will simply stall at the merge block.
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Assuming a consensus client is connected in advance, the two clients will automatically
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handle the merge together with no disruption to the user.
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## Using Geth after The Merge
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Although The Merge is a profound change to Ethereum's underlying achitecture, there will be minimal changes to how Geth
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users interact with Ethereum. At The Merge responsibility for consensus logic and block propagation are handed over to
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the consensus layer, but all of Geth’s other functionality remains intact. This means transactions, contract deployments
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and data queries can still be handled by Geth using the same methods as before. This includes interacting with Geth using
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the JSON_RPC_API directly using tools such as [curl](https//curl.se), third party libraries such as
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[Web3.js][web3js-link] or [Web3.py][web3py-link] in development frameworks, e.g. [Truffle][truffle-link], [Hardhat][hardhat-link],
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[Brownie][brownie-link], [Foundry][foundry-link] or using Web3.js in Geth's built-in Javascript console.
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Much more information about the Javascript console is available on the [Javascript Console page](/docs/interface/javascript-console).
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## Summary
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The Merge is an upcoming upgrade to Ethereum that swaps the existing PoW consensus mechanism for a PoS consensus
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mechanism. This changes the client software organization for Ethereum nodes. After The Merge, nodes are required to
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run both execution and consensus clients that each have responsibility for specific parts of the Ethereum protocol
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and communicate with each other over a local RPC connection. Although there is some necessary configuration in advance
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of The Merge, the experience for Geth users should change minimally as a result of The Merge.
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[engine-api-link]: https://github.com/ethereum/execution-apis/blob/main/src/engine/specification.md
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[cl-list]: https://ethereum.org/en/developers/docs/nodes-and-clients/#consensus-clients
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[web3py-link]: https://web3py.readthedocs.io/en/stable/web3.main.html
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[web3js-link]: https://web3js.readthedocs.io/en/v1.2.9/
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[brownie-link]: https://eth-brownie.readthedocs.io/en/stable/
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[truffle-link]: https://trufflesuite.com/
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[hardhat-link]: https://hardhat.org/
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[foundry-link]: https://github.com/foundry-rs/foundry)
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[con-client-link]:https://ethereum.org/en/glossary/#consensus-client
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