Ethereum Core Developer Meeting Update 007: The Merge Architecture and Beyond

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The Ethereum ecosystem continues its transformation toward a more scalable, secure, and sustainable future. In this update from the 124th core developer meeting, we dive into the progress toward The Merge, the upcoming transition from proof-of-work (PoW) to proof-of-stake (PoS). With key milestones like Arrow Glacier and advancements in client interoperability through events like Amphora, the network edges closer to one of the most significant upgrades in blockchain history.

This article unpacks the post-merge architecture, explores the roles of execution and consensus layers, explains the new Engine API, and outlines what developers and users should expect in the lead-up to 2025.

Arrow Glacier: Delaying the Difficulty Bomb

At the 124th core developer meeting, participants reached consensus on delaying the difficulty bomb—a mechanism designed to gradually increase mining difficulty and render PoW mining unviable. The decision was made to push its activation to June 2022 via the Arrow Glacier network upgrade.

👉 Discover how Ethereum’s transition impacts network stability and validator rewards.

This upgrade, defined by EIP-4345, will activate at block 13,773,000—expected around December 8, 2021. The primary goal? To buy time for The Merge without forcing another emergency hard fork before the full transition.

It's crucial to understand that The Merge and the difficulty bomb are independent. The Merge activates based on a predefined total difficulty threshold (TERMINAL_TOTAL_DIFFICULTY), not on the bomb’s detonation. This means:

With Arrow Glacier likely being the last PoW-focused upgrade, Ethereum is signaling its final steps away from energy-intensive mining.

The Post-Merge Architecture: Execution and Consensus Layers

After The Merge, Ethereum will operate using two distinct but interconnected layers:

Both layers rely on existing, battle-tested clients. Node operators who previously ran only an execution or consensus client will now need to run both to maintain a full validating node.

Danny Ryan’s widely shared architectural diagrams—now immortalized as NFTs with proceeds funding core developers—illustrate this duality perfectly. Let’s explore each component.

The Beacon Node: Guardian of Consensus

Currently, beacon nodes achieve consensus on empty blocks containing only attestation data, deposits, and slashing records. Post-Merge, these blocks will include ExecutionPayloads—the PoS equivalent of traditional Ethereum blocks.

Here’s how it works:

  1. The beacon node monitors the PoW chain.
  2. When a block reaches or exceeds TERMINAL_TOTAL_DIFFICULTY, it becomes the final PoW block.
  3. All subsequent blocks are produced by validators on the beacon chain.

To validate new blocks, beacon nodes communicate with their paired execution client via the Engine API—a secure, private interface separate from public RPC endpoints.

Engine API: Bridging Two Worlds

The Engine API enables seamless communication between consensus and execution layers. It introduces three critical methods:

engine_executePayload

Requests the execution client to validate an ExecutionPayload. The response can be:

Validation is recursive—each block depends on its parent’s validity—so clients may fetch historical data if needed.

engine_forkchoiceUpdated

Informs the execution layer about changes in chain head or finalized blocks. If a validator needs to propose a new block, this call includes payloadAttributes, which specify:

Upon receiving this, the execution engine updates its view of the chain and begins building a candidate payload.

engine_getPayload

Requests the best available ExecutionPayload from the execution engine. Validators use this when they’re selected to produce a block.

These three methods form the backbone of cross-layer coordination—ensuring consistency, security, and continuity during and after The Merge.

The Execution Engine: Powering State and Transactions

Post-Merge, execution clients (like Geth or Nethermind) become execution engines, stripped of mining logic but retaining core responsibilities:

Block Format Changes (via EIP-3675)

Several PoW-specific fields are zeroed out:

This streamlines the block structure for PoS while maintaining backward compatibility.

Reward System Overhaul

Block rewards no longer exist at the execution layer. Instead:

Validators must ensure senders pay at least the current baseFeePerGas. This maintains economic security and prevents spam.

Network Behavior Adjustments

👉 Learn how staking evolves after The Merge and what it means for yield opportunities.

These changes reflect Ethereum’s shift from decentralized mining to decentralized validation—where economic stake replaces computational work.

P2P Networking, APIs, and Synchronization

Post-Merge, both layers coexist on the peer-to-peer network:

User-facing APIs remain separate:

Synchronization strategies are under active development. Clients must handle edge cases like merging mid-sync or recovering from long-range attacks. These efforts will likely be detailed in future technical deep dives.

What’s Next? Finalizing Specs and Testnets

Following the successful Amphora interoperability milestone, focus has shifted to finalizing specifications and testing multi-client setups on development networks like Pithos.

Pithos runs daily test scenarios with diverse client combinations, ensuring robustness across implementations. A community call is scheduled soon to align infrastructure providers and tooling teams ahead of broader deployment.

The specification is nearing finalization—with minimal expected changes moving forward. This stability paves the way for wider testnet rollouts and eventual mainnet activation.

Frequently Asked Questions (FAQ)

Q: What happens to miners after The Merge?
A: Mining will cease as Ethereum transitions to proof-of-stake. Miners can repurpose hardware or explore other PoW chains, but ETH block production will be fully validator-driven post-Merge.

Q: Do I need to do anything as an ETH holder?
A: No action is required. Your funds remain safe, and balances are unaffected. The transition is seamless at the user level.

Q: Will gas fees decrease after The Merge?
A: Not immediately. The Merge focuses on consensus change, not scalability. Fee reductions depend on future upgrades like sharding and rollups.

Q: Can The Merge be delayed again?
A: Yes. While timelines aim for 2025, technical challenges could prompt adjustments. However, development momentum remains strong.

Q: How does staking work after The Merge?
A: Stakers continue validating blocks and earning rewards. Post-Merge improvements enhance security and reduce centralization risks in validator selection.

Q: Is there a risk of chain splits?
A: Minimal. Multi-client diversity and extensive testing reduce split risk. The community has shown strong coordination around upgrades historically.

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With core developers closing in on specification finalization, Ethereum stands at the threshold of a new era—one defined by efficiency, sustainability, and long-term scalability. As we approach 2025, all signs point to a successful Merge, setting the stage for further innovation across decentralized applications, DeFi, and Web3 infrastructure.