What is a Modular Blockchain?
A modular blockchain separates the traditionally combined responsibilities of a blockchain—execution, consensus, settlement, and data availability—into distinct, specialized layers that can be independently optimized, upgraded, and combined. This stands in contrast to monolithic blockchains (like Bitcoin, Solana, or early Ethereum), where all four functions are handled by a single integrated protocol. The modular thesis, popularized by the data availability layer Celestia (founded by Mustafa Al-Bassam, John Adler, and others in 2019), argues that decoupling these functions enables faster innovation, better scalability, and more efficient resource allocation than the monolithic approach.
The four core functions of any blockchain are:
- Execution: Processing transactions and updating the state (running smart contracts, transferring tokens, etc.)
- Consensus: Agreeing on the ordering of transactions and the canonical chain
- Settlement: Providing a finality guarantee—confirming that a state transition is valid and irreversible
- Data Availability (DA): Ensuring that the transaction data needed to reconstruct the state is actually accessible to all participants
In a monolithic blockchain, all four functions are tightly coupled—every node must execute every transaction, participate in consensus, store all data, and provide settlement. This coupling means that improving one function (e.g., faster execution) is constrained by the limitations of the others (e.g., slow consensus or data storage). In a modular architecture, each function is handled by a specialized layer that can be optimized independently.
The modular approach has become one of the dominant design paradigms in Web3. Ethereum itself has evolved from a monolithic chain toward a modular architecture through its rollup-centric roadmap, where L1 focuses on consensus, settlement, and DA while rollups handle execution. Celestia, EigenDA, and Avail are purpose-built DA layers. Rollups like Arbitrum, Optimism, and zkSync are execution layers. The settlement function is evolving through smart contract standards like OP Stack’s fault proof system.
How It Works
The Four Pillars
Execution Layer handles transaction processing and state management. In the modular stack, this is the role of rollups (Arbitrum, Optimism, zkSync) and app-chains (Cosmos zones, Avalanche subnets). The execution layer receives transactions, orders them (via a sequencer), executes them, and produces state commitments. It does NOT need to handle consensus or data availability directly—those are delegated to other layers.
Key properties of a good execution layer:
- High throughput (thousands to millions of TPS)
- Fast confirmation times (sub-second to seconds)
- Programmability (EVM support, custom VMs)
- Flexible upgrade paths (can upgrade without L1 hard fork)
Consensus Layer orders transactions and ensures that all honest participants agree on the canonical chain. In the modular stack, the consensus layer is typically the L1 blockchain (Ethereum, Celestia) or a dedicated consensus protocol. The consensus layer does NOT need to execute transactions—it only needs to order and timestamp data commitments.
Settlement Layer provides the finality guarantee. It is the layer where state roots or validity proofs are posted and verified, making transaction outcomes irreversible. Settlement is conceptually the “court of last resort”—if there is a dispute about the correct state, the settlement layer resolves it. On Ethereum, settlement happens through L1 smart contracts that verify fraud proofs or validity proofs.
Data Availability Layer ensures that the transaction data needed to verify or reconstruct the state is published and accessible. This is the most critical innovation in modular architectures. Without guaranteed data availability, users cannot verify rollup state transitions or force-exit their funds. The DA layer provides this guarantee, typically through techniques like erasure coding and data availability sampling (DAS).
The Modular Stack in Practice
A typical modular transaction flow:
- User submits a transaction to the execution layer (rollup sequencer)
- Execution layer processes the transaction, updates its local state, and posts the transaction data (compressed) to the data availability layer
- The execution layer posts a state commitment (state root) to the settlement layer
- The consensus layer (typically embedded in the DA or settlement layer) orders these commitments
- Verifiers (light clients, full nodes, or provers) can check the DA layer to confirm data is available, and check the settlement layer to confirm the state commitment is valid
- If the state commitment is disputed (optimistic) or invalid (ZK), the settlement layer’s verification mechanism resolves it
Monolithic vs. Modular: A Detailed Comparison
| Dimension | Monolithic (Solana) | Modular (Ethereum + Rollups) |
|---|---|---|
| Execution | On-chain, single chain | Off-chain, multiple rollups |
| Consensus | Integrated (Tower BFT) | L1 (Casper FFG + LMD GHOST) |
| Settlement | Same as consensus | L1 contracts (rollup bridges) |
| Data availability | Integrated | L1 blobs / Celestia / EigenDA |
| Max throughput | Limited by single chain | Scales with number of rollups |
| Upgrade flexibility | Hard fork required | Each layer upgrades independently |
| State bloat | All state on one chain | State distributed across L2s |
| Developer choice | One execution environment | Choose any VM (EVM, SVM, MoveVM) |
Real-World Examples
Celestia is the first purpose-built modular data availability layer, launched as a mainnet in October 2023. It uses a novel consensus mechanism (Tendermint) combined with data availability sampling (DAS) to provide cheap, scalable DA. Celestia’s block space is used by rollups like Rollkit (sovereign rollups), Dymension (rollapp hub), and Eclipse (SVM rollup on Celestia DA). Celestia’s key innovation is Namespaced Merkle Trees (NMTs), which allow multiple rollups to share the same block space while maintaining separate data namespaces.
Ethereum has become increasingly modular through its rollup-centric roadmap. EIP-4844 (March 2024) introduced blob-carrying transactions as the first step of modular DA. Future upgrades (PeerDAS, full danksharding) will expand this into a fully sharded DA layer. Ethereum’s settlement role has also evolved—L1 contracts like the OptimismPortal (Optimism) and the Bridge contract (Arbitrum) provide on-chain settlement for L2 states.
EigenDA (by EigenLayer) takes a different approach to modular DA. Instead of running a separate blockchain, EigenDA restakes Ethereum validators to provide DA services. Validators who are already staking ETH on the Beacon Chain can optionally opt in to provide DA verification for rollups. This approach leverages Ethereum’s existing security without requiring a separate token or validator set.
Polygon 2.0 (later rebranded as Polygon CDK—Chain Development Kit) represents a modular framework where each component (execution via zkEVM, DA via Polygon Avail or custom DA, sequencer via custom or shared) can be independently configured. Polygon Avail (now an independent project) launched its own DA-focused mainnet in July 2024.
OP Stack (by Optimism) is a modular rollup framework that separates execution (the rollup node), settlement (OptimismPortal on L1), consensus (sequencer ordering), and data availability (calldata or blobs). Multiple chains built on the OP Stack (Base, Zora, Mode, Metal, ZKsync’s ZK stack is separate) form the “Superchain,” sharing a common settlement layer and interoperability protocol.
Key Risks / Considerations
- Interoperability complexity: In a modular world, cross-rollup communication requires bridging, which adds latency, cost, and security risk. The Superchain and IBC aim to solve this, but cross-modular communication remains immature.
- DA centralization: If most rollups use the same DA layer, a DA layer outage or attack could simultaneously affect all dependent rollups. Celestia’s validator set is smaller than Ethereum’s, raising centralization concerns.
- Settlement fragmentation: Different rollups settle on different L1 contracts with different verification mechanisms, creating a fragmented settlement landscape.
- Economic sustainability: Modular layers must each have their own economic model and fee revenue. If execution layers (rollups) don’t generate enough fee revenue to pay for DA and settlement costs, the modular stack may not be economically viable long-term.
- Complexity for users: Users must understand which layers they’re interacting with, manage gas on multiple networks, and navigate bridging UX—a significantly more complex experience than a monolithic chain.
Frequently Asked Questions
Q: Is Ethereum monolithic or modular? A: Ethereum is transitioning from monolithic to modular. It still handles consensus, settlement, and DA at the L1 level, but execution has been largely offloaded to rollups. With the implementation of danksharding, Ethereum’s L1 will function primarily as a DA and settlement layer—fully modular in the data availability dimension.
Q: Is Solana modular? A: No. Solana is explicitly monolithic—all four functions (execution, consensus, settlement, DA) are handled by a single integrated protocol. Solana’s design philosophy argues that modularity adds latency and complexity that can be avoided by vertical integration. The debate between monolithic and modular architectures is one of the most active in Web3.
Q: What is a “sovereign rollup”? A: A sovereign rollup is a rollup that settles on a DA layer (like Celestia) rather than on a smart contract on a settlement layer (like Ethereum). Sovereign rollups define their own rules for state transitions and validity proofs, rather than relying on a settlement contract. This gives them more flexibility but less composability with other rollups that share the same settlement layer.
Q: Can you mix and match modular components? A: Yes—that’s the entire point. A rollup could use Celestia for DA, Ethereum for settlement, and its own sequencer for execution. The OP Stack allows swapping DA providers (currently calldata and blobs, with EigenDA integration planned). This composability is the core value proposition of modularity.