Understanding the fundamental mechanisms that enable Layer 2 networks to scale Ethereum by processing transactions off-chain.
LABS
Guides
Batch Transactions in Layer 2 DeFi
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core_concepts
Core Concepts of Transaction Batching
01
State Commitment
State roots are cryptographic commitments of the L2's state posted to the L1. This creates a verifiable anchor point for the entire system's data, including account balances and contract storage. The L1 acts as a secure ledger of record, allowing anyone to verify the correctness of the L2's execution by checking these commitments against fraud or validity proofs.
02
Data Availability
Transaction data must be published and accessible on-chain for security. For optimistic rollups, this data is posted in calldata or blobs, allowing independent verifiers to reconstruct the L2 state and submit fraud proofs. Without guaranteed data availability, the system cannot be trustlessly verified, making it a critical component for censorship resistance and security.
03
Proof Systems
Validity proofs (ZK) and fraud proofs (Optimistic) are cryptographic methods to verify batch correctness. ZK-rollups generate a succinct proof for every batch, while optimistic rollups assume correctness unless challenged. The proof system defines the trust model, finality time, and computational overhead, directly impacting user experience and security guarantees.
04
Sequencer Role
The sequencer is a node that orders transactions into a batch for submission to L1. It provides instant soft confirmations to users and can offer front-running protection. While often centralized for efficiency, decentralized sequencer sets are an active area of research to enhance censorship resistance and liveness of the L2 network.
05
Batch Finality
Finality refers to the irreversible inclusion of a transaction. L1 finality is achieved when the batch is confirmed on Ethereum. L2 soft finality is immediate from the sequencer. For optimistic rollups, a challenge period (e.g., 7 days) is required for full economic finality, during which fraud proofs can be submitted.
06
Cost Economics
Batching amortizes fixed L1 costs (calldata, proof verification) across many transactions. Gas efficiency is achieved by compressing data and moving computation off-chain. The cost model involves L1 data fees, L2 execution fees, and sequencer/prover operational costs, which determine the ultimate savings passed to the end-user.
Batching Implementations Across L2s
Understanding Transaction Batching
Transaction batching is a core scaling technique where multiple user operations are grouped into a single batch before being submitted to the base layer (like Ethereum). This drastically reduces the cost per transaction for users. Instead of paying for individual on-chain verification, users share the fixed cost of posting the batch's data or proof.
How It Works on Different L2s
- Optimistic Rollups (Arbitrum, Optimism): Transactions are executed off-chain and the results are posted as a batch of calldata to Ethereum. A fraud proof window allows challenges, ensuring security.
- ZK-Rollups (zkSync Era, Starknet): Transactions are executed off-chain and a cryptographic validity proof (ZK-proof) is generated for the entire batch. Only this compact proof is posted to L1, offering finality.
- Validiums (Immutable X): Similar to ZK-Rollups, but transaction data is stored off-chain with a data availability committee, further reducing costs but with different trust assumptions.
Practical Example
When swapping tokens on a DEX like Uniswap deployed on Arbitrum, your trade is combined with hundreds of others in a batch. You only pay a fraction of the L1 gas fee, making the swap affordable even during network congestion.
User Workflow for Batched DeFi Interactions
Process overview for planning, constructing, and executing a batch of transactions on an L2.
1
Define the Transaction Sequence
Plan the logical order of operations for your DeFi strategy.
Detailed Instructions
Identify the interdependent actions that form your strategy, such as a leveraged yield farm. The sequence is critical for state consistency and gas efficiency. For example, you cannot supply collateral you do not yet own.
- Sub-step 1: Map dependencies: List each contract call and its required inputs. A flash loan must precede the swap, which must precede the liquidity provision.
- Sub-step 2: Check approval states: Determine which tokens need ERC-20
approve()calls for the involved protocols. Batch these approvals first. - Sub-step 3: Simulate locally: Use a forked mainnet environment (e.g., Foundry's
forge) to test the logic and identify potential reverts before committing funds.
javascript// Example sequence array for a simple batch const txSequence = [ approve(USDC, router, '1000000'), swap(USDC, WETH, '500000'), addLiquidity(WETH, USDC, '1.0', '1500') ];
Tip: Use a dependency graph to visualize the flow. Tools like Tenderly's Simulation API can help validate the path.
2
Construct the Multicall Data
Encode the individual function calls into a single calldata payload.
Detailed Instructions
Use a multicall contract (like Uniswap V3's Multicall2 or a custom aggregator) to bundle calls. Each call is encoded as (target, value, data).
- Sub-step 1: Encode each call: Use ethers.js
interface.encodeFunctionData()or web3.pyencode_function_call()with the function signature and arguments. - Sub-step 2: Aggregate into an array: Compile the encoded calls into the format your multicall executor expects. Ensure the order matches your planned sequence.
- Sub-step 3: Calculate total value: If any call involves a native token transfer (e.g.,
msg.value), sum these amounts. The batch transaction'svaluemust equal this total.
solidity// Example Multicall2 aggregate function call function aggregate(Call[] memory calls) public payable returns (uint256 blockNumber, bytes[] memory returnData) { blockNumber = block.number; returnData = new bytes[](calls.length); for(uint256 i = 0; i < calls.length; i++) { (bool success, bytes memory ret) = calls[i].target.call{value: calls[i].value}(calls[i].data); require(success, "Multicall aggregate: call failed"); returnData[i] = ret; } }
Tip: For complex batches, consider using a relayer pattern where a smart contract holds temporary custody to execute the entire sequence atomically.
3
Simulate and Estimate Gas
Test the batch execution and calculate required gas on the target L2.
Detailed Instructions
Simulation is non-negotiable to avoid costly reverts. L2s like Arbitrum and Optimism have distinct gas calculation models.
- Sub-step 1: Use an RPC
eth_call: Simulate the entire multicall transaction with your wallet address asmsg.senderto check for logic errors. - Sub-step 2: Parse revert reasons: If the simulation fails, decode the revert string or custom error from the returned data to debug.
- Sub-step 3: Estimate L2-specific gas: Call
eth_estimateGas. On Optimism, account for the L1 data fee. On Arbitrum, note the gas price includes L1 cost. Add a 10-20% buffer for safety.
bash# Example curl for simulation curl -X POST \ -H "Content-Type: application/json" \ --data '{"jsonrpc":"2.0","method":"eth_call","params":[{"from":"0xYourAddress","to":"0xMulticallAddr","data":"0xEncodedBatch"},"latest"],"id":1}' \ https://optimism-mainnet.infura.io/v3/YOUR_KEY
Tip: For time-sensitive arbitrage, use private mempool services (e.g., Flashbots Protect) even on L2s to front-run protection and ensure execution.
4
Execute and Monitor the Batch
Send the transaction and verify its atomic completion.
Detailed Instructions
Submit the signed batch transaction to the network. Atomicity is key—if any sub-call fails, the entire batch should revert.
- Sub-step 1: Set appropriate gas parameters: Use the estimated gas, a competitive max fee per gas (e.g., 0.1 Gwei on Arbitrum), and a high enough priority fee for timely inclusion.
- Sub-step 2: Submit via your wallet provider: Use
sendTransactionwith the multicall contract address and the aggregated calldata. Ensure the wallet has sufficient ETH for gas and any aggregatedvalue. - Sub-step 3: Verify all sub-calls succeeded: Upon confirmation, decode the transaction receipt logs. Check that the multicall contract did not emit a failure event and that each expected event (e.g.,
Swap,Deposit) is present.
javascript// Example post-execution verification with ethers.js const receipt = await provider.getTransactionReceipt(txHash); const iface = new ethers.utils.Interface(multicallABI); const log = iface.parseLog(receipt.logs[0]); console.log(`Batch executed at block: ${receipt.blockNumber}`); console.log(`All calls successful: ${!log.args.hasFailures}`);
Tip: Use a transaction receipt listener to programmatically trigger the next action in your workflow (e.g., withdrawing profits) immediately upon batch success.
Gas Cost Analysis: Batched vs. Single Transactions
Comparison of gas efficiency and costs for common DeFi operations on Optimism.
| Transaction Type | Single Tx (ETH) | Batched 5 Txs (ETH) | Batched 10 Txs (ETH) |
|---|---|---|---|
ERC-20 Transfer | 0.00015 | 0.00045 | 0.00075 |
Uniswap V3 Swap | 0.00035 | 0.00110 | 0.00195 |
AAVE Deposit | 0.00055 | 0.00160 | 0.00285 |
Compound Borrow | 0.00060 | 0.00175 | 0.00310 |
NFT Mint (ERC-721) | 0.00080 | 0.00225 | 0.00390 |
Staking Reward Claim | 0.00025 | 0.00070 | 0.00120 |
Gas Savings per Tx | 0% | ~40% | ~50% |
developer_tools
Developer Tools and SDKs
Essential libraries and frameworks for building, testing, and deploying batch transaction logic on Layer 2 networks.
01
Ethers.js & Viem
Provider and Signer abstraction for interacting with EVM chains. These libraries handle RPC calls, contract interactions, and transaction signing.
- Viem's Batch Calls: Use
multicallto bundle multipleeth_callRPC requests into one, reducing latency for state reads. - Ethers Batch Signer: Prepare multiple transactions for a single user signature using
populateTransaction. - Essential for constructing the raw transaction data before submitting to a bundler or sequencer.
02
Account Abstraction SDKs
Smart contract wallet toolkits like ZeroDev, Biconomy, and Alchemy's Account Kit. They abstract away EOAs, enabling batch transactions via a single UserOperation.
- Bundler APIs: Submit batches of UserOperations to a dedicated mempool for inclusion.
- Paymaster Integration: Sponsor gas fees for users in batch, improving UX.
- Session Keys: Allow predefined batches of actions (e.g., weekly DCA) to be executed without repeated signing.
03
Aggregator SDKs
Liquidity routing SDKs such as 1inch Fusion, CowSwap, and ParaSwap. They find optimal trade routes and can settle multiple swaps in a single atomic transaction.
- Intent-Based Architectures: Users submit a desired outcome (e.g., "swap X for Y at best price"), and the solver batches the necessary steps.
- CoW Protocol Batch Auctions: Aggregate multiple users' orders into a single settlement transaction, minimizing MEV.
- Critical for building efficient DeFi dashboards that optimize multi-step workflows.
04
Gas Estimation Libraries
Advanced fee calculators like Gas Station Network (GSN) and Blocknative's Mempool API. They predict Layer 2 gas costs for complex batches.
-
Batch Gas Overhead: Calculate the non-linear gas cost of adding more operations to a batch, as L2s often have fixed overhead per batch.
-
Priority Fee Estimation: For L2s with EIP-1559, estimate the optimal
maxPriorityFeePerGasto ensure batch inclusion in the next block. -
Prevents failed transactions and optimizes cost for end-users.
05
Testing Frameworks
Foundry and Hardhat with L2-specific plugins. They enable local simulation of batch transaction execution and state changes.
- Forking Mainnet & L2: Test batch interactions against a forked version of Arbitrum or Optimism.
- Gas Report Snapshots: Compare the gas cost of a batched operation versus its individual components.
- Invariant Testing: Assert that a batch of actions (e.g., deposit, swap, stake) always maintains specific protocol invariants.
06
Indexing & Subgraphs
Querying tools like The Graph and Goldsky for analyzing historical batch transaction data. They index event logs from batched contract calls.
- Track Batch Success Rates: Query for failed user operations within a batched settlement.
- Aggregate Volume Metrics: Analyze total value executed through a specific batcher contract over time.
- User Flow Analysis: Reconstruct complex multi-contract interactions from a single transaction hash for debugging.
Frequently Asked Questions
The core advantage is gas cost amortization. Submitting multiple operations in a single transaction allows you to pay the fixed base fee for L1 data availability and L2 execution only once, rather than per operation. This drastically reduces the effective cost per action. For example, bundling five token swaps might cost 0.001 ETH total, whereas executing them individually could cost 0.0005 ETH each, totaling 0.0025 ETH. This efficiency is critical for complex DeFi strategies like portfolio rebalancing or multi-step arbitrage, where individual gas fees would be prohibitive.