The Builder simulates bundles and transactions against the latest chain state to create valid Signet rollup blocks and submits them to the configured host chain as an EIP-4844 transaction.

Bundles are treated as MEV-style bundles, meaning that the Builder should respect transaction ordering, bundle atomicity, and the specified revertability.

🚀 System Design

The Builder orchestrates a series of asynchronous actors that work together to build blocks for every assigned slot.

1. Env - watches the latest host and rollup blocks to monitor gas rates and block updates.
2. Cache - polls bundle and transaction caches and adds them to the cache.
3. Simulator - simulates transactions and bundles against rollup state and block environment to build them into a cohesive block.
4. Submit - handles preparing and submitting the simulated block to all configured MEV relay/builder endpoints concurrently.
5. Metrics - records block and tx data over time.

%%{ init : { "theme" : "dark" } }%%
flowchart TD
   %% Initialization
   Start(["Start"]) --> Init["Init tracing & logging"]
   Init --> Config["Load BuilderConfig from env"]

   Inbound["📥 Transactions &
   📦 Bundles"] --> Cache["🪏 Cache Task"]
   Quincey["🖊️ Quincey (Signer)"]

   %% Block building loop
   subgraph BuilderLoop["Block Building Loop"]
      Env["🔢 Env Task"]
      Cache["🪏 Cache Task"]
      Simulator["💾 Simulator Task"]
      Metrics["📏 Metrics Task"]
      SubmitTask["📥 Submit Task"]
   end

   %% Signing
   SubmitTask --hash--> Quincey
   Quincey -- signature --> SubmitTask

   %% Config wiring
   Config -.rollup rpc.-> Env
   Config -.host rpc.-> Env
   Config -.host rpc.-> Simulator
   Config -.rollup rpc.-> Simulator
   Config -.host rpc.-> Metrics
   Config -.host rpc.-> SubmitTask

   %% Core flow
   Env ==block env==> Simulator
   Cache ==sim cache==> Simulator
   Simulator ==built block==> SubmitTask

   %% Network submission
   SubmitTask ==>|"tx bundle"| Relays["🛡️ MEV Relays / Builders"]
   Relays ==> Ethereum

   %% Metrics
   SubmitTask ==rollup block tx hash==> Metrics

💾 Simulation Task

The block building loop waits until a new block environment has been received, and then kicks off the next attempt.

When the Builder receives a new block, it takes a reference to the transaction cache, calculates a simulation deadline for the current slot with a buffer of 1.5 seconds, and begins constructing a block for the current slot.

Transactions enter through the cache, and then they're sent to the simulator, where they're run against the latest chain state and block environment. If they're successfully applied, they're added to the block. If a transaction fails to be applied, it is simply ignored.

When the deadline is reached, the simulator is stopped, and all open simulation threads are cancelled. The built block is then bundled with the block environment and the previous host header that it was simulated against, and all three are passed along to the submit task.

✨ Submit Task

The submit task prepares a MEV bundle from the Signet block and its host transactions, then fans it out to all configured relay/builder endpoints concurrently (SUBMIT_ENDPOINTS). At least one successful relay acceptance is required; individual relay failures are tolerated and logged. The blob sidecar is always forwarded to Pylon regardless of relay outcome.

If the block received from simulation is empty, the submit task will ignore it.

If it's non-empty, the submit task attempts to get a signature for the block, and if it fails due to a 403 error, it will skip the current slot and begin waiting for the next block.

⚙️ Configuration

The Builder is configured via environment variables. The following values are supported for configuration.

💻 Recommended Specs

Note: Builder prefers clock speed over core count, recommended 2.8Ghz+

💾 EVM Behavior

🗿 Inherited Header Values

PREVRANDAO is set to a random byte string for each block.

// `src/tasks/env.rs`
prevrandao: Some(B256::random()),

TIMESTAMP - Block timestamps are set to the same value as the current Ethereum block.

Blob gas values excess_blob_gas and blob_gasprice are also set to 0 for all Signet blocks.

🔢 Disabled Opcodes

BLOBHASH - EIP-4844 is not supported on Signet. BLOBBASEFEE - EIP4844 is not supported.

⛽ Transaction Submission

When a completed, non-empty Signet block is received by the Submit task, it prepares the block data into a blob transaction and submits it to the network.

If it fails, it will retry up to 3 times with a 12.5% bump on each retry.

The previous header's basefee is tracked through the build loop and used for gas estimation purposes in the Submit Task.

🛠️ Development

Requirements

• Rust ≥ 1.85
• AWS CLI
• A private key or AWS KMS key for signing transactions

✅ Testing

Unit Tests

Integration Tests

Integration tests require network access (RPC endpoints or Anvil) and are gated behind the test-utils Cargo feature. They are not compiled by default.

make test-all                                            # Run all tests (unit + integration)
cargo test --features test-utils --test block_builder_test  # Run a specific integration test

Deployment Verification

1. Build the Docker image:

docker build -t builder:latest .

2. Push to your container registry:

docker push <registry>/builder:latest

3. Update your deployment manifests with the new image.

4. Verify expected behavior in your target network.

• This should typically include sending a test transaction and verifying it is simulated and built into a block.

🪪 License

This project is licensed under the MIT License.