Consensus contracts - sequencing

Transactions flowing through the system reach the smart contract environment after one of two contract call use cases:

• Sequence batches requests coming from the sequencer component in the node.
• Verifying batches requests coming from the aggregator component in the node.

This section focuses on the sequencing workflow.

The diagram below shows the sequencing workflow for rollup (non-validium) stacks which calls sequenceBatches(...) and onSequenceBatches(...).

sequenceBatches()

The sequenceBatches() function is called on the PolygonZkEVMEtrog.sol contract:

sequenceBatches(batches, maxSequenceTs, initSequenceBatch, l2Coinbase)

The rollup sequencer component calls the sequenceBatches function on the PolygonZkEVMEtrog.sol contract which inherits the function from PolygonRollupBaseEtrog.sol.

The function takes an array of BatchData structs from one of the consensus contracts. Each struct contains L2 Ethereum transactions data, and some forced state information.

struct BatchData {
    bytes transactions;
    bytes32 forcedGlobalExitRoot;
    uint64 forcedTimestamp;
    bytes32 forcedBlockHashL1;
}

The function validates arguments, checks and organizes the batches, and appends them in the correct sequence while computing an accumulated hash.

Finally, the function emits a SequenceBatches event which sends a newly sequenced batch of transactions to the PolygonRollupManager.sol contract after the onSequenceBatches(...) function returns successfully.

Stepwise, the function does the following:

1. Validates arguments.
2. Tells the bridge to update the global exit root by calling globalExitRootManager.updateExitRoot(L1LocalExitRoot) which creates a new global exit root with the newest L1 local exit root.
3. Gets L1 info root and other variables needed for computation.
4. Goes through the batches to compute the accumulated hash with keccak(batch.transaction) and keccak(accInputHash, txHash, l1InfoRoot, maxSequenceTs, l2Coinbase, bytes32(0)).
5. Stores the accumulated hash.
6. Caller pays the rollup manager in POL.
7. Calls the PolygonRollupManager.onSequenceBatches(...) function which waits for an OnSequenceBatches(...) event callback.
8. Emits SequenceBatches(...) event.

onSequenceBatches()

The onSequenceBatches() function is called on the PolygonRollupManager.sol contract:

onSequenceBatches(newSequencedBatches, newAccInputHash)

It takes the sequenced batches and the accumulated hash from the calling contract, adds the batches to the correct stack, and updates the batch count.

Stepwise, the function does the following:

1. Validates the arguments and the caller contract.
2. Updates the totalSequencedBatches storage variable.
3. Updates the lastBatchSequenced and adds a new SequencedBatchData struct for the rollup that called sequenceBatches.
4. Attempts to consolidate pending state for the rollup by updating lastVerifiedBatch, batchNumToStateRoot[], and lastLocalExitRoot state variables, and also by updating globalExitRootManager.updateExitRoot(L2sLocalExitRoot), after which it emits a ConsolidatePendingState(...) event.
5. Emits an OnSequenceBatches(...) event back to the original sequenceBatches(...) call.

sequenceBatchesValidium()

The sequenceBatchesValidium() function is called on the PolygonValidiumEtrog.sol contract:

sequenceBatchesValidium(batches, l2Coinbase, dataAvailabilityMessage)

Info

• This function is not included in the sequence diagram above.
• The differences, however, are minimal.

The sequencing logic is nearly the same as for the rollup sequenceBatches(...) function except the function takes a ValidiumBatchData[] array instead of BatchData[]. This means that, instead of passing the actual transaction data, the struct passes the hashes of the transactions.

struct ValidiumBatchData {
    bytes32 transactionsHash;
    bytes32 forcedGlobalExitRoot;
    uint64 forcedTimestamp;
    bytes32 forcedBlockHashL1;
}

It also has a dataAvailabilityMessage parameter instead of the sequence information. This parameter contains all the ECDSA address signatures of the committee in ascending order and is used for verification.

Stepwise, the function has identical logic to the PolygonRollupBaseEtrog.sequenceBatches(...) function apart from the following steps:

1. ValdiumBatchData instead of BatchData.
2. Accumulates the txHash into accumulatedNonForcedTransactionHash.
3. Adds a validity check with dataAvailabilityProtocol.verifyMessage(accumulatedNonForcedTransactionHash, dataAvailabilityMessage).

Comments