L2 Sequencer Uptime Feeds

Optimistic rollups (e.g., Arbitrum, Optimism) and many ZK-rollups rely on sequencers to efficiently manage transaction ordering, execution, and batching before submitting them to Layer 1 (L1) blockchains like Ethereum. The sequencer plays a crucial role in optimizing transaction throughput, reducing fees, and ensuring fast transaction confirmations on L2 networks, making it a key component of their scalability and performance.

However, if the sequencer becomes unavailable, users will lose access to the standard read/write APIs, preventing them from interacting with applications on the L2 network. Although the L2 chain's security and state commitments remain enforced by Layer 1, no new batched blocks will be produced by the sequencer. Users with sufficient technical expertise can still interact directly with the network through the underlying rollup contracts on L1. However, this process is more complex and costly, creating an unfair advantage for those who can bypass the sequencer. This imbalance in access can lead to disruptions or distortions in applications, such as liquidations or market operations that rely on timely transactions.

To mitigate these risks, your applications can integrate a Sequencer Uptime Data Feed, which continuously monitors and records the last known status of the sequencer. By utilizing this feed, you can:

Detect sequencer downtime in real time.
Implement a grace period to prevent mass liquidations or unexpected disruptions.
Ensure fair access to services by temporarily pausing operations during sequencer failures.

This proactive approach enhances the resilience and fairness of applications operating on L2 networks, ensuring a more stable and equitable user experience.

Supported Networks

You can find proxy addresses for the L2 sequencer feeds at the following addresses:

Real-time Monitoring Process

Arbitrum

The diagram below shows how these feeds update and how a consumer retrieves the status of the Arbitrum sequencer.

Chainlink nodes trigger an OCR round every 30s and update the sequencer status by calling the validate function in the ArbitrumValidator contract by calling it through the ValidatorProxy contract.
The ArbitrumValidator checks to see if the latest update is different from the previous update. If it detects a difference, it places a message in the Arbitrum inbox contract.
The inbox contract sends the message to the ArbitrumSequencerUptimeFeed contract. The message calls the updateStatus function in the ArbitrumSequencerUptimeFeed contract and updates the latest sequencer status to 0 if the sequencer is up and 1 if it is down. It also records the block timestamp to indicate when the message was sent from the L1 network.
A consumer contract on the L2 network can read these values from the ArbitrumUptimeFeedProxy contract, which reads values from the ArbitrumSequencerUptimeFeed contract.

Handling Arbitrum Outages

If the Arbitrum network becomes unavailable, the ArbitrumValidator contract continues to send messages to the L2 network through the delayed inbox on L1. This message stays there until the sequencer is back up again. When the sequencer comes back online after downtime, it processes all transactions from the delayed inbox before it accepts new transactions. The message that signals when the sequencer is down will be processed before any new messages with transactions that require the sequencer to be operational.

Other Supported Networks

On BASE, Mantle, Metis, OP, Scroll, Soneium and zkSync, the sequencer's status is relayed from L1 to L2 where the consumer can retrieve it.

On the L1 network:

A network of node operators runs the external adapter to post the latest sequencer status to the AggregatorProxy contract and relays the status to the Aggregator contract. The Aggregator contract calls the validate function in the OptimismValidator contract.
The OptimismValidator contract calls the sendMessage function in the L1CrossDomainMessenger contract. This message contains instructions to call the updateStatus(bool status, uint64 timestamp) function in the sequencer uptime feed deployed on the L2 network.
The L1CrossDomainMessenger contract calls the enqueue function to enqueue a new message to the CanonicalTransactionChain.
The Sequencer processes the transaction enqueued in the CanonicalTransactionChain contract to send it to the L2 contract.

On the L2 network:

The Sequencer posts the message to the L2CrossDomainMessenger contract.
The L2CrossDomainMessenger contract relays the message to the OptimismSequencerUptimeFeed contract.
The message relayed by the L2CrossDomainMessenger contains instructions to call updateStatus in the OptimismSequencerUptimeFeed contract.
Consumers can then read from the AggregatorProxy contract, which fetches the latest round data from the OptimismSequencerUptimeFeed contract.

Handling Other Supported Network Outages

If the sequencer is down, messages cannot be transmitted from L1 to L2 and no L2 transactions are executed. Instead, messages are enqueued in the CanonicalTransactionChain on L1 and only processed in the order they arrived later when the sequencer comes back up. As long as the message from the validator on L1 is already enqueued in the CTC, the flag on the sequencer uptime feed on L2 will be guaranteed to be flipped prior to any subsequent transactions. The transaction that flips the flag on the uptime feed will be executed before transactions that were enqueued after it. This is further explained in the diagrams below.

When the Sequencer is down, all L2 transactions sent from the L1 network wait in the pending queue.

Transaction 3 contains Chainlink’s transaction to set the status of the sequencer as being down on L2.
Transaction 4 is a transaction made by a consumer that is dependent on the sequencer status.

After the sequencer comes back up, it moves all transactions in the pending queue to the processed queue.

Transactions are processed in the order they arrived so Transaction 3 is processed before Transaction 4.
Because Transaction 3 happens before Transaction 4, Transaction 4 will read the status of the Sequencer as being down and responds accordingly.

Example Consumer Contract

This example code works on any network that supports Solidity. Create the consumer contract for sequencer uptime feeds similarly to the contracts that you use for other Chainlink Data Feeds. Configure the constructor using the following variables:

Configure the sequencerUptimeFeed object with the sequencer uptime feed proxy address for your L2 network.
Configure the dataFeed object with one of the Data Feed proxy addresses that are available for your network.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import {AggregatorV2V3Interface} from "@chainlink/contracts/src/v0.8/shared/interfaces/AggregatorV2V3Interface.sol";

/**
 * THIS IS AN EXAMPLE CONTRACT THAT USES HARDCODED VALUES FOR CLARITY.
 * THIS IS AN EXAMPLE CONTRACT THAT USES UN-AUDITED CODE.
 * DO NOT USE THIS CODE IN PRODUCTION.
 */

contract DataConsumerWithSequencerCheck {
    AggregatorV2V3Interface internal dataFeed;
    AggregatorV2V3Interface internal sequencerUptimeFeed;

    uint256 private constant GRACE_PERIOD_TIME = 3600;

    error SequencerDown();
    error GracePeriodNotOver();

    /**
     * Network: OP Mainnet
     * Data Feed: BTC/USD
     * Data Feed address: 0xD702DD976Fb76Fffc2D3963D037dfDae5b04E593
     * Uptime Feed address: 0x371EAD81c9102C9BF4874A9075FFFf170F2Ee389
     * For a list of available Sequencer Uptime Feed proxy addresses, see:
     * https://docs.chain.link/docs/data-feeds/l2-sequencer-feeds
     */
    constructor() {
        dataFeed = AggregatorV2V3Interface(
            0xD702DD976Fb76Fffc2D3963D037dfDae5b04E593
        );
        sequencerUptimeFeed = AggregatorV2V3Interface(
            0x371EAD81c9102C9BF4874A9075FFFf170F2Ee389
        );
    }

    // Check the sequencer status and return the latest data
    function getChainlinkDataFeedLatestAnswer() public view returns (int) {
        // prettier-ignore
        (
            /*uint80 roundID*/,
            int256 answer,
            uint256 startedAt,
            /*uint256 updatedAt*/,
            /*uint80 answeredInRound*/
        ) = sequencerUptimeFeed.latestRoundData();

        // Answer == 0: Sequencer is up
        // Answer == 1: Sequencer is down
        bool isSequencerUp = answer == 0;
        if (!isSequencerUp) {
            revert SequencerDown();
        }

        // Make sure the grace period has passed after the
        // sequencer is back up.
        uint256 timeSinceUp = block.timestamp - startedAt;
        if (timeSinceUp <= GRACE_PERIOD_TIME) {
            revert GracePeriodNotOver();
        }

        // prettier-ignore
        (
            /*uint80 roundID*/,
            int data,
            /*uint startedAt*/,
            /*uint timeStamp*/,
            /*uint80 answeredInRound*/
        ) = dataFeed.latestRoundData();

        return data;
    }
}

Open in Remix What is Remix?

The sequencerUptimeFeed object returns the following values:

answer: A variable with a value of either 0 or 1
- 0: The sequencer is up
- 1: The sequencer is down
startedAt: This timestamp indicates when the sequencer feed changed status. When the sequencer comes back up after an outage, wait for the GRACE_PERIOD_TIME to pass before accepting answers from the data feed. Subtract startedAt from block.timestamp and revert the request if the result is less than the GRACE_PERIOD_TIME.
- The startedAt variable returns 0 only on Arbitrum when the Sequencer Uptime contract is not yet initialized. For L2 chains other than Arbitrum, startedAt is set to block.timestamp on construction and startedAt is never 0. After the feed begins rounds, the startedAt timestamp will always indicate when the sequencer feed last changed status.

If the sequencer is up and the GRACE_PERIOD_TIME has passed, the function retrieves the latest answer from the data feed using the dataFeed object.

L2 Sequencer Uptime Feeds

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