OP Stack
Protocol
Rollup
Forced Transaction

Forced Transaction Flow

This guide explains the nuances of forced transactions during sequencer downtime.

Users are able to force-include deposit transactions, which can initiate withdrawals, at any time. However, there are important nuances to understand about the chain derivation pipeline and sequencer behavior.

Key Concepts

  • Sequencing Window: A 12-hour rolling window to include L2 transactions, including native L2 transactions and deposit transactions.
  • Max Time Drift: 30 minutes, the maximum delay for including a deposit transaction, relative to the L2 chain.
  • Sequencer Downtime: Period when the sequencer is offline or not producing blocks.

Normal Operation

Under normal circumstances:

  1. Deposit transactions are included within 30 minutes.
  2. The sequencer processes transactions and produces blocks regularly.

Sequencer Downtime Scenarios

Short Downtime (< 30 minutes)

  • Deposits are still included within the 30-minute max time drift.
  • Regular L2 transactions may be delayed.

Extended Downtime (30 minutes to 12 hours)

  1. Deposits are force-included within 30 minutes.
  2. The L2 chain state remains uncertain until:
    • The sequencer comes back online, or
    • The 12-hour sequencing window expires.

Prolonged Downtime (> 12 hours)

  1. After 12 hours, nodes start generating blocks deterministically.
  2. These blocks only include deposit transactions.
  3. When the sequencer returns:
    • All deposit transactions are included first.
    • No regular L2 transactions are included until the L2 chain is within 12 hours of the chain.

Important Considerations

  • Forced transactions, through deposits (no need for deposited value), ensure timely execution of actions, mitigating risks like DEX price divergence during sequencer downtime.
  • Actions remain speculative for up to 12 hours due to the sequencing window.
  • The 12-hour window is a balance between operational reliability and minimizing potential L2 reorganizations.

Example Scenario

If a deposit is initiated after the 30-minute mark but before the sequencing window expires:

  1. The deposit will be effective when the sequencing window expires (up to ~11 hours later).
  2. Nodes reading data from L1 will produce a block with the deposit after the sequencing window expires.
  3. The eventual L2 chain will include the deposit in a block with an onchain timestamp close to the L1 block where the deposit originated.

The sequencing window is a rolling 12-hour delay from when an L1 block is first known. This design allows the sequencer a grace period to come back online without causing L2 chain reorganizations.

Conclusion

The forced transaction mechanism on OP Stack chains provides a robust way to handle sequencer downtime, ensuring that critical transactions are included in a timely manner. While the 12-hour sequencer window introduces a degree of uncertainty during downtime, the system is designed to guarantee eventual consistency and transaction inclusion.