> For the complete documentation index, see [llms.txt](https://docs.usefarlight.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.usefarlight.com/risk/sequencer-and-chain.md).

# Surviving sequencer failure

How Robinhood Chain's single sequencer and its configuration can fail, and the mechanisms that keep Farlight working through each case.

Robinhood Chain is an Arbitrum Nitro rollup, and its one sequencer is operated by Robinhood. That is normal for an L2 at this stage. It also means a specific set of failure modes, and a lending protocol has to plan for them instead of hoping they stay theoretical. Farlight's answer is to make sure every position can still be steered when the chain itself goes quiet.

## How it can fail

| Scenario                              | What a user sees                                                                                                                            |
| ------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------- |
| Sequencer downtime                    | No transaction lands on the L2 until it is back, while the underlying stock market keeps trading.                                           |
| Sequencer censorship                  | Specific transactions, a repayment for example, are never included.                                                                         |
| Slow L1 finality                      | Withdrawals to Ethereum take the standard 7 days. Loan operations on the L2 are not affected.                                               |
| A change to the chain's configuration | A new DA mode or validator set, or Timeboost or BoLD being switched on, could change what the protocol assumes about finality and ordering. |

## Defences

### No liquidations for an hour after an outage

`PriceGate` reads the Chainlink L2 Sequencer Uptime Feed. When the feed shows the sequencer has only just come back, no liquidation auction can be started for a grace period of **one hour**. That gives borrowers a window to top up collateral or repay before anyone can act on a price that moved while nobody could transact.

### Every function is reachable from L1

Each state-changing function in Farlight accepts transactions submitted through Arbitrum's delayed inbox on Ethereum. If the sequencer has not included such a transaction within 24 hours it can be force-included. In practice:

* a censoring sequencer cannot stop a borrower from repaying,
* a censoring sequencer cannot stop a lender from cancelling an offer,
* a borrower who expects an outage can queue a top-up from L1 ahead of time.

Combined with the grace period, force inclusion becomes a route people can actually use rather than a footnote.

### Timestamps, not block numbers

On Arbitrum-stack chains `block.number` returns a value derived from L1. Interest accrual, expiries and auction curves therefore rely on `block.timestamp` only. Where a real L2 block height is needed, the protocol calls `ArbSys.arbBlockNumber()`.

### Compact calldata

The L1 data component of gas is charged on the compressed size of calldata. Offer structs are packed tightly and signatures use the compact 64-byte form wherever they can, which keeps an `originate` call that carries twenty offers affordable.

## Configuration watch

Two questions turn on whether Timeboost (express-lane ordering) or BoLD (permissionless validation) is active on Robinhood Chain: the order in which liquidators reach an auction, and how quickly a force-included transaction can be trusted. Farlight tracks both. If Timeboost is switched on, the auction design is reviewed to confirm that express-lane access cannot systematically front-run keeper transactions, and any change that follows is published in the governance log.


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