Offchain Labs says Arbitrum wasn't down for an hour, despite transaction issues

Offchain Labs, the team behind the Arbitrum Layer 2 project, says the network was not down for an hour yesterday.

"Was Arbitrum One down for an hour? No," CTO Harry Kalodner said Friday on social media. "Did the batch poster stop posting for an hour? Yes."

Many users had presumed Arbitrum was down for an hour because the Ethereum Layer 2 network didn't seem to be processing transactions. However, Kalodner explained that the outage was due to the batch poster.

"What happens if the batch poster stops posting? The sequencer continues accepting and ordering transactions along with publishing confirmations, but batches won't be posted," Kalodner explained. "For most users the chain will operate as normal, though anyone waiting for full finality will pause."

"The batch poster hit an edge case in geth's mempool implementation which caused it to reject transactions if the total cost of all transactions in the mempool was greater than the balance of the sender," he continued. "This blocked the batch poster from escalating the fees of any transaction. This is made particularly complex since the Arbitrum One batch poster's main ETH balance is stored in a separate smart contract called the gas refunder which reimburses it for ETH spent on batch posting to increase security of its funds."

The batch poster would've had enough ETH to send all the transactions, but geth rejected its attempts," Kalodner added. "Did geth have a bug? Sort of, but not really. The batch poster consumes about 2% of Ethereum's gas limit and the gas refunder is unusual. It's no wonder it hits weird edge cases."

What happened

About 22 hours ago, Arbitrum experienced an hour-long pause between batches 316,002 and 316,003, according to the blockchain tracker Arbiscan. 

Some social media users claimed that this meant they couldn't submit transactions during this one-hour dark period, leading them to believe the whole blockchain either froze or was down.

The Arbitrum Foundation, the organization building out Arbitrum, did not immediately respond to a request for comment from The Block.