Klaytn Downtime Due to “Consensus and Algorithm Problem”

The block generation stoppage on Kakao and Ground X’s Klaytn blockchain last month on the 29th was identified as a ‘block consensus delay’ problem. Ground X announced that they plan to improve the consensus algorithm to prevent any more consensus delays in the future.

The block generation failure lasted for about 13 hours from 11:30pm on the 29th until 12:30 pm the next day. As a result of the stoppage, the blockchain’s ability to story transaction data was disabled, and the entire network was stopped up. Services operating on the Klaytn main net were also negatively affected by the stoppage since users were entirely unable to withdraw tokens from exchanges.

Ground X explained that the stoppage was caused by a delay in the consensus of block 24002380. For some reason, the stop-gap measure in place to prevent such problems due to consensus delay did not respond. Normally after consensus is delayed by more than a second, the next consensus process should start automatically. On the 29th, however, the unstable node that caused the delay stopped the entire network.

This problem was magnified by the delayed block consensus on the next round which caused the nodes to be out of sync. The downed node and failed consensus conspired against the entire network and forced two groups to be formed among the nodes where proposers for new blocks were proposing block numbers that did not match up with each other’s and caused a 13-hour disagreement.

The issue was essentially blamed on a Byzantine Fault Tolerance (BFT) algorithm issue. While BFT algorithms are well-known to be highly secure, they require the consensus of more than two thirds of the nodes for block generation. When the whole community of nodes are split into two groups, consensus becomes impossible under the security measures of a BFT algorithm since that two-thirds consensus cannot be reached.

Naturally Ground X vows to make improvements to the consensus model to ensure that this problem does not happen again. One concession that is likely to be made is to favor block proposers over preventing block generation.

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