Polygon hit by 157-block ‘reorg’ despite hard-fork to reduce reorgs

On Wednesday, Polygon experienced a blockchain quirk affecting 157 blocks or approximately five minutes worth of network activity.

After blocks appeared to have stopped being produced for several minutes, the Ethereum side-chain’s co-founder Sandeep Nailwal tweeted that (block explorer) Polygonscan was “having some issues.”

In actual fact, the interruption was down to a chain reorganization or ‘reorg’, a problem that Polygon is attempting to fix.

Reorgs occur when network nodes fall out of sync with each other, and two distinct chains of blocks are produced concurrently. This may be due to a bug, network latency, or even malicious activity. When nodes sync once again, one canonical version of the chain is kept, and the blocks included in the invalid ‘fork’ are ignored.

Potential consequences of reorgs can include delays in achieving transaction finality, reverted transactions, or theoretically, a 51% attack on a (reduced) validator set.

Reorgs of a few blocks are not uncommon, and generally have no effect on users. But this case caused concern due to the ‘depth’ of the reorganization, which included 157 blocks. This could potentially have affected hundreds of users’ transactions. 

The following day, Uniswap founder Hayden Adams publicly called out the network, referring to both Wednesday’s disruption and another 120-block reorg in December.

Nailwal’s fellow co-founders Mihailo Bjelic and Jaynti Kanani stressed that this case was down to a specific bug and that current efforts are already underway to address the issue.

Read more: Explained: Is Polygon truly decentralized?

Polygon is well-established as a popular, low-cost alternative to Ethereum mainnet. The network saw a huge increase in its user base during the Spring of 2021 as gas prices on Ethereum became exorbitant.

However, as a cheap Ethereum side-chain with less demanding users, Polygon has often faced rather less scrutiny than alternative Layer 1 blockchains, such as Solana or Avalanche.

As well as lengthy reorgs, Polygon has been prone to (relatively) high gas fees. These can be due to spam transactions — which are cheaper to send than on mainnet — or blockchain-based games, such as last year’s Sunflower Farms, which take up significant network resources and cause congestion.