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The Ethereum network experienced a drop in the number of validators (computers that approve transactions) after the major Fusaka upgrade was rolled out. The Ethereum network faced a tense moment, but fortunately, the issue was resolved quickly. What happened and why is the number of validators important for the network?
Prysm Bug #
The Fusaka upgrade caused a stir when it was revealed that as much as 25% of validators were no longer voting on whether a block was approved. This was due to a bug in the Prysm Ethereum Client (software used by validators to approve blocks), which caused validators to stop functioning correctly.
Their advice on X was to restart their client with an additional command: --disable-last-epoch-targets. By doing this, validators could resume participating in block approval and bypass the bug.
How Ethereum Block Approval Works #
On Ethereum, approximately 66% of validators must approve a block; otherwise, it is rejected. With 25% not participating, the network was close to a standstill.
Block approval works as follows: a new block arrives containing all transactions on the Ethereum network from the previous block period. Validators then vote, and if there is consensus (agreement) on approval or rejection, the process continues. If more than 66% approve, the block is added to the Ethereum blockchain; otherwise, it is not.
Currently, 99% of validators are participating again. This means that almost all validators have applied the above solution, which is remarkable within a day.
What Happens with Less Than 66% Approval? #
If a block to be added to the Ethereum blockchain receives less than 66% approval from validators, it is not added. If this becomes permanent due to a bug, the Ethereum network faces a serious problem. Blocks can still be produced, but they are not finalized.
A finalized block is immutable. A non-finalized block can still be altered. For a large blockchain like Ethereum, this would be a huge risk, as would be for other blockchains dependent on Ethereum’s finality, such as Arbitrum.
This scenario has occurred twice before on Ethereum. In early May 2023, bugs in Prysm and Teku caused a loss of finality on the Ethereum mainnet twice within 24 hours. The problem then was that Prysm was running on 68% of all validators. This issue persists today, but now it is with Lighthouse, which runs on over 50% of validators. A problem with this client could immediately suspend Ethereum’s finality.