What Is Checkpointing?
Also known as: Checkpoint
In short
Checkpointing is a blockchain security method in which certain blocks are marked as finalized, preventing the chain history before that point from being reorganized (reorg) and making attacks such as double spending harder.
Checkpoints can be implemented in different ways. In Ethereum's Proof of Stake design, the first block of each epoch (a period of 32 slots) is a checkpoint; when validators representing at least two-thirds of the total stake vote on these checkpoints, they are first justified and then finalized. Reverting a finalized block would require being willing to accept the slashing of at least one-third of the total stake.
Checkpoints are also used for nodes newly joining the network. With checkpoint sync in Ethereum, instead of verifying the chain from the very beginning, a node can start synchronizing from a recent finalized state obtained from a source it trusts. Some networks, such as Polygon PoS, periodically record the state of their own chain on the Ethereum mainnet as checkpoints. Bitcoin clients have also used checkpoints embedded in the code in the past.
The limitation of the method lies in who determines the checkpoint and how. Checkpoints that developers hard-code manually or that are obtained from a single source require trust in that source and may mean a compromise on decentralization.
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