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Opti sync: elaborate on why sync optimistically #2958

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Aug 9, 2022
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Opti sync: elaborate on why sync optimistically #2958

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Elaborate on why sync optimistically
mkalinin Jul 28, 2022
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mkalinin Aug 2, 2022
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Expand on opti-sync in context of block sync
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17 changes: 17 additions & 0 deletions sync/optimistic.md
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Expand Up @@ -270,6 +270,23 @@ optimistic blocks (and vice-versa).

## Design Decision Rationale

### Why sync optimistically?

Most execution engines use state sync as a default sync mechanism on Ethereum Mainnet
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because executing blocks from genesis takes several weeks on commodity hardware.

State sync requires the knowledge of the current head of the chain to converge eventually.
If not constantly fed with the most recent head, state sync won't be able to complete
because the recent state soon becomes unavailable due to state trie pruning.

Optimistic block import (i.e. import without waiting for a payload to be executed by the engine)
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breaks a deadlock between the state sync and processing a beacon block by skipping payload
execution as long as execution engine is syncing.
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Optimistic sync works nicely with execution engines using block execution as a default
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sync mechanism (e.g. Erigon). It makes optimistic sync a *generalized* solution for
interaction between consensus and execution engine during the sync process.

### Why `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY`?

Nodes can only import an optimistic block if their justified checkpoint is
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