Optimistic sync
Optimistic sync is the process of syncing an execution and consensus client concurrently, without having the consensus client wait for the execution client. During optimistic sync, the consensus client quickly syncs up to the latest consensus but delays verifying block execution payloads: it continuously informs the execution client of the latest consensus head, allowing the execution client to perform a snapshot sync directly to the latest state.
Once the execution client has caught up, the consensus and execution clients work in lock-step each validating the block.
Both execution and consensus clients must be fully synced to perform validation duties: while optimistically synced, validator duties (attestation, sync committee and block production work) are skipped.
Running without execution client
Nimbus continues to sync optimistically when the execution client is not available thanks to its built-in execution payload verifier.
Identifying optimistic sync
An optimistically synced node can be identified by examining the "Slot start" log message.
When optimistically synced, the sync key will have a /opt suffix, indicating that it's waiting for the execution client to catch up: