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Bloklake serves blocks up to the chain head. The newest blocks can still be replaced by a reorg: the chain switches to a different version of its recent history, and blocks you already read are replaced by others with different hashes and contents.

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Three numbers in every response

Rows at or below finalized_block are final. Rows between finalized_block and archive_height might still be replaced.

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Strategy 1: only read finalized data

The simplest correct approach. Cap every range at finalized_block:
Your data lags the head by the finality delay, and you never handle a reorg.

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Strategy 2: read to the head, re-check the unfinalized tail

For live data:
  1. Read up to the head. Store rows at or below finalized_block as final.
  2. Keep rows above it as tentative, together with the response’s to_block and to_block_hash.
  3. On the next poll, re-read from the first tentative block. If the block at the old to_block now has a different hash, a reorg replaced it: drop your tentative rows and use the new ones.
The complete loop, tested against a reorg, is in Following the head. Replacing the tentative rows on every poll is the simplest correct implementation: you never need to diff, because you always re-read everything that could have changed.
A reorg can change blocks without changing archive_height: same height, different block. Don’t detect reorgs by watching the height. Compare to_block_hash, or re-read the unfinalized range as above.

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No snapshot isolation

Two requests can see different heads. A block visible in one response might not exist yet, or might have been replaced, in the next. If you need a stable, repeatable view, pin toBlock to a block at or below finalized_block: the same request then always returns the same rows.

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Polling cadence

Poll about once per block time: every 12 seconds on Ethereum, every second or so on Robinhood Chain. Polling faster returns mostly empty responses, which still count toward your rate limit and are billed at the 10 KB minimum. See Following the head for a complete loop.