How to Test Blockchain Indexer Recovery from Chain Reorganizations

Blockchain indexers can silently serve stale or incorrect data after a chain reorganization, even while appearing healthy, because their checkpoints may reference abandoned branches. A tutorial published on DEV Community demonstrates how to build an acceptance test using Python and SQLite to verify that an indexer correctly recovers and matches a canonical chain replay. The guide highlights that a checkpoint storing only a block height is insufficient, as two competing blocks can share the same position, making it necessary to record both block number and hash. It also warns that Ethereum log subscriptions can emit duplicate events across competing histories, so storage must be designed to prevent duplication while accurately tracking events that appear in different blocks. The tutorial is intentionally scoped to a synthetic model and does not connect to a live node, but its acceptance criteria — requiring canonical data, active events, aggregates, and checkpoints to agree at a committed boundary — are intended to apply to production indexers.