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What is a blockchain indexer?
An indexer is a service that reads a blockchain block by block, pulls out the events and data it cares about, and stores them in a searchable database. Apps and explorers use indexers to answer questions such as 'every transfer to this address' that a node cannot answer quickly on its own.
What it means for you
The transaction history, token lists and portfolio totals most apps show you come from an indexer, not straight from the chain. If the indexer lags, misses a token or goes offline, you can see stale or missing data while your funds are unaffected. When something looks wrong, look up the address or transaction hash on a block explorer before assuming funds are gone.
How it works
Blockchains store data as an append-only chain of blocks with no tables or relationships, and nodes expose only low-level RPC methods such as eth_getLogs and eth_call, with no native way to filter, aggregate, sort or join. Finding every transfer for one user means scanning the chain block by block, and querying past state needs an archive node because most nodes prune history. An indexer does that work once: it follows chosen contracts, decodes their event logs, turns them into structured records and serves them through a query API, so apps do not have to rescan the chain.
Sources: The Graph docs: About indexing, ethereum.org: Data and analytics, ethereum.org: Anatomy of smart contracts (events and logs) · checked 4 October 2026
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