43. Building an indexer: polling new blocks

An indexer polls (or subscribes to) new blocks, extracts relevant transactions/events, and writes them into a database so an application can query rich, filtered data without hitting the node for every request.

js
async function pollBlocks(rpcCall, db, state) {
  const latestHex = await rpcCall("eth_blockNumber");
  const latest = parseInt(latestHex, 16);
  for (let n = state.lastProcessed + 1; n <= latest; n++) {
    const block = await rpcCall("eth_getBlockByNumber", [`0x${n.toString(16)}`, true]);
    await db.saveBlock(block);
    state.lastProcessed = n;
  }
}

setInterval(() => pollBlocks(rpcCall, db, state).catch(console.error), 5000);

Real indexers must also handle chain reorganizations (a previously-seen block being replaced) by re-checking recent block hashes and rolling back/re-applying affected data — naively assuming block numbers only ever increase leads to corrupted data after a reorg.

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