Building Off-Chain Services for Ergo dApps

1. Watchers / Monitors

Services that scan new blocks or the mempool for relevant events.

Example: SigmaUSD's reserve monitoring system watches SigRSV/SigUSD token amounts and triggers UI updates.

// Fleet SDK watcher (simplified)
import { ErgoNodeApi } from "@fleet-sdk/common";
const nodeApi = new ErgoNodeApi({ url: "http://your-node-ip:9053" });
let lastHeight = 0;

async function watchForBoxes() {
  try {
    const info = await nodeApi.getNodeInfo();
    const currentHeight = info.fullHeight;
    if (currentHeight > lastHeight) {
      for (let h = lastHeight + 1; h <= currentHeight; h++) {
        const headerIds = await nodeApi.getHeaderIdsAtHeight(h);
        if (!headerIds?.length) continue;
        const block = await nodeApi.getBlockById(headerIds[0]);
        if (!block?.blockTransactions) continue;
        for (const tx of block.blockTransactions) {
          for (const box of tx.outputs) {
            if (box.assets.some(a => a.tokenId === "YOUR_TOKEN_ID")) {
              console.log(`Found box: ${box.boxId} at height ${h}`);
            }
          }
        }
      }
      lastHeight = currentHeight;
    }
  } catch (e) { console.error(e); }
  setTimeout(watchForBoxes, 30000);
}
watchForBoxes();

Cross-chain example: Rosen Bridge Watchers monitor deposit events on other blockchains and relay to Ergo.

2. Bots / Agents

Automated services that construct and submit transactions.

Example: Oracle Pool bots post new price data to the chain.

// Fleet SDK bot (conceptual)
import { OutputBuilder, TransactionBuilder, ErgoNodeApi, ErgoProver } from "@fleet-sdk/core";
const nodeApi = new ErgoNodeApi({ url: "http://your-node-ip:9053" });
const botProver = new ErgoProver(process.env.BOT_SECRET_KEY);
async function updateOraclePrice() {
  try {
    const price = await getExternalPrice("ERG-USD");
    const newPriceLong = BigInt(Math.round(price * 100));
    const oracleBox = await nodeApi.getBoxById("YOUR_ORACLE_POOL_BOX_ID");
    const creationHeight = await nodeApi.getCurrentHeight();
    const unsignedTx = new TransactionBuilder(creationHeight)
      .from([oracleBox])
      .to(new OutputBuilder(oracleBox.value, oracleBox.ergoTree)
        .setAdditionalRegisters({ R4: SLong(newPriceLong).toString() })
        .addTokens(oracleBox.assets))
      .sendChangeTo(botProver.getAddress())
      .payFee(RECOMMENDED_MIN_FEE)
      .build();
    const signedTx = await botProver.sign(unsignedTx);
    await nodeApi.submitTransaction(signedTx);
  } catch (e) { console.error(e); }
  setTimeout(updateOraclePrice, 300000);
}
updateOraclePrice();

See: Oracle Pools V2

3. Backend APIs for dApps

Central API for frontends to handle state, indexing, and wallet orchestration.

Example: Auction House backend provides endpoints for auctions, bids, and transaction construction.

// Express.js backend API with Fleet SDK (conceptual)
import express from 'express';
import { ErgoNodeApi } from '@fleet-sdk/common';
const app = express();
app.use(express.json());
const nodeApi = new ErgoNodeApi({ url: "http://your-node-ip:9053" });
app.post('/api/build-auction-tx', async (req, res) => {
  try {
    const { sellerAddress, tokenId, startPrice, auctionLength } = req.body;
    const currentHeight = await nodeApi.getCurrentHeight();
    const deadline = currentHeight + auctionLength;
    const auctionOutput = {
      value: startPrice.toString(),
      ergoTree: "YOUR_AUCTION_CONTRACT_TREE",
      assets: [{ tokenId, amount: "1" }],
      additionalRegisters: {
        R4: SGroupElement(ErgoAddress.fromBase58(sellerAddress).getPublicKey()).toHex(),
        R5: SLong(BigInt(deadline)).toHex()
      },
      creationHeight: currentHeight
    };
    res.json({ auctionOutput, fee: RECOMMENDED_MIN_FEE.toString(), creationHeight });
  } catch (e) { res.status(500).json({ error: 'Failed to build tx params' }); }
});
app.listen(3000, () => console.log('API running'));

4. Indexer Services

Dedicated services for processing/storing blockchain data efficiently. See: Indexing Guide