feat(api): cache fee estimates that hit rpc

packages/bitcore-node/src/routes/api/fee.ts

@@ -1,17 +1,23 @@
 import { Request, Response } from 'express';
 import { ChainStateProvider } from '../../providers/chain-state';
 const router = require('express').Router({ mergeParams: true });
+const feeCache = {};
 
 router.get('/:target', async (req: Request, res: Response) => {
   let { target, chain, network } = req.params;
-  if (network === 'regtest') {
-    return res.json({ feerate: 0.0002 }); // default 20 sat/byte for regtest
+  if (target < 0 || target > 100) {
+    return res.status(400).send('invalid target specified');
+  }
+  const cachedFee = feeCache[`${chain}:${network}:${target}`];
+  if (cachedFee && cachedFee.date > Date.now() - 10 * 1000) {
+    return res.json(cachedFee.fee);
   }
   try {
     let fee = await ChainStateProvider.getFee({ chain, network, target});
     if (!fee) {
       return res.status(404).send('not available right now');
     }
+    feeCache[`${chain}:${network}:${target}`] = { fee, date: Date.now() };
     return res.json(fee);
   } catch (err) {
     return res.status(500).send(err);