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// Wrapper around Esplora-compatible block explorer APIs (mempool.space and
// Blockstream — same REST shape). Every outgoing request goes through one
// rate-aware proxy (#run) so that:
// - offline mode can be enforced in a single place,
// - requests are serialized and spaced (one at a time), and
// - a 429 from ANY request immediately backs off ALL subsequent requests
// (global exponential pause), easing back as requests succeed.
const BACKENDS = {
mainnet: ['https://mempool.space/api', 'https://blockstream.info/api'],
testnet: ['https://mempool.space/testnet/api', 'https://blockstream.info/testnet/api'],
};
export class Api {
constructor(net = 'mainnet') {
this.net = net;
this.offline = false;
this._hosts = BACKENDS[net] || BACKENDS.mainnet;
this._rr = 0;
// Serialized scheduler: one request at a time, min gap between starts.
this._active = 0;
this._maxConcurrent = 1;
this._minGapMs = 500;
this._nextStart = 0;
this._queue = [];
// Global rate-limit state, shared across every request and host.
this._pauseUntil = 0; // no request may start before this time
this._penalty = 0; // current backoff level
this._okStreak = 0; // consecutive successes (used to ease the penalty)
}
explorerTx(txid) {
const path = this.net === 'testnet' ? 'testnet/tx' : 'tx';
return `https://mempool.space/${path}/${txid}`;
}
// ---- rate-aware global scheduler --------------------------------------
#schedule(task) {
return new Promise((resolve, reject) => {
this._queue.push({ task, resolve, reject });
this.#pump();
});
}
#pump() {
while (this._active < this._maxConcurrent && this._queue.length) {
const { task, resolve, reject } = this._queue.shift();
this._active++;
const now = Date.now();
// Respect both the per-request spacing AND any global backoff pause.
const startAt = Math.max(now, this._nextStart, this._pauseUntil);
this._nextStart = startAt + this._minGapMs;
setTimeout(() => {
Promise.resolve()
.then(task)
.then(resolve, reject)
.finally(() => {
this._active--;
this.#pump();
});
}, startAt - now);
}
}
// A 429 anywhere → pause every outgoing request, growing the pause each time.
#penalize() {
this._penalty = Math.min(this._penalty + 1, 6);
this._okStreak = 0;
const backoff = Math.min(2000 * 2 ** (this._penalty - 1), 30000); // 2s→30s
this._pauseUntil = Date.now() + backoff;
}
#reward() {
if (this._penalty === 0) return;
if (++this._okStreak >= 4) {
this._penalty--;
this._okStreak = 0;
}
}
// The single choke-point: fetch through the scheduler with 429 handling.
async #run(url, opts) {
if (this.offline) throw new Error('offline');
return this.#schedule(async () => {
const res = await fetch(url, opts);
if (res.status === 429) {
this.#penalize();
const e = new Error('rate limited');
e.rateLimited = true;
throw e;
}
this.#reward();
return res;
});
}
#host() {
return this._hosts[this._rr++ % this._hosts.length];
}
async #get(path, asText = false) {
let lastErr;
for (let attempt = 0; attempt < 6; attempt++) {
try {
const res = await this.#run(this.#host() + path);
if (!res.ok) throw new Error(`${res.status} ${res.statusText}: ${path}`);
return asText ? await res.text() : await res.json();
} catch (e) {
lastErr = e;
// On a 429 the global pause already delays the retry; for other errors
// add a small wait before trying the other host.
if (!e.rateLimited) await sleep(300);
}
}
throw lastErr;
}
// chain_stats / mempool_stats tell us whether an address has ever been used.
addressInfo(address) {
return this.#get(`/address/${address}`);
}
addressUtxos(address) {
return this.#get(`/address/${address}/utxo`);
}
addressTxs(address) {
return this.#get(`/address/${address}/txs`);
}
async feeRates() {
for (const base of this._hosts) {
const isMempool = base.includes('mempool');
try {
const res = await this.#run(base + (isMempool ? '/v1/fees/recommended' : '/fee-estimates'));
if (!res.ok) continue;
const data = await res.json();
if (isMempool && data && data.halfHourFee) return data;
if (!isMempool && data) return mapEsploraFees(data);
} catch {
/* try next host */
}
}
return { fastestFee: 20, halfHourFee: 10, hourFee: 5, economyFee: 2, minimumFee: 1 };
}
async broadcast(hexTx) {
let lastErr;
for (const base of this._hosts) {
try {
const res = await this.#run(base + '/tx', { method: 'POST', body: hexTx });
const body = await res.text();
if (!res.ok) {
lastErr = new Error(body || `${res.status} broadcast failed`);
continue;
}
return body.trim(); // txid
} catch (e) {
lastErr = e;
}
}
throw lastErr;
}
}
// Esplora /fee-estimates is { blockTarget: sat/vB }. Map to the named tiers the
// UI expects. Round up so we never underpay.
function mapEsploraFees(est) {
const at = (n, d) => Math.max(1, Math.ceil(est[n] || d));
return {
fastestFee: at(1, 10),
halfHourFee: at(3, 5),
hourFee: at(6, 3),
economyFee: at(144, 2),
minimumFee: at(1008, 1),
};
}
function sleep(ms) {
return new Promise((r) => setTimeout(r, ms));
}
// Run async jobs with a small concurrency cap. (The global scheduler is the
// real limiter; this just bounds how many are queued at once.)
export async function pool(items, worker, concurrency = 4) {
const results = new Array(items.length);
let next = 0;
async function run() {
while (next < items.length) {
const i = next++;
results[i] = await worker(items[i], i);
}
}
await Promise.all(Array.from({ length: Math.min(concurrency, items.length) }, run));
return results;
}
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