feat(async): add jitter to retry exponential backoff (#3379)

async/retry.ts

@@ -1,6 +1,8 @@
 // Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
 // This module is browser compatible.
 
+import { assert } from "../_util/asserts.ts";
+
 export class RetryError extends Error {
   constructor(cause: unknown, count: number) {
     super(`Exceeded max retry count (${count})`);
@@ -20,7 +22,7 @@ export interface RetryOptions {
   minTimeout?: number;
 }
 
-const defaultRetryOptions = {
+const defaultRetryOptions: Required<RetryOptions> = {
   multiplier: 2,
   maxTimeout: 60000,
   maxAttempts: 5,
@@ -46,7 +48,7 @@ const defaultRetryOptions = {
  *  maxAttempts: 5,
  *  minTimeout: 100,
  * });
-```
+ * ```
  */
 export async function retry<T>(
   fn: (() => Promise<T>) | (() => T),
@@ -57,9 +59,11 @@ export async function retry<T>(
     ...opts,
   };
 
-  if (options.maxTimeout >= 0 && options.minTimeout > options.maxTimeout) {
-    throw new RangeError("minTimeout is greater than maxTimeout");
-  }
+  assert(options.maxTimeout >= 0, "maxTimeout is less than 0");
+  assert(
+    options.minTimeout <= options.maxTimeout,
+    "minTimeout is greater than maxTimeout",
+  );
 
   let timeout = options.minTimeout;
   let error: unknown;
@@ -69,14 +73,26 @@ export async function retry<T>(
       return await fn();
     } catch (err) {
       await new Promise((r) => setTimeout(r, timeout));
-      timeout *= options.multiplier;
-      timeout = Math.max(timeout, options.minTimeout);
-      if (options.maxTimeout >= 0) {
-        timeout = Math.min(timeout, options.maxTimeout);
-      }
+
+      timeout = _exponentialBackoffWithJitter(
+        options.maxTimeout,
+        options.minTimeout,
+        i,
+        options.multiplier,
+      );
+
       error = err;
     }
   }
 
   throw new RetryError(error, options.maxAttempts);
 }
+
+export function _exponentialBackoffWithJitter(
+  cap: number,
+  base: number,
+  attempt: number,
+  multiplier: number,
+) {
+  return Math.random() * Math.min(cap, base * multiplier ** attempt);
+}

async/retry_test.ts

@@ -1,6 +1,6 @@
 // Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
-import { retry } from "./retry.ts";
-import { assert, assertRejects } from "../testing/asserts.ts";
+import { _exponentialBackoffWithJitter, retry } from "./retry.ts";
+import { assertEquals, assertRejects } from "../testing/asserts.ts";
 
 function generateErroringFunction(errorsBeforeSucceeds: number) {
   let errorCount = 0;
@@ -19,7 +19,7 @@ Deno.test("[async] retry", async function () {
   const result = await retry(threeErrors, {
     minTimeout: 100,
   });
-  assert(result === 3);
+  assertEquals(result, 3);
 });
 
 Deno.test("[async] retry fails after max errors is passed", async function () {
@@ -31,11 +31,80 @@ Deno.test("[async] retry fails after max errors is passed", async function () {
   );
 });
 
-Deno.test("[async] retry throws if minTimeout is less than maxTimeout", async function () {
-  await assertRejects(() =>
-    retry(() => {}, {
-      minTimeout: 1000,
-      maxTimeout: 100,
-    })
-  );
+Deno.test(
+  "[async] retry throws if minTimeout is less than maxTimeout",
+  async function () {
+    await assertRejects(() =>
+      retry(() => {}, {
+        minTimeout: 1000,
+        maxTimeout: 100,
+      })
+    );
+  },
+);
+
+Deno.test(
+  "[async] retry throws if minTimeout is less than 0",
+  async function () {
+    await assertRejects(() =>
+      retry(() => {}, {
+        maxTimeout: -1,
+      })
+    );
+  },
+);
+
+// test util to ensure deterministic results during testing of backoff function by polyfilling Math.random
+function prngMulberry32(seed: number) {
+  return function () {
+    let t = (seed += 0x6d2b79f5);
+    t = Math.imul(t ^ (t >>> 15), t | 1);
+    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
+    return ((t ^ (t >>> 14)) >>> 0);
+  };
+}
+
+// random seed generated with crypto.getRandomValues(new Uint32Array(1))[0]
+const INITIAL_SEED = 3460544849;
+
+const expectedTimings: readonly (readonly number[] & { length: 10 })[] & {
+  length: 10;
+} = [
+  [69, 83, 56, 791, 131, 2140, 5480, 7706, 6036, 17908],
+  [54, 16, 23, 381, 145, 2717, 3195, 4374, 3149, 21390],
+  [32, 183, 334, 155, 391, 2954, 2890, 8202, 25202, 38387],
+  [54, 89, 26, 174, 741, 1245, 1021, 12191, 19834, 17559],
+  [74, 71, 113, 43, 496, 3196, 3843, 7860, 8943, 44312],
+  [20, 129, 52, 555, 857, 3072, 3955, 7078, 5640, 1339],
+  [75, 154, 59, 302, 998, 851, 5034, 8401, 23920, 41925],
+  [86, 26, 211, 491, 139, 2263, 4502, 10713, 15976, 32328],
+  [35, 10, 18, 449, 774, 698, 743, 8833, 24537, 7446],
+  [11, 122, 178, 132, 573, 1803, 5107, 4505, 11523, 17598],
+] as const;
+
+Deno.test("[async] retry - backoff function timings", async (t) => {
+  const originalMathRandom = Math.random;
+
+  await t.step("_exponentialBackoffWithJitter", () => {
+    let nextSeed = INITIAL_SEED;
+
+    for (const row of expectedTimings) {
+      const randUint32 = prngMulberry32(nextSeed);
+      nextSeed = prngMulberry32(nextSeed)();
+      Math.random = () => randUint32() / 0x100000000;
+
+      const results: number[] = [];
+      const base = 100;
+      const cap = Infinity;
+
+      for (let i = 0; i < 10; ++i) {
+        const result = _exponentialBackoffWithJitter(cap, base, i, 2);
+        results.push(Math.round(result));
+      }
+
+      assertEquals(results as typeof row, row);
+    }
+  });
+
+  Math.random = originalMathRandom;
 });