control-repetition-with-schedule

description: Use Schedule to create composable policies for controlling the repetition and retrying of effects. globs: "**/*.ts" alwaysApply: true

Control Repetition with Schedule

Rule: Use Schedule to create composable policies for controlling the repetition and retrying of effects.

Example

This example demonstrates composition by creating a common, robust retry policy: exponential backoff with jitter, limited to 5 attempts.

import { Effect, Schedule, Duration } from "effect";

// A simple effect that can fail
const flakyEffect = Effect.try({
  try: () => {
    if (Math.random() > 0.2) {
      throw new Error("Transient error");
    }
    return "Operation succeeded!";
  },
  catch: (error: unknown) => {
    Effect.logInfo("Operation failed, retrying...");
    return error;
  },
});

// --- Building a Composable Schedule ---

// 1. Start with a base exponential backoff (100ms, 200ms, 400ms...)
const exponentialBackoff = Schedule.exponential("100 millis");

// 2. Add random jitter to avoid thundering herd problems
const withJitter = Schedule.jittered(exponentialBackoff);

// 3. Limit the schedule to a maximum of 5 repetitions
const limitedWithJitter = Schedule.compose(withJitter, Schedule.recurs(5));

// --- Using the Schedule ---
const program = Effect.gen(function* () {
  yield* Effect.logInfo("Starting operation...");
  const result = yield* Effect.retry(flakyEffect, limitedWithJitter);
  yield* Effect.logInfo(`Final result: ${result}`);
});

// Run the program
Effect.runPromise(program);

---

Explanation:
While you could write manual loops or recursive functions, Schedule provides a much more powerful, declarative, and composable way to manage repetition. The key benefits are:

• Declarative: You separate the what (the effect to run) from the how and when (the schedule it runs on).
• Composable: You can build complex schedules from simple, primitive ones. For example, you can create a schedule that runs "up to 5 times, with an exponential backoff, plus some random jitter" by composing Schedule.recurs, Schedule.exponential, and Schedule.jittered.
• Stateful: A Schedule keeps track of its own state (like the number of repetitions), making it easy to create policies that depend on the execution history.