id	example-async-fetcher
type	example
title	Async Batch Fetcher with Bounded Concurrency and Retries
summary	Implement a resilient async task batcher with Semaphore concurrency control, exponential backoff, and error isolation.
lang	en-US
content_version	1
status	reviewed
reviewed_on	2026-09-02

Async Batch Fetcher with Bounded Concurrency and Retries

Unbounded async concurrency (asyncio.gather) leads to socket exhaustion, 429 rate-limiting, and cascading server failures. A production batch fetcher must constrain concurrency with a Semaphore, retry transient failures (429, 503, timeouts) with backoff, fail fast on permanent errors (400, 404), and isolate individual item failures from crashing the batch.

python examples/async-fetcher/verify.py starter --expect-failure
python examples/async-fetcher/verify.py solution

The first command reports expected failures from the naive starter. The second verifies the resilient solution. Give TASK.md to a coding agent, ask it to fix only starter/fetcher.py, and run:

python examples/async-fetcher/verify.py starter