feat: retry on 503, add Retry-After date (RFC1123) support

[hhsnopek]

Description

This PR introduces a with503Retry higher-order function that wraps fetch and provides robust, spec-aligned retry handling for HTTP 503 Service Unavailable responses. It retries only on 503s, passes all other responses through untouched, and fully honors any provided AbortSignal.

Key Features

1. 503-only retry behavior

• Non-503 responses return immediately.
• Up to MAX_RETRIES = 5 retry attempts for 503 responses.

2. Standards-compliant and defnesive handling of Retry-After:

• Supports both delta-seconds and HTTP-date formats.
• Returns a millisecond delay (getRetryAfterMs) or seconds (getRetryAfterSeconds) with clear unit types in name.
• Past dates resolve to 0 (retry immediately).
• Invalid headers are ignored (treated as if missing).
• Upper bound clamped to MAX_DELAY_MS = 60_000 to avoid pathological server values.

3. Exponential backoff with jitter

• If no valid Retry-After header is present:
• Delay is randomized between 0 and 2^attempt * BASE_DELAY_MS.
• BASE_DELAY_MS = 500
• Helps reduce retry storms and load cascades.

4. Abort-safe retry waits

• All waits use waitForTimeoutOrCancel, which:
  • Resolves on timeout.
  • Rejects immediately with a DOMException "AbortError" if an AbortSignal fires.
  • Ensures retry loops terminate fast on caller cancellation.

5. No change to global fetch

• The wrapper returns a new function and does not mutate globals, making it safe for tests and modular usage.

Motivations

• 503 responses represent transient server overload conditions; retrying is generally safe and recommended.
• Retry-After support ensures compliance with server instructions and avoids piling onto a recovering subsystem.
• Exponential backoff + jitter reduces thundering herds.
• Abort propagation ensures requests do not hang during application shutdown, navigation, or controller-driven cancellation.

Implementation Notes

• The retry loop is explicit and easy to audit.
• getRetryAfterMs is strict, predictable, and does not attempt to interpret exotic date formats beyond what Date can parse.
• The wrapper’s return type remains a normal fetch Promise<Response>, so no API changes for consumers.

Next Steps / Possible Future Improvements

• test cases to validate integration with client
• make wrapper more generic

Checklist

• [x] pnpm test runs as expected.
• [x] pnpm build runs as expected.
• [ ] (If applicable) JSDoc comments have been added or updated for any package exports
• [ ] (If applicable) Documentation has been updated

Type of change

• [ ] 🐛 Bug fix
• [x] 🌟 New feature
• [ ] 🔨 Breaking change
• [ ] 📖 Refactoring / dependency upgrade / documentation
• [ ] other:

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Latest commit: 9fba31bee7509bc612bf9ea41c464f9b007d3fbb

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✨ Finishing touches

🧪 Generate unit tests (beta)

• [ ] Create PR with unit tests
• [ ] Post copyable unit tests in a comment
• [ ] Commit unit tests in branch henry/add-retry-after-date-support

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Project	Deployment	Preview	Comments	Updated (UTC)
clerk-js-sandbox	Ready	Preview	Comment	Dec 5, 2025 11:09pm