Retry Policy

Retry with Exponential Backoff

Network calls and flaky integrations need a retry wrapper, but plain retries hammer the same endpoint and amplify outages. Exponential backoff doubles the wait between attempts so transient failures recover fast and persistent failures don't DDoS the upstream. This snippet covers the canonical retry, a jittered version that prevents thundering-herd retries across clients, and a policy-driven variant that lets the caller decide which errors are retryable.

Poll Until a Condition Is True

Polling shows up everywhere systems are eventually consistent: waiting for a job status to flip to `done`, for a file to appear, for a deploy to roll out. This snippet walks from a basic fixed-interval poller to one with a deadline, then to exponential backoff with jitter so a thundering herd does not hammer the upstream. Reach for it any time you need to wait for a remote condition without writing the same retry loop again.

Design a Notification Service

Design a multi-channel notification service that delivers 10B push, email, and SMS notifications per day across three independent provider networks (APNs, FCM, SendGrid, Twilio) with priority queues, per-user rate limits, and idempotent retries. The interview centerpiece is the fan-out from a single application event to multiple channels and providers, each with its own rate limits, failure modes, and delivery semantics. We cover priority queues for transactional vs marketing traffic, retry policies with exponential backoff, deduplication of duplicate triggers, user preference enforcement, and the device token lifecycle that quietly invalidates tens of millions of tokens per day.

Validate JSON output and recover from failures

Implement a dependency-free validator for a ticket contract, classify each fixture failure by layer, and design a two-attempt recovery that never commits invalid output.

Explain latency, throughput, and rate limits

Explain why a slow model provider raises concurrency and retry load, using latency, throughput, rate limits, deadlines, and a bounded response strategy.

Implement a bounded retry policy

Implement a pure, deterministic retry policy with attempt limits, capped jittered backoff, retry-after, and a deadline check, then fit it into an executor that handles cancellation, partial streams, and uncertain writes.