On-callMediumoc-g074

Subject Rate limit from vendorLevel Mid–Senior~30 minCommon in Networking & APIs interviewsIndustries Technology, Software development

Question

Your transactional email pipeline (password resets, receipts) starts lagging at 11:00 — emails that should send in seconds now take 20+ minutes, and some fail. Dashboards: your email vendor (SendGrid-style) API returns a rising share of HTTP 429 'rate limit exceeded' with Retry-After headers; your send queue depth is climbing steadily; your app is otherwise healthy. Recent context: at 10:45 a one-off marketing batch job started blasting a 2M-recipient newsletter through the same API key / account as your transactional mail. How do you triage and mitigate?

What a strong answer looks like

Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.

Learn the concepts

Diagram & narrate the incident

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Run or narrate your approach, then ask the coach.