Q: You have a Lambda function that's running fine at 10 invocations/second but fails at 1000/second with throttling errors. How do you handle this?
Lambda has a concurrency limit — default 1000 concurrent executions per region per account.
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🎙️ Candidate Opening & Architectural Context
""In our AWS cloud environment, we managed high-traffic microservices where this exact scenario occurred. When addressing this question, I walk the interviewer through our production incident runbook: isolating the blast radius, checking diagnostic logs and metrics, and applying a safe fix.""
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🛠️ Production Runbook & Step-by-Step Resolution
1️⃣
Initial Diagnostics & Root Cause Analysis
Lambda has a concurrency limit — default 1000 concurrent executions per region per account.
- Request a concurrency limit increase via AWS Support.
- Reserved concurrency — reserve a portion of the account limit for critical functions, prevent other functions from taking it all.
- Provisioned concurrency — pre-warm N instances of the function. No cold starts, no throttling up to that N.
2️⃣
Remediation & Permanent Safeguards
At 1000 req/s with a 1-second function, you need 1000 concurrent = hitting the limit. Solutions:
- Queue-based architecture — put an SQS queue in front. Lambda reads from the queue with controlled concurrency. Requests buffer in SQS instead of being throttled.
- Reduce function duration — faster functions = lower concurrent execution count needed.
💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)
"Pro-Tip: Request a concurrency limit increase via AWS Support.."
⚡ 60-Second Elevator Pitch Talking Points
- Request a concurrency limit increase via AWS Support.
- Reserved concurrency — reserve a portion of the account limit for critical functions, prevent oth...
- Provisioned concurrency — pre-warm N instances of the function. No cold starts, no throttling up ...
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