When should engineering teams use AWS Lambda with Provisioned Concurrency vs the default on demand mode? What are the performance and cost trade-offs?

Teams debate whether always-warm Lambdas are worth the cost; this Q&A helps guide when it makes sense.

A: • With default on demand Lambda, cold starts (especially in Java/C# runtimes or heavy initializers) can add latency (up to hundreds of milliseconds). • Provisioned Concurrency (PC) ensures a set number of execution environments stay warm and ready, eliminating cold starts for those slots. AWS initializes code and runtime ahead of requests. • Ideal when: o User-facing APIs where < 50 ms latency matters, o Low-traffic but high SLAs (e.g. real-time streams, chatbots, IoT triggers), o Daily burst traffic patterns (e.g. 9–5 for business apps). • Cost trade-offs: You pay for warm environments regardless of usage; PC must be allocated per Lambda alias/version. • Fallback: When PC scale is exceeded, additional invocations use on demand and may incur cold starts. • Optimizations: For Java, AWS offers SnapStart (init snapshotting), which helps cold start latency without PC. Use CloudWatch metrics (ConcurrentExecutions) to model average vs peak load and allocate ≈(avg concurrency × average duration) +10% buffer. • Community caution: Ping based warming hacks are unreliable (e.g. warmers via cron/rules). Provisioned Concurrency is the only AWS supported way to guarantee cold start reduction. Tags: AWS Lambda, provisioned concurrency, cold start, performance, cost model