Monitor Lambda cold start durations with CloudWatch

AWS Lambda provides an Initialization subsegment in X-Ray traces that represents the function's initialization code run before the handler, resolving dependencies and initializing global variables. This step is executed only once, but its duration can be significant, especially for functions with many dependencies. To optimize cold start durations, focus on reducing this initialization time, and AWS has introduced support for monitoring it through async-custom-metrics SAR since v1.5.1, allowing users to track the InitDuration metric.

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