Aws Lambda Snapstart: Which Lambda Control to Use When

# 1. Reserved Concurrency
# What: Maximum concurrent instances for one function
# Cold starts: NOT eliminated
# Cost: FREE
# Use: Isolate functions, prevent one function consuming all account concurrency

aws lambda put-function-concurrency \
  --function-name my-fn \
  --reserved-concurrent-executions 100
# Requests beyond 100 get 429 TooManyRequests

# 2. Provisioned Concurrency
# What: Pre-initialize N instances — always warm
# Cold starts: ELIMINATED for provisioned count
# Cost: $0.000004646/GB-hour (always running, even idle)
# 1 unit, 512MB, 24/7: $1.63/month. 10 units: $16.30/month

# 3. SnapStart (Java 11/17/21 only)
# What: Snapshot JVM after init, restore on cold start
# Cold starts: Reduced 4-8s to 100-400ms
# Cost: FREE

# Scale PC up before peak traffic, down after — avoid paying for idle
Resources:
  PCTarget:
    Type: AWS::ApplicationAutoScaling::ScalableTarget
    Properties:
      MaxCapacity: 20
      MinCapacity: 2
      ResourceId: !Sub "function:${MyFn}:live"
      ScalableDimension: lambda:function:ProvisionedConcurrency
      ServiceNamespace: lambda

  PCPolicy:
    Type: AWS::ApplicationAutoScaling::ScalingPolicy
    Properties:
      PolicyType: TargetTrackingScaling
      ScalingTargetId: !Ref PCTarget
      TargetTrackingScalingPolicyConfiguration:
        TargetValue: 0.7
        PredefinedMetricSpecification:
          PredefinedMetricType: LambdaProvisionedConcurrencyUtilization

  # Scheduled scale-up at 8AM:
  MorningScale:
    Type: AWS::ApplicationAutoScaling::ScheduledAction
    Properties:
      Schedule: "cron(0 8 * * ? *)"
      ScalableTargetAction:
        MinCapacity: 10
        MaxCapacity: 10

# Use this decision tree:

if runtime == "java":
    try SnapStart first (free, 96% of PC benefit)
    only add PC if you need sub-200ms guaranteed
elif latency_sla_under_50ms:
    use Provisioned Concurrency + Auto Scaling
elif need_function_isolation:
    use Reserved Concurrency
elif traffic_spiky and cold_starts_are_visible:
    use Provisioned Concurrency with scheduled scaling
else:
    optimize code (lazy loading, smaller packages) before paying for PC