Load balancer Capacity Unit Reservation for your Application Load Balancer - Elastic Load Balancing

Load balancer Capacity Unit Reservation for your Application Load Balancer

Load balancer Capacity Unit (LCU) reservation is a capability that allows you to reserve a static minimum capacity for your load balancer. Application Load Balancers automatically scale to support detected workloads and meet capacity needs. When minimum capacity is configured your load balancer will continue scaling up or down based on the traffic received, but will prevent the capacity from going lower than the minimum capacity configured.

Consider using LCU reservation in following situations:

  • You have an upcoming event that will have a sudden, unusual high traffic and want to ensure your load balancer can support the sudden traffic spike during the event.

  • You have unpredictable spiky traffic due to the nature of your workload for a short period.

  • You are setting up your load balancer to on-board or migrate your services at a specific start time and need start with a high capacity instead of waiting for auto-scaling to take effect.

  • You need to maintain a minimum capacity to meet service level agreements or compliance requirements.

  • You are migrating workloads between load balancers and want to configure the destination to match the scale of the source.

Estimate LCU reservation needed

When determining the amount of capacity you should reserve for your load balancer, we recommend performing load testing or reviewing historical workload data that represents the upcoming traffic you expect. Using the Elastic Load Balancing console, you can estimate how much capacity you need to reserve based on the reviewed traffic.

Alternatively, you can utilize the CloudWatch metric PeakLCUs to determine the level of capacity needed. The PeakLCUs metric accounts for peaks in your traffic pattern that the load balancer must scale across all scaling dimensions to support your workload. The PeakLCUs metric is different from the ConsumedLCUs metric, which only aggregates the billing dimensions of your traffic. Using the PeakLCUs metric is recommended to ensure your LCU reservation is adequate during load balancer scaling. When estimating capacity, use Sum(PeakLCUs) if the per-minute PeakLCUs CloudWatch metric is available. Otherwise, use Max(PeakLCUs) * (samplecount / PERIOD(metric) * 60) to estimate.

If you don't have historical workload data to reference and cannot perform load testing, you can estimate capacity needed using the LCU reservation calculator. The LCU reservation calculator uses data based on historical workloads AWS observe and may not represent your specific workload. For more information, see Load Balancer Capacity Unit Reservation Calculator.

LCU reservation service quotas

Your account has quotas related to LCUs. For more information, see LCU quotas.