ALB LCU reservation
You can set an ALB LCU reservation to establish a baseline of Load Balancer Capacity Unit (LCU) capacity for an ALB instance. In high-traffic scenarios, the ALB instance can immediately use the reserved capacity to handle traffic spikes without waiting for auto-scaling, which improves the availability and stability of your services. The ALB instance continues to use auto-scaling, but its performance will not drop below the reserved capacity. You are billed hourly for the number of reserved LCUs.
Use cases for an LCU reservation:
You are preparing for promotional events that are expected to cause sudden traffic spikes, and you want to ensure your ALB instance can handle the peak traffic.
Your services have bursty workloads with unpredictable traffic spikes.
You are launching or migrating services that require an ALB instance to have high initial performance, rather than waiting for auto-scaling.
You need to maintain a deterministic capacity to meet your business requirements.
You are performing a migration between load balancers and want the target load balancer's performance to match the source.
Usage notes
The ALB LCU reservation feature is disabled by default. To use it, apply for the required quota in the Quota Center.
Only an upgraded ALB instance supports LCU reservations. You can use ALB instance cloning to manually migrate services from an existing ALB instance to an upgraded one.
LCU reservations are supported only on Standard Edition and WAF-enabled ALB instances, not Basic Edition instances. You can upgrade the edition of an ALB instance.
Configure an LCU reservation
To configure an LCU reservation, the instance must have at least one listener in the Running state.
An LCU reservation does not include bandwidth for the elastic IP address (EIP) used by an Internet-facing ALB instance. If you require more bandwidth, purchase and add the EIP to an Internet Shared Bandwidth instance. If you use an Anycast EIP, increase its peak bandwidth.
The reserved LCU capacity is distributed evenly across the zones where the ALB instance is deployed. To achieve optimal performance with an LCU reservation, we recommend deploying the ALB instance in at least two zones, configuring an equal number of backend services in each, and enabling cross-zone load balancing.
Console
In the ALB console, click the ID of the target instance to go to the Instance Details page. On the LCU Reservation tab, click Edit LCU Reservation.
Select a method for Capacity Estimation:
Historic-Reference Based Estimate: If historical traffic data is available, select the target ALB instance from the Reference ALB Instance drop-down list. You can view the historical Peak LCU in the chart below. The Peak LCU value reflects the maximum processing performance that the ALB instance consumed during historical traffic peaks.
Manual Estimate: If no historical traffic data is available, enter the estimated Maximum Bandwidth and New Connections for the ALB instance. The system calculates and displays the Estimated LCUs.
Based on the estimate, enter a value for Load Balancer Capacity Units (LCU) and click OK. You must reserve at least 100 LCUs. The maximum capacity is limited by your quota.
For latency-sensitive applications, such as financial trading, you can deploy the LCU reservation in a deployment set with a low-latency or proximity-based placement strategy to achieve lower forwarding latency. To use this feature, contact your account manager.
API
Call the ModifyCapacityReservation operation to configure an LCU reservation.
View an LCU reservation
Console
In the ALB console, navigate to the Instance Details page of the target instance and click the LCU Reservation tab.
In the LCU Reservation section, view information such as Reservation Status and Reserved LCU.
In the ALB LCU Usage section, compare the Reserved LCU and Peak LCU values to check the usage of the Reserved LCU.
For more information, see Metrics for LCU reservations.
API
Call the DescribeCapacityReservation operation to query the status of an LCU reservation.
The following table describes the possible statuses of an LCU reservation.
Status | Description | Allowed actions |
Pending | The LCU reservation is being configured. | No |
Ready | The reserved capacity is ready for use. | Yes |
Failed | The LCU reservation request cannot be completed at this time. | Yes |
Rebalancing | The load balancer is rebalancing its capacity because a zone was added or removed. Any change to the number of zones for the ALB instance triggers an automatic rebalance to evenly redistribute the total capacity across the zones. | No |
Modify or cancel an LCU reservation
You can increase the reserved LCU capacity for an ALB instance an unlimited number of times. However, you can decrease or cancel the reservation a combined maximum of twice per day.
If the performance metrics that correspond to your reserved capacity exceed the maximum auto-scaling performance of the ALB instance, you cannot decrease or cancel the reservation for seven days after it is created.
Console
In the ALB console, click the ID of the target instance to go to the Instance Details page and select the LCU Reservation tab.
To modify the reserved LCU capacity, click Edit LCU Reservation, change the value for Load Balancer Capacity Units (LCU), and click OK.
To cancel the LCU reservation, click Cancel Capacity and then click OK.
API
Call the ModifyCapacityReservation operation to modify or cancel an LCU reservation.
Billing
ALB LCU fees are billed hourly. A billing cycle is one hour. If you use the service for less than one hour in a billing cycle, the usage is rounded up to one hour. Within a billing cycle, the system compares the initial number of reserved LCUs with the number of reserved LCUs that become ready after a modification, and uses the maximum value to calculate the reserved LCU fee.
If actual LCU usage is less than or equal to the reserved LCU amount, your bill includes only the reserved LCU fee:
reserved LCU fee = unit price per LCU × number of reserved LCUs
If actual LCU usage exceeds the reserved LCU amount, your bill includes both the LCU fee and the reserved LCU fee:
LCU fee = unit price per LCU × (actual LCU usage - number of reserved LCUs)reserved LCU fee = unit price per LCU × number of reserved LCUs
The unit price per reserved LCU is the same as the unit price per LCU. The actual price is subject to the price displayed on the buy page.
Resource plans cannot pay for reserved LCU capacity. They can only cover LCU usage that exceeds the reserved capacity.
The following table shows billing examples for an ALB instance with different usage and reservation configurations in each billing cycle.
Billing cycle | Scenario | Actual LCU usage | Reserved LCUs | Fee |
10:00:00–10:59:59 |
| 20 | - | The LCU fee for this hour is USD 0.007/unit × 20 = USD 0.14. |
11:00:00–11:59:59 |
| 30 | 100 | The reserved LCU fee for this hour is USD 0.007/unit × 100 = USD 0.7. |
12:00:00–12:59:59 |
| 150 | 100 | The LCU fee for this hour is USD 0.007/unit × (150 - 100) = USD 0.35. The reserved LCU fee for this hour is USD 0.007/unit × 100 = USD 0.7. |
13:00:00–13:59:59 |
| 110 | 120 | The reserved LCU fee for this hour is USD 0.007/unit × 120 = USD 0.84. |
14:00:00–14:59:59 |
| 30 | 120 | The reserved LCU fee for this hour is USD 0.007/unit × 120 = USD 0.84. |
Quotas
To request a quota increase, contact your account manager.
Quota name | Description | Default |
alb_quota_reserved_capacity_units_per_loadbalancer | Maximum reserved LCU capacity per ALB instance | 5,000 |
alb_quota_reserved_capacity_units_per_region | Maximum reserved LCU capacity per region | 20,000 |
FAQ
Determining LCU capacity and consequences
Reserving too much capacity leads to unnecessary costs because you are charged for reserved LCUs even if they are not used. Reserving too little may not fully cover traffic peaks, which means some traffic might still need to wait for auto-scaling, affecting service stability. We recommend that you estimate your resource needs as described in the Configure an LCU reservation section and perform stress tests on your services to determine a suitable reservation amount. You can adjust the reservation later based on observed LCU usage.
Effective time of billing changes
A decrease or cancellation takes effect at the beginning of the next billing cycle. For example, if you cancel an LCU reservation at 14:30, you are still billed based on the maximum number of reserved LCUs that were active during the 14:00:00–14:59:59 billing cycle. Billing for the reservation stops at 15:00.