You can set a resource reservation to serve as the Load Balancer Capacity Unit (LCU) capacity baseline for your Application Load Balancer (ALB) instance. During high-load scenarios, ALB can instantly use the reserved capacity to handle traffic peaks. This eliminates the need to wait for auto-scaling to take effect and improves service availability and stability. The ALB instance still scales automatically, but its performance will not drop below the configured reserved capacity. You are charged for the reserved LCU quantity. Fees are calculated on an hourly basis.
Resource reservation scenarios:
You are preparing to launch an operational activity that is expected to cause a sudden traffic spike. You want to ensure that ALB can support the peak traffic during the event.
Your business is burstable, and you cannot effectively predict traffic spikes.
A service that is being published or migrated requires ALB to have high performance from the start, instead of waiting for auto-scaling.
You need to maintain deterministic capacity to meet business requirements.
You are migrating between Server Load Balancer instances and want the performance of the destination Server Load Balancer to match the source Server Load Balancer.
Applicability
The resource reservation feature is not enabled by default. To use this feature, contact your account manager to request access.
Only upgraded ALB instances support resource reservation. ALB instances that have not been upgraded do not support this feature. You can manually migrate services from an existing ALB instance to an upgraded one using ALB instance cloning.
Only Standard Edition and WAF-enhanced ALB instances support resource reservation. Basic Edition ALB instances do not support this feature. For more information, see Upgrade the functional version of an ALB instance.
Configure a resource reservation
To configure a resource reservation, the instance must have at least one listener in the Running state.
An Internet-facing ALB instance provides public network capabilities through elastic IP addresses (EIPs) by default. Resource reservation does not include EIP bandwidth capacity reservation. To obtain more bandwidth, you can purchase and add your EIPs to an Internet Shared Bandwidth instance. If you use an Anycast EIP, see increase the peak bandwidth of the Anycast EIP.
The reserved LCU capacity is evenly distributed among the zones where the ALB instance is deployed. To achieve optimal performance for an ALB instance with resource reservation, we recommend that you deploy the ALB instance in at least two zones, configure the same number of backend services in each zone, and enable cross-zone load balancing.
Console
In the ALB console, click the ID of the target instance to go to the Instance Details page. Select the Resource Reservation tab and click LCU Resource Reservation.
Select a method for Reserved Capacity Estimation:
Estimate Based on Reference Value: If you have historical traffic data, select the target ALB instance from the Reference Server Load Balancer Instance drop-down list. The historical Peak LCU is displayed in the chart below. The Peak LCU value reflects the maximum processing performance that the ALB instance consumed during historical traffic peaks.
Manual Estimation: If you do not have historical traffic data, enter the Peak Bandwidth and New Connections based on your traffic estimates for the ALB instance. Then, refer to the system-calculated Estimated LCU.
Enter a value for Total Reserved LCUs based on the reference value and click OK. You must configure at least 100 LCUs for resource reservation. The maximum configurable capacity is limited by the quota.
If your business involves latency-sensitive scenarios, such as financial transactions, resource reservation supports deploying resources in one of your deployment sets with a low-latency network policy or a proximity spread policy to achieve lower forwarding latency. To use this feature, contact your account manager to request access.
API
Call the ModifyCapacityReservation operation to configure a resource reservation.
View a resource reservation
Console
In the ALB console, click the ID of the target instance to go to the Instance Details page. Select the Resource Reservation tab.
In the Resource Reservation section, you can view information such as Reservation Status and Reserved LCUs.
In the Load Balancer LCU Usage section, you can compare the Reserved LCU and Peak LCU values to check the usage of the Reserved LCU.
For more information, see Metrics for resource reservation.
API
Call the DescribeCapacityReservation operation to query the status of a resource reservation.
The following table describes the possible status values of a resource reservation.
Status | Description | Can be modified or canceled |
Pending | The resource reservation is being configured. | No |
Ready | The reserved capacity is ready and available for use. | Yes |
Failed | The resource reservation request cannot be completed at the moment. | Yes |
Rebalancing | A zone has been added or removed, and the Server Load Balancer instance is rebalancing its capacity. Any change in the number of zones for an ALB instance triggers automatic rebalancing to evenly redistribute the total capacity across the zones. | No |
Modify or cancel a resource reservation
You can upgrade the reserved capacity for each ALB instance an unlimited number of times. However, you can downgrade or cancel the reservation a maximum of two times per day in total.
If the underlying performance metrics that correspond to your reserved capacity exceed the maximum auto-scaling performance of the ALB instance, you cannot downgrade or cancel the reservation for seven days after it is successfully created.
Console
In the ALB console, click the ID of the target instance to go to the Instance Details page. Select the Resource Reservation tab.
To modify the reserved LCU capacity, click LCU Resource Reservation, reset the value of Total Reserved LCUs, and click OK.
To cancel the resource reservation, click Cancel Resource Reservation and then click OK.
API
Call the ModifyCapacityReservation operation to modify or cancel a resource reservation.
Billing details
ALB LCU fees are charged on an hourly basis. The billing cycle is one hour. If the usage duration is less than one hour within a billing cycle, it is counted as one hour. Within a billing cycle, the system compares the initial number of reserved LCUs with the number of LCUs that are ready after a resource reservation modification. The system uses the maximum value to calculate the reserved LCU fee.
If the actual LCU usage is less than or equal to the number of reserved LCUs, the bill includes only the reserved LCU fee:
Reserved LCU fee = Unit price per reserved LCU × Number of reserved LCUs
If the actual LCU usage is greater than the number of reserved LCUs, the 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 reserved LCU × Number of reserved LCUs
The unit price of a reserved LCU is the same as the unit price of an LCU. The actual purchase price is subject to the price displayed on the purchase page.
Resource plans cannot be used to offset fees for reserved LCU capacity. Only the portion of the actual LCU usage that exceeds the reserved LCU quantity can be offset by resource plans.
The following table describes the billing details for an ALB instance with different resource usage and reservation configurations in each billing cycle.
Billing cycle | Key operations and events | Actual LCU usage | Number of reserved LCUs | Fee |
10:00:00 to 10:59:59 |
| 20 | - | LCU fee for this hour = USD 0.007/unit × 20 = USD 0.14 |
11:00:00 to 11:59:59 |
| 30 | 100 | Reserved LCU fee for this hour = USD 0.007/unit × 100 = USD 0.7 |
12:00:00 to 12:59:59 |
| 150 | 100 | LCU fee for this hour = USD 0.007/unit × (150 - 100) = USD 0.35 Reserved LCU fee for this hour = USD 0.007/unit × 100 = USD 0.7 |
13:00:00 to 13:59:59 |
| 110 | 120 | Reserved LCU fee for this hour = USD 0.007/unit × 120 = USD 0.84 |
14:00:00 to 14:59:59 |
| 30 | 120 | Reserved LCU fee for this hour = USD 0.007/unit × 120 = USD 0.84 |
Quotas
You can contact your account manager to request an adjustment.
Quota name | Description | Default value |
alb_quota_reserved_capacity_units_per_loadbalancer | The maximum configurable reserved LCU capacity for each ALB instance | 5000 |
alb_quota_reserved_capacity_units_per_region | The maximum reserved LCU capacity per region | 20000 |
FAQ
How do I determine the LCU capacity to reserve? What are the consequences of reserving too much or too little?
Reserving too much capacity results in wasted costs because you are billed for reserved LCUs even if they are not used. Reserving too little capacity may not fully cover traffic peaks. This may cause some traffic to wait for auto-scaling, which affects service stability. You can estimate your resource needs as described in the Configure a resource reservation section. Combine this estimation with stress testing to determine a reasonable value. You can adjust the reservation later based on your observed LCU usage.
When does billing change after downgrading or canceling a resource reservation?
A downgrade or cancellation takes effect at the beginning of the next billing cycle. For example, if you successfully cancel a resource reservation at 14:30, you are still billed for the maximum number of reserved LCUs that were active during the 14:00:00 to 14:59:59 billing cycle. Billing for the reservation stops at 15:00:00.