A resource reservation sets a baseline Load Balancer Capacity Unit (LCU) for your Application Load Balancer (ALB) instance. During high-load scenarios, ALB instantly uses this reserved capacity to handle traffic spikes without waiting for auto-scaling. This improves service availability and stability. The ALB instance still scales automatically but guarantees a performance baseline equivalent to the configured reserved capacity. You are billed hourly for the number of reserved LCUs.
Resource reservation scenarios:
You are launching an operational activity that is expected to cause a sudden traffic spike and need to ensure that ALB can support the peak traffic during the event.
Your business is burstable, and you cannot effectively predict traffic surges.
A service is being published or migrated and requires ALB to deliver high performance from the start, instead of waiting for auto-scaling.
You need to maintain deterministic capacity to meet business requirements.
You are performing a migration between Server Load Balancer instances and want the target instance’s performance to match the source instance.
Scope
The resource reservation feature is not enabled by default. You can go to the Quota Hub to request a privilege quota.
Resource reservation is supported only on upgraded ALB instances. Pre-upgrade ALB instances do not support this feature. You can use ALB instance cloning to manually migrate services from an existing ALB instance to an upgraded one.
Resource reservation is supported only on Standard Edition and WAF-enhanced ALB instances. Basic Edition ALB instances do not support this feature. You can upgrade the edition 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 access through elastic IP addresses (EIPs) by default. Resource reservation does not include EIP bandwidth reservation. To obtain more bandwidth, you can purchase and add your EIPs to an Internet Shared Bandwidth instance. If you use an Anycast EIP, you can increase the peak bandwidth of the Anycast EIP.
The reserved LCU capacity is evenly distributed across the zones where the ALB instance is deployed. For optimal performance with resource reservation, 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, find the target instance and click its ID. On the Instance Details page, select the Resource Reservation tab and click LCU Resource Reservation.
Select a Reserved Capacity Estimation method:
Estimate Based on Reference Value: If you have historical traffic data, select the target ALB instance from the Reference SLB Instance drop-down list. The chart shows the historical Peak LCU. 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, estimate the traffic of the ALB instance and enter the Peak Bandwidth and New Connections. You can use the system-calculated Estimated LCU as a reference.
Enter the Total Reserved LCUs based on the reference value and click OK. The minimum reservation is 100 LCUs. The maximum capacity is limited by your 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 loose affinity policy to obtain lower forwarding latency. Contact your account manager to request access.
API
Call ModifyCapacityReservation to configure a resource reservation.
View a resource reservation
Console
In the ALB console, find the target instance and click its ID. On the Instance Details page, select the Resource Reservation tab.
In the Resource Reservation section, you can view information such as Reservation Status and Reserved LCU.
In the Load Balancer LCU Usage section, you can compare Reserved LCU and Peak LCU to confirm Reserved LCU usage.
For more information, see Metrics for resource reservation.
API
Call DescribeCapacityReservation to query resource reservation status.
The following table describes the possible resource reservation status values.
Status | Status descriptions | 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 this time. | Yes |
Rebalancing | A zone has been added or removed, and the load balancer is rebalancing capacity. Any change in the number of zones for an ALB instance triggers automatic rebalancing to evenly redistribute total capacity across 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 combined maximum of two times per day.
If the underlying performance metrics corresponding to your reserved capacity exceed the ALB instance’s maximum auto-scaling performance, you cannot downgrade or cancel the reservation for seven days after it becomes ready.
Console
In the ALB console, find the target instance and click its ID. On the Instance Details page, select the Resource Reservation tab.
To modify the reserved LCU capacity, click LCU Resource Reservation, set a new value for Total Reserved LCUs, and then click OK.
To cancel the resource reservation, click Cancel Resource Reservation and then click OK.
API
Call ModifyCapacityReservation to modify or cancel a resource reservation.
Billing details
ALB LCU fees are charged hourly. The billing cycle is one hour. Usage for less than one hour in a billing cycle is billed as a full hour. Within a billing cycle, the system compares the initial reserved LCU count with the count after any modification. The system uses the maximum value to calculate the reserved LCU fee.
If actual LCU usage ≤ reserved LCUs, the bill includes only the reserved LCU fee:
Reserved LCU fee = Unit price per reserved LCU × Number of reserved LCUs
If actual LCU usage > reserved LCUs, the bill includes both the LCU fee and the reserved LCU fee:
LCU fee = Unit price per LCU × (Actual LCU usage − Reserved LCUs)Reserved LCU fee = Unit price per reserved LCU × Number of reserved LCUs
The unit price for reserved LCUs is the same as the LCU unit price. The actual price is displayed on the purchase page.
Resource plans cannot offset fees for reserved LCU capacity. A resource plan can offset fees only for LCU usage that exceeds the reserved capacity.
The following table shows examples of how an ALB instance is billed based on different usage and reservation configurations in each billing cycle.
Billing cycle | Key operations and events | Actual LCU usage | Reserved LCUs | Fee |
10:00:00–10:59:59 |
| 20 | - | LCU fee for this hour = USD 0.007/unit × 20 = USD 0.14 |
11:00:00–11:59:59 |
| 30 | 100 | Reserved LCU fee for this hour = USD 0.007/unit × 100 = USD 0.7 |
12:00:00–12:59:59 |
| 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 |
13:00:00–13:59:59 |
| 110 | 120 | Reserved LCU fee for this hour = USD 0.007/unit × 120 = USD 0.84 |
14:00:00–14:59:59 |
| 30 | 120 | Reserved LCU fee for this hour = USD 0.007/unit × 120 = USD 0.84 |
Quotas
Contact your account manager to request a quota adjustment.
Quota name | Description | Default value |
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
How do I determine the LCU capacity to reserve? What happens if I reserve too much or too little?
Reserving too much capacity results in unnecessary costs because you pay for reserved LCUs even if they are not used. Reserving too little capacity may not fully cover traffic peaks, which forces some traffic to wait for auto-scaling and can affect service stability. You can estimate resources as described in Configure a resource reservation and combine this with stress testing to determine a reasonable value. You can adjust the value later based on the observed reserved LCU usage.
After I downgrade or cancel a resource reservation, when does billing change?
A successful downgrade or cancellation takes effect at the start of the next billing cycle. For example, if you cancel a reservation at 14:30, the 14:00:00–14:59:59 billing cycle is still billed based on the highest reserved LCU count that was active during that hour. The reserved LCU fees are no longer charged from 15:00 onward.