Multi-modal autoscaling with Amazon EC2 Auto Scaling: adding signals for faster, more reliable scaling

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AWS Compute Blog Multi-modal autoscaling with Amazon EC2 Auto Scaling: adding signals for faster, more reliable scaling How do you handle unpredictable workload patterns that spike during promotional events or seasonal peaks? Multi-modal autoscaling with Amazon EC2 Auto Scaling combines infrastructure metrics like CPU with application-level signals, so a group scales on the demand its users create and not only on how busy the servers look. Those signals track the load that drives your business outcomes, such as sales or sign-ups. CPU-based autoscaling works well for many workloads, but some demand does not register as CPU right away. Adding signals such as request counts and application metrics lets a group respond to the load its users create. By publishing Amazon CloudWatch custom metrics and application-driven triggers, you give Auto Scaling more information to act on. In our testing, a group that scaled only on CPU rejected about 7,000 checkout sessions during a demand spike, and a stronger baseline that added Application Load Balancer request count still rejected about 6,900. A group that added an application metric rejected none, and it held p99 latency to about 0.43 seconds against 2.25 seconds for the CPU-only group. Predictive scaling can add a forecasting layer for cyclical demand, but it needs days of history to be useful, so we treat it as a complement. In this post, we show you how to implement multi-modal autoscaling on EC2 Auto Scaling, with code samples and results from a controlled test. Prerequisites To follow along, you need access to the following AWS services with appropriate permissions: - EC2 Auto Scaling, for scaling policies and group management. - CloudWatch, for metrics, alarms, and dashboards. - AWS CloudFormation, for infrastructure deployment. Expanding beyond single-metric scaling The default target tracking policy in EC2 Auto Scaling uses average CPU utilization, a practical starting point because CPU usage is a universal characteristic of compute workloads. Adding complementary signals, such as application-level metrics or predictive forecasting, gives Auto Scaling more information to make timely capacity decisions. For workloads that need a faster response from target tracking alone, see Faster scaling with Amazon EC2 Auto Scaling target tracking. In distributed architectures, different components can have distinct scaling characteristics. An API gateway might correlate well with request rate, while a background processor scales better on queue depth. With multi-modal scaling, you can match each component’s policy to its actual workload pattern. For containerized workloads, consider event-driven autoscaling with KEDA on Amazon Elastic Kubernetes Service (Amazon EKS). Multi-modal autoscaling architecture Multi-modal autoscaling combines three approaches to capacity management. Reactive scaling responds to current CloudWatch metrics, such as CPU utilization, memory, network throughput, response times, and custom application indicators. Application-metric scaling brings workload-specific signals into the decision, using custom CloudWatch metrics like active user sessions, queue depth, or transaction volume. These application metrics are often the closest measurable proxy for business activity such as orders or sign-ups. Predictive scaling uses machine learning in EC2 Auto Scaling to forecast capacity needs from historical patterns, so infrastructure scales before demand increases. With application-metric scaling, applications can scale on signals that infrastructure metrics miss. An ecommerce platform might scale on active checkout sessions, while a streaming service scales on concurrent stream counts. In the test later in this post, we use active checkout sessions as the custom metric. Implementing multi-modal autoscaling This section builds the configuration in layers. Start with CPU target tracking as a baseline that every group keeps, then add a custom application metric that reflects real user load. The test later in this post compares these signals against a request-count baseline. Predictive scaling is an optional forecasting layer described at the end. Step 1: CPU target tracking Start with the foundation that most workloads already use: a target tracking policy on average CPU utilization. Target tracking is a managed policy that adjusts capacity to keep a metric at or near a target value. It supports predefined metrics, including CPU utilization and request count per target, and custom CloudWatch metrics. When multiple target tracking policies are active, Auto Scaling coordinates them: it scales out if any policy requires it, but scales in only when all policies agree, which helps prevent oscillation. Our test also included a second infrastructure baseline, a target tracking policy on the load balancer’s request count per target. It uses the same structure with a predefined metric: Step scaling is another option for spike handling. With step scaling, you can define different capacity increments for different alarm thresholds. It keeps evaluating the alarm during scaling activities, which can make it react faster than target tracking’s default evaluation window. Step scaling policies do not coordinate with each other. Step 2: Add a custom application metric Next, add a second target tracking policy on a custom CloudWatch metric that reflects application load. In our test, instances publish an active checkout sessions metric at a 10-second resolution. To act on that resolution, set a Period of 10 seconds on the policy. Without it, the policy waits for three 1-minute datapoints like any other and the high-resolution metric only adds publishing cost. With it, a scale-out can begin in about 30 seconds. The policy includes the Auto Scaling group dimension so it tracks the metric for the right group. We set the target to 100 active sessions per instance, about 75 percent of the measured per-instance capacity of 135. This leaves headroom to absorb a spike while new instances boot. Step 3: Add predictive scaling Predictive scaling is an optional forecasting layer. It uses machine learning in EC2 Auto Scaling to analyze historical load and scale ahead of recurring, cyclical demand, using customized metric specifications in ForecastAndScale mode. You need to provide several days of history for it to forecast well, so it complements reactive signals rather than replacing them. Start in ForecastOnly mode to watch the forecast before it drives any scaling. Monitoring Use CloudWatch dashboards to track how each policy contributes to scaling decisions, and set alarms on the metrics that matter for your workload, such as per-instance load or latency. Enable detailed monitoring on the launch template, with Monitoring set to true, so that the system publishes CPU metrics every minute. Without it, you cannot complete the CPU policy’s scale-in evaluation and your group will stop scaling in. Watching the policies side by side is what surfaced this scale-in behavior. Performance results We compared three Auto Scaling groups under an identical load profile in a single 75-minute test in the us-east-1 Region. Each group used c8g.large instances with a minimum of 6 and a maximum of 40 instances, and every group carried the same CPU target tracking policy at 70 percent as a fallback: - ASG A: CPU target tracking only. This is the single-signal infrastructure baseline. - ASG B: CPU target tracking plus an Application Load Balancer request-count policy. Request rate is a stronger infrastructure baseline than CPU alone. - ASG C: CPU target tracking plus the custom checkout-sessions metric at 10-second resolution, published with a Period of 10 seconds. All the groups received the same load at the same time. During the shared ramp, arrival rate rose and every group scaled correctly, which makes the comparison fair. CPU crossed 70 percent on the CPU group, request count crossed its target of 300 on the request-count group, and all three converged to a similar size. Then arrival rate was held flat while the number of concurrent checkout sessions kept rising, a shape that infrastructure signals cannot see. The next table reports that divergence phase, measured directly from CloudWatch and the load balancer. The difference is what each group could see. Arrival rate was held flat while the number of concurrent sessions rose, so CPU and request count stayed in range while the application saturated. The CPU-only and request-count groups held at 11 instances and rejected 7,064 and 6,886 checkouts. Their CPU even fell, from about 67 percent to about 45 percent, because a rejected request never reaches the work it would have done, so a policy targeting 70 percent saw spare capacity at the moment the application was failing users. The application-metric group read the rising sessions directly and scaled from 11 to 22 instances, rejecting none. Effect on latency and errors We measured latency and rejected checkouts on the load balancer during the test. At rest, all groups were identical. The gap opened only in the divergence phase, when concurrency rose without a matching change in arrival rate. Session slots are the scarce resource here, so sessions per instance is the causal driver of latency. The application group scales on sessions and we report latency as the outcome, rather than scaling on latency directly, which is not recommended for target tracking. The latency figures come from the load balancer’s TargetResponseTime at the end of the divergence phase. A client-side number measured over the internet would reflect network round-trip rather than the service. The application-metric group, ASG C, kept per-instance load near its target and rejected no checkouts. Its average latency at the end of the divergence phase was 0.284 seconds against 1.122 for the CPU-only group, and its p99 was 0.431 seconds against 2.254. The request-count group, ASG B, tracked its own signal within range the whole time, which is exactly why it could not react: request rate was flat while concurrency climbed. Once every group has enough capacity, they perform the same. The value of the application signal is in the transition, the gap between when demand arrives and when the fleet is ready, which the infrastructure signals here never detected. Handling known high-traffic events For planned events like flash sales, scheduled scaling can pre-scale capacity ahead of time. Multi-modal scaling complements scheduled scaling by handling unplanned spikes and organic traffic that does not follow a fixed schedule. Understanding cost implications Running the application signal requires more instances. During the spike it held about 22 instances, against 11 on the infrastructure-only groups. That extra capacity is what kept sessions per instance near the target and stopped the group from turning checkouts away. For your own workload, the question is whether a spike’s worth of extra instances costs less than the checkouts you would otherwise reject. Conclusion Multi-modal autoscaling combines infrastructure metrics with application-level signals so a group scales on the demand its users create, not only on how busy its servers look. In our test, a group that scaled only on CPU rejected about 7,000 checkout sessions during a demand spike, and a stronger baseline that added load balancer request count still rejected about 6,900. A group that added a custom application metric rejected none, and held p99 latency near 0.43 seconds against 2.25 seconds for the CPU-only group. Its CPU even fell while the infrastructure groups were failing requests, which shows why an infrastructure signal alone can miss the demand that matters. Start with CPU target tracking as a fallback. Add a signal that reflects the load your users create, and pick the one that tracks closest to a business outcome like orders or active users. Set a Period on a high-resolution custom metric so the policy can act on it, and enable detailed monitoring so scale-in works. Predictive scaling is worth adding for demand you can forecast, once the group has days of history to learn from. To implement multi-modal autoscaling, you can open Amazon EC2 Auto Scaling in the AWS Management Console and add a second scaling signal to one of your existing groups, following the configuration steps in this post. For a deeper look at target tracking behavior, see Faster scaling with Amazon EC2 Auto Scaling target tracking. The Amazon EC2 Auto Scaling User Guide covers predictive scaling policies, custom metrics, and scaling cooldowns in detail.