Horizontal Autoscaling (HAS) is a powerful feature that allows your application to automatically adjust its computing resources based on ongoing usage. This guide will walk you through the benefits, ideal use cases, and best practices for implementing HAS in your Aptible deployments.

By leveraging HAS, you can optimize resource utilization, improve application performance, and potentially reduce costs. Whether you’re running a web service, API, or any other scalable application, understanding and properly configuring HAS can significantly enhance your infrastructure’s efficiency and reliability.

Let’s dive into the key aspects of Horizontal Autoscaling and how you can make the most of this feature for your Aptible-hosted applications.

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Key Benefits of Horizontal Autoscaling

• Cost efficiency & Performance: Ensure your App Services are always using the optimal amount of containers. Scale loads with periods of high and low usage that can be parallelized - efficiently.
• Greater reliability: Reduce the likelihood of an expensive computation (ie. a web request) consuming all of your fixed size processing capability
• Reduced engineering time: Save time manually scaling your app services with greater automation

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What App Services are good candidates for HAS?

First, let’s consider what sort of process is NOT a candidate:

• Job workers, unless your jobs are idempotent and/or your queue follows exactly-once semantics
• Services that have a costly startup time
  • Scaling up happens during times of increased load, so a restart that takes a long time to complete during these times is not ideal
• Services that cannot be easily parallelized
  • If your workload is not easily parallelized, you could end up in a scenario where all the load is on one container and the others do near-zero work.

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So what’s a good candidate?

• Services that have predictable and well-understood load patterns
  • We talk about this more in How to set thresholds and container minimums for App Services
• Services that have a workload that can be easily parallelized
  • Web workers as an example, since each web request is completely independent from another
• Services that experience periods of high/low load
  • However, there’s no real risk to setting up HAS on any service just in case they ever experience higher load than expected, as long as having multiple processes running at the same time is not a problem (see above for processed that are not candidates).

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How to set thresholds and container minimums for App Services

Horizontal Autoscaling is configured per App Service. Guidelines to keep in mind for configuration:

• Minimum number of containers - Should be set to 2 as a minimum if you want High-Availability
• Max number of containers - This one depends on how many requests you want to be able to handle under load, and will differ due to specifics of how your app behaves. If you’ve done load testing with your app and understand how many requests your app can handle with the container size you’re currently using, it is a matter of calculating how many more containers you’d want.
• Min CPU threshold - You should set this to slightly above the CPU usage your app exhibits when there’s no/minimal usage to ensure scale downs happen, any lower and your app will never scale down. If you want scale downs to happen faster, you can set this threshold higher.
• Max CPU threshold - A good rule of thumb is 80-90%. There is some lead time to scale ups occurring, as we need a minimum amount of metrics to have been gathered before the next scale-up event happens, so setting this close to 100% can lead to bottlenecks. If you want scale ups to happen faster, you can set this threshold lower.
• Scale Up, and Scale Down Steps - These are set to 1 by default, but you are able to modify the values if you want autoscaling events to jump up or down by more than 1 container at a time.

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Let’s go through an example:

We have a service that exhibits periods of load and periods of near-zero use. It is a production service that is critical to us, so we want a high-availability setup, which means our minimum containers will be 2. Metrics for this service are as follows:

Since our low usage is 12%, the HAS default of 0.1 won’t work for us - we would never scale down. Let’s set it to 0.2 to be conservative

At 84% usage when under load, we’re near the limit but not quite topped out. Usually this would mean you need to validate whether this service would actually benefit from having more containers running. In this case, let’s say our monitoring tools have surfaced that request queueing gets high during these times. We could set our scale up threshold to 0.8, the default, or set it a bit lower if we want to be conservative.

With this, we can expect our service to scale up during periods of high load, up to the maximum number of containers if necessary. If we had set our max CPU limit to something like 0.9, the scaling up would be unlikely to trigger in this particular scenario.

With the metrics look-back period set to 10 minutes and our scale-up cooldown set to a minute(the default), we can expect our service to scale up by 1 container every 5 minutes as long as our load across all containers stays above 80%, until we reach the maximum containers we set in the configuration. Note the 5 minutes between each event - that is currently a hardcoded minimum cooldown.

Since we set a min CPU (scale down) threshold high enough to be above our containers minimal usage, we have guaranteed scale downs will occur. We could set our scale-down threshold higher if we want to be more aggressive about maximizing container utility.