Many software-as-a-service (SaaS) providers adopt throttling as a common technique to protect a distributed system from spikes of inbound traffic that might compromise reliability, reduce throughput, or increase operational cost. Multi-tenant SaaS systems have an additional concern of fairness; excessive traffic from one tenant needs to be selectively throttled without impacting the experience of other tenants. This is also known as “the noisy neighbor” problem. AWS itself enforces some combination of throttling and quota limits on nearly all its own service APIs. SaaS providers building on AWS should design and implement throttling strategies in all of their APIs as well.

In this two-part blog series, we will explore tiering and throttling strategies for multi-tenant REST APIs and review tenant isolation models with hands-on sample code. In part 1, we will look at why a tiering and throttling strategy is needed and show how Amazon API Gateway can help by showing sample code. In part 2, we will dive deeper into tenant isolation models as well as considerations for production.

We selected Amazon API Gateway for this architecture since it is a fully managed service that helps developers to create, publish, maintain, monitor, and secure APIs. First, let’s focus on how Amazon API Gateway can be used to throttle REST APIs with fine granularity using Usage Plans and API Keys. Usage Plans define the thresholds beyond which throttling should occur. They also enable quotas, which sets a maximum usage per a day, week, or month. API Keys are identifiers for distinguishing traffic and determining which Usage Plans to apply for each request. We limit the scope of our discussion to REST APIs because other protocols that API Gateway supports — WebSocket APIs and HTTP APIs — have different throttling mechanisms that do not employ Usage Plans or API Keys.

SaaS providers must balance minimizing cost to serve and providing consistent quality of service for all tenants. They also need to ensure one tenant’s activity does not affect the other tenants’ experience. Throttling and quotas are a key aspect of a tiering strategy and important for protecting your service at any scale. In practice, this impact of throttling polices and quota management is continuously monitored and evaluated as the tenant composition and behavior evolve over time.

Architecture Overview

Figure 1. Cloud Architecture of the sample code.

Figure 1 – Architecture of the sample code

To get a firm foundation of the basics of throttling and quotas with API Gateway, we’ve provided sample code in AWS-Samples on GitHub. Not only does it provide a starting point to experiment with Usage Plans and API Keys in the API Gateway, but we will modify this code later to address complexity that happens at scale. The sample code has two main parts: 1) a web frontend and, 2) a serverless backend. The backend is a serverless architecture using Amazon API Gateway, AWS Lambda, Amazon DynamoDB, and Amazon Cognito. As Figure I illustrates, it implements one REST API endpoint, GET /api, that is protected with throttling and quotas. There are additional APIs under the /admin/* resource to provide Read access to Usage Plans, and CRUD operations on API Keys.

All these REST endpoints could be tested with developer tools such as curl or Postman, but we’ve also provided a web application, to help you get started. The web application illustrates how tenants might interact with the SaaS application to browse different tiers of service, purchase API Keys, and test them. The web application is implemented in React and uses AWS Amplify CLI and SDKs.

Prerequisites

To deploy the sample code, you should have the following prerequisites:

For clarity, we’ll use the environment variable, ${TOP}, to indicate the top-most directory in the cloned source code or the top directory in the project when browsing through GitHub.

Detailed instructions on how to install the code are in ${TOP}/INSTALL.md file in the code. After installation, follow the ${TOP}/WALKTHROUGH.md for step-by-step instructions to create a test key with a very small quota limit of 10 requests per day, and use the client to hit that limit. Search for HTTP 429: Too Many Requests as the signal your client has been throttled.

Figure 2: The web application (with browser developer tools enabled) shows that a quick succession of API calls starts returning an HTTP 429 after the quota for the day is exceeded.

Figure 2: The web application (with browser developer tools enabled) shows that a quick succession of API calls starts returning an HTTP 429 after the quota for the day is exceeded.

Responsibilities of the Client to support Throttling

The Client must provide an API Key in the header of the HTTP request, labelled, “X-Api-Key:”. If a resource in API Gateway has throttling enabled and that header is missing or invalid in the request, then API Gateway will reject the request.

Important: API Keys are simple identifiers, not authorization tokens or cryptographic keys. API keys are for throttling and managing quotas for tenants only and not suitable as a security mechanism. There are many ways to properly control access to a REST API in API Gateway, and we refer you to the AWS documentation for more details as that topic is beyond the scope of this post.

Clients should always test for the response to any network call, and implement logic specific to an HTTP 429 response. The correct action is almost always “try again later.” Just how much later, and how many times before giving up, is application dependent. Common approaches include:

  • Retry – With simple retry, client retries the request up to defined maximum retry limit configured
  • Exponential backoff – Exponential backoff uses progressively larger wait time between retries for consecutive errors. As the wait time can become very long quickly, maximum delay and a maximum retry limits should be specified.
  • Jitter – Jitter uses a random amount of delay between retry to prevent large bursts by spreading the request rate.

AWS SDK is an example client-responsibility implementation. Each AWS SDK implements automatic retry logic that uses a combination of retry, exponential backoff, jitter, and maximum retry limit.

SaaS Considerations: Tenant Isolation Strategies at Scale

While the sample code is a good start, the design has an implicit assumption that API Gateway will support as many API Keys as we have number of tenants. In fact, API Gateway has a quota on available per region per account. If the sample code’s requirements are to support more than 10,000 tenants (or if tenants are allowed multiple keys), then the sample implementation is not going to scale, and we need to consider more scalable implementation strategies.

This is one instance of a general challenge with SaaS called “tenant isolation strategies.” We highly recommend reviewing this white paper ‘SasS Tenant Isolation Strategies‘. A brief explanation here is that the one-resource-per-customer (or “siloed”) model is just one of many possible strategies to address tenant isolation. While the siloed model may be the easiest to implement and offers strong isolation, it offers no economy of scale, has high management complexity, and will quickly run into limits set by the underlying AWS Services. Other models besides siloed include pooling, and bridged models. Again, we recommend the whitepaper for more details.

Figure 3. Tiered multi-tenant architectures often employ different tenant isolation strategies at different tiers. Our example is specific to API Keys, but the technique generalizes to storage, compute, and other resources.

Figure 3- Tiered multi-tenant architectures often employ different tenant isolation strategies at different tiers. Our example is specific to API Keys, but the technique generalizes to storage, compute, and other resources.

In this example, we implement a range of tenant isolation strategies at different tiers of service. This allows us to protect against “noisy-neighbors” at the highest tier, minimize outlay of limited resources (namely, API-Keys) at the lowest tier, and still provide an effective, bounded “blast radius” of noisy neighbors at the mid-tier.

A concrete development example helps illustrate how this can be implemented. Assume three tiers of service: Free, Basic, and Premium. One could create a single API Key that is a pooled resource among all tenants in the Free Tier. At the other extreme, each Premium customer would get their own unique API Key. They would protect Premium tier tenants from the ‘noisy neighbor’ effect. In the middle, the Basic tenants would be evenly distributed across a set of fixed keys. This is not complete isolation for each tenant, but the impact of any one tenant is contained within “blast radius” defined.

In production, we recommend a more nuanced approach with additional considerations for monitoring and automation to continuously evaluate tiering strategy. We will revisit these topics in greater detail after considering the sample code.

Conclusion

In this post, we have reviewed how to effectively guard a tiered multi-tenant REST API hosted in Amazon API Gateway. We also explored how tiering and throttling strategies can influence tenant isolation models. In Part 2 of this blog series, we will dive deeper into tenant isolation models and gaining insights with metrics.

If you’d like to know more about the topic, the AWS Well-Architected SaaS Lens Performance Efficiency pillar dives deep on tenant tiers and providing differentiated levels of performance to each tier. It also provides best practices and resources to help you design and reduce impact of noisy neighbors your SaaS solution.

To learn more about Serverless SaaS architectures in general, we recommend the AWS Serverless SaaS Workshop and the SaaS Factory Serverless SaaS reference solution that inspired it.