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Version: 2.33.1

Per-user Concurrency Limiting


The following policy is based on the Concurrency Limiting blueprint.


Concurrency limiting is a critical strategy for managing the load on an API. By setting limits on the number of concurrent requests API consumers can make, concurrency limiting ensures balanced resource utilization, preventing a single user or small groups of users from monopolizing resources, which can lead to overloading the API. This approach is key to maintaining service reliability and fair access for all API consumers.

Aperture enforces per-key concurrency limits, offering precise control over API usage. Each unique key is assigned a maximum concurrency limit, which dictates the maximum number of concurrent requests allowed. When this limit is reached new requests are rejected.

flowchart RL classDef TokenCounter fill:#F8773D,stroke:#000000,stroke-width:2px; classDef Service fill:#56AE89,stroke:#000000,stroke-width:2px; subgraph Aperture_Cloud ["Aperture Cloud"] TB[\Token Counter/] class TB TokenCounter end class Aperture_Cloud Service TB <-- "counting tokens" --> SDK subgraph "SDK" end class SDK Service

The diagram shows how the Aperture SDK interacts with Aperture Cloud to determine whether to allow or reject incoming requests based on the defined maximum concurrency parameters.


Before exploring Aperture's concurrency limiting capabilities, make sure that you have signed up to Aperture Cloud and set up an organization. For more information on how to sign up, follow our step-by-step guide.

Concurrency Limiting with Aperture SDK

The first step to using the Aperture SDK is to import and set up Aperture Client:

You can obtain your organization address and API Key within the Aperture Cloud UI by clicking the Aperture tab in the sidebar menu.

The next step is making a startFlow call to Aperture. For this call, it is important to specify the control point (concurrency-limiting-feature in our example) and the labels that will align with the concurrency limiting policy, which we will create in Aperture Cloud in one of the next steps.

Now, the next part is assessing whether a request is permissible or not by checking the decision returned by the ShouldRun call. Developers can leverage this decision to either reject requests made by abusive users or to allow requests if the concurrent limit has not been crossed. While our current example only logs the request, in real-world applications, you can execute relevant business logic when a request is allowed. It is important to make the end call made after processing each request, in order to send telemetry data that would provide granular visibility for each flow.

Create a Concurrency Limiting Policy

Navigate to the Policies tab on the sidebar menu, and select Create Policy in the upper-right corner. Next, choose the Rate Limiting blueprint, select Concurrency and complete the form with these specific values:

  1. Policy name: Unique for each policy, this field can be used to define policies tailored for different use cases. Set the policy name to concurrency-limit-test.
  2. Max concurrency: Configures the maximum number of concurrent requests allowed. Set Max concurrency to 20.
  3. Limit by label key: Determines the specific label key used for enforcing concurrency limits. We'll use user_id as an example.
  4. Max inflight duration: Configures the time duration after which a flow is assumed to have ended in case end call is missed. Set Max inflight duration to 60s.
  5. Control point: It can be a particular feature or execution block within a service. We'll use concurrency-limiting-feature as an example.

Concurrency Limit Test

Once you've entered these six fields, click Continue and then Apply Policy to finalize the policy setup.

For this policy, users are permitted to make up to 10 concurrent requests in before hitting the concurrency limit. Additional requests made after the maximum number of concurrent requests is reached are rejected.

Next, we'll proceed to run an example to observe the newly implemented policy in action.

Concurrency Limiting in Action

Begin by cloning the Aperture JS SDK.

Switch to the example directory and follow these steps to run the example:

  1. Install the necessary packages:
    • Run npm install to install the base dependencies.
    • Run npm install @fluxninja/aperture-js to install the Aperture SDK.
  2. Run npx tsc to compile the TypeScript example.
  3. Run node dist/concurrency_limit_example.js to start the compiled example.

Once the example is running, it will prompt you for your Organization address and API Key. In the Aperture Cloud UI, select the Aperture tab from the sidebar menu. Copy and enter both your Organization address and API Key to establish a connection between the SDK and Aperture Cloud.

Monitoring Concurrency Limiting Policy

After running the example for a few minutes, you can review the telemetry data in the Aperture Cloud UI. Navigate to the Aperture Cloud UI, and click the Policies tab located in the sidebar menu. Then, select the concurrency-limiter-test policy that you previously created.

Once you've clicked on the policy, you will see the following dashboard:

Concurrency Limiter Graph 1 Concurrency Limiter Graph 2

These panels provide insights into how the policy is performing by monitoring the number of total, accepted and rejected requests along with the acceptance percentage. Observing these graphs will help you understand the effectiveness of your concurrency limiting setup and help in making any necessary adjustments or optimizations.