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Version: development

Concurrency Limiter

The Concurrency Limiter component is used to enforce in-flight request concurrency to prevent overloads. It can also be used to enforce limits per entity such as a user to ensure fair access across users. Requests are allowed or denied based on whether the in-flight requests are within the configured limit. Instead of measuring the number of requests, the Concurrency Limiter can also be configured to measure the number of tokens associated with a request. Tokens can be sent as flow labels using Aperture SDKs.


Only accepted requests are counted towards the in-flight concurrency.

Lifecycle of a Request

The Concurrency Limiter maintains a ledger of in-flight requests. The ledger is updated by the Agents based on the flow start and end calls made from the SDKs. Alternatively, for proxy integrations, the flow end is inferred as the access log stream is received from the underlying middleware or proxy.

Distributed Request Ledgers

For each configured Concurrency Limiter Component, every matching Aperture Agent instantiates a copy of the Concurrency Limiter. Although each agent has its own copy of the component, they all share the in-flight request ledger through a distributed cache. This means that they work together as a single Concurrency Limiter, providing seamless coordination and control across Agents. The Agents within an agent group constantly share state and detect failures using a gossip protocol.

Max In-flight Duration

In case of failures at the SDK or middleware/proxy, the flow end call might not be made. To prevent stale entries in the ledger, the Concurrency Limiter allows the definition of a maximum in-flight duration. This can be set according to the maximum time a request is expected to take. If the request exceeds the configured duration, it is automatically removed from the ledger by the Concurrency Limiter.