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Version: 0.2.0

Introduction

Welcome to the official documentation for Aperture!

Aperture is an open-source flow control and reliability management platform for modern web applications.

Why is flow control needed?

Modern web-scale apps are a complex network of inter-connected microservices that implement features such as account management, search, payments & more. This decoupled architecture has advantages but introduces new complex failure modes. When traffic surges, it can result in a queue buildup on a critical service, kick-starting a positive feedback loop and causing cascading failures. The application stops serving responses in a timely manner and critical end-user transactions are interrupted.

Absence of flow controlAbsence of flow control

Applications are governed by Little’s Law, which describes the relationship between concurrent requests in the system, arrival rate of requests, and response times. For the application to remain stable, the concurrent requests in the system must be limited. Indirect techniques to stabilize applications such as rate-limiting and auto-scaling fall short in enabling good user experiences or business outcomes. Rate-limiting individual users are insufficient in protecting services. Autoscaling is slow to respond and can be cost-prohibitive. As the number of services scales, these techniques get harder to deploy.

Reliability with flow controlReliability with flow control

This is where flow control comes in. Applications can degrade gracefully in real-time when using flow control techniques with Aperture, by prioritizing high-importance features over others.

How Aperture Works

At the fundamental level, Aperture enables flow control through observing, analyzing, and actuating, facilitated by agents and a controller.

Aperture Control LoopAperture Control Loop

Aperture Agents

Aperture Agents live next to your service instances as a sidecar and provide powerful flow control components such as a weighted fair queuing scheduler for prioritized load-shedding and a distributed rate-limiter for abuse prevention. A flow is the fundamental unit of work from the perspective of an Aperture Agent. It could be an API call, a feature, or even a database query.

Graceful degradation of services is achieved by prioritizing critical application features over background workloads. Much like when boarding an aircraft, business class passengers get priority over other passengers; every application has workloads with varying priorities. A video streaming service might view a request to play a movie by a customer as a higher priority than running an internal machine learning workload. A SaaS product might prioritize features used by paid users over those being used by free users. Aperture Agents schedule workloads based on their priorities helping maximize user experience or revenue even during overload scenarios.

Aperture Agents monitor golden signals using an in-built telemetry system and a programmable, high-fidelity flow classifier used to label requests based on attributes such as customer tier or request type. These metrics are analyzed by the controller.

Aperture Controller

The controller is powered by always-on, dataflow-driven policies that continuously track deviations from service-level objectives (SLOs) and calculate recovery or escalation actions. The policies running in the controller are expressed as circuits, much like circuit networks in the game Factorio.

For example, a gradient control circuit component can be used to implement AIMD (Additive Increase, Multiplicative Decrease) style counter-measure that limits the concurrency on a service when response times deteriorate. Advanced control components like PID can be used to further tune the concurrency limits.

Aperture’s Controller is comparable in capabilities to autopilot in aircraft or adaptive cruise control in some automobiles.

Deploying Aperture

Aperture can be inserted into service instances with either Service Meshes or SDKs:

  • Service Mesh: Aperture can be deployed with no changes to application code, using Envoy. It latches onto Envoy’s External Authorization API for control purposes and collects access logs for telemetry purposes. On each request, Envoy sends request metadata to the Aperture Agent for a flow control decision. Inside the Aperture Agent, the request traverses classifiers, rate-limiters, and schedulers, before the decision to accept or drop the request is sent back to Envoy. Aperture participates in the OpenTelemetry tracing protocol as it inserts flow classification labels into requests, enabling visualization in tracing tools such as Jaeger.
  • Aperture SDKs: In addition to service mesh insertion, Aperture provides SDKs that can be used by developers to achieve fine-grained flow control at the feature level inside service code. For example, an e-commerce app may prioritize users in the checkout flow over new sessions when the application is experiencing an overload. The Aperture Controller can be programmed to degrade features as an escalated recovery action when basic load shedding is triggered for several minutes.