Self-adaptive Threshold-based Policy for Microservices Elasticity

The microservice architecture structures an application as a collection of loosely coupled and distributed services. Since application workloads usually change over time, the number of replicas per microservice should be accordingly scaled at run-time. The most widely adopted scaling policy relies on statically defined thresholds, expressed in terms of system-oriented metrics. This policy might not be well-suited to scale multi-component and latency-sensitive applications, which express requirements in terms of response time. In this paper, we present a two-layered hierarchical solution for controlling the elasticity of microservice-based applications. The higher-level controller estimates the microservice contribution to the application performance, and informs the lower-level components. The latter accordingly scale the single microservices using a dynamic threshold-based policy. So, we propose MB Threshold and QL Threshold, two policies that employ respectively model-based and model-free reinforcement learning approaches to learn threshold update strategies. These policies can compute different thresholds for the different application components, according to the desired deployment objectives. A wide set of simulation results shows the benefits and flexibility of the proposed solution, emphasizing the advantages of using dynamic thresholds over the most adopted policy that uses static thresholds.