Coordinated Self-Adaptation in Large-Scale Peer-to-Peer Overlays

Self-adaptive systems typically rely on a closed control loop which detects when the current behavior deviates too much from the optimal one, determines new optimal values for system parameters, and applies changes to the system configuration. In decentralized systems, implementing each of these steps is challenging, especially when nodes need to coordinate their local configurations. In this paper, we propose a decentralized method to automatically tune global system parameters in a coordinated manner. We use gossip-based protocols to continuously monitor system properties and to disseminate parameter updates. We show that this method applied to a decentralized resource selection service allows the system to quickly adapt to changes in workload types and node properties, and only incurs a negligible communication overhead

Adam2: Reliable Distribution Estimation in Decentralised Environments

To enable decentralised actions in very large distributed systems, it is often important to provide the nodes with global knowledge about the values of attributes across all nodes. This paper shows how, given an attribute whose values are distributed across a large decentralised system, each node can efficiently estimate the statistical distribution of these values. Simulations using heavily skewed real-world node attribute distributions show that our estimation methods outperform the state-of-the-art heuristics by an order of magnitude with an average error of 0.05% and a maximum error of 2%. To obtain this accuracy, each node sends on average just 120 kB of data independent of the system size. Our algorithms also achieve this accuracy in the presence of heavy churn of system membership. Furthermore, our algorithm enables self-tuning by continuously estimating the accuracy of its own distribution approximation