Delos: Simple, flexible storage for the Facebook control plane

Introduced Delos, a new architecture for replicated storage systems designed for control plane services. Delos features a modular, layered design separating API materialization, update ordering (VirtualLog/SimpleLogs), and reconfiguration. This allows for fast deployment (8 months for initial version), rapid evolution (10x latency reduction via hot-swapping ordering mechanisms), and flexibility in API and performance/reliability guarantees. It uses a VirtualLog abstraction that chains SimpleLogs, enabling dynamic switching between different ordering mechanisms (e.g., ZooKeeper-based, native converged, disaggregated LogDevice) based on latency SLAs.