Chapter 2
Introducing Krustlet
Step into the vanguard of cloud-native innovation with Krustlet-a groundbreaking project that redefines how Kubernetes interacts with WebAssembly workloads. This chapter unveils the origins, design decisions, and foundational architecture of Krustlet, demonstrating how it seamlessly integrates with the Kubernetes ecosystem to unlock new possibilities for workload orchestration. By examining Krustlet's distinct approach, supported runtimes, and evolving feature set, we lay the groundwork for understanding how this technology is catalyzing the next wave of cloud-native applications.
2.1 Project Overview and Architecture
Krustlet emerged from a compelling necessity to extend Kubernetes' native scheduling and orchestration capabilities to WebAssembly (Wasm) workloads. Originating as an experimental project, its foundational aim was to bridge the widening gap between cloud-native infrastructure and the lightweight, secure execution environments embodied by Wasm modules. By enabling Kubernetes to schedule Wasm code directly onto nodes through an innovative interface, Krustlet fundamentally redefines the traditional node agent abstraction, thereby enriching the Kubernetes ecosystem with new execution paradigms and resource models.
At the core, Krustlet reconceptualizes the notion of a kubelet-the primary Kubernetes component responsible for managing pods on a node-transforming it into a platform-agnostic Wasm scheduler and runtime orchestrator. Unlike conventional kubelets that manage container runtimes such as Docker or containerd, Krustlet delegates workload execution to a specialized Wasm runtime. This architectural shift introduces a series of abstractions tailored to Wasm's unique characteristics, including lightweight sandboxing, language-agnostic execution, and fine-grained capability control. Consequently, Krustlet decouples Kubernetes scheduling from container-based infrastructure, enabling Wasm workloads to coexist with traditional containerized applications seamlessly.
The architectural blueprint of Krustlet can be understood through three primary abstractions: Node Provider, Wasm Runner, and Kubernetes Runtime Interface.
Node Provider encapsulates the interactions with the Kubernetes control plane, authorizing the Krustlet agent to join the cluster as a legitimate node. It manages node registration, heartbeat signaling, and coordinates resource advertisement. This abstraction establishes Krustlet as a first-class Kubernetes node, capable of receiving and responding to pod scheduling requests.
Wasm Runner represents the execution backend responsible for the instantiation and lifecycle management of Wasm modules. Krustlet is runtime-agnostic and can interchangeably operate with multiple Wasm runtimes such as Wasmtime or Wasmer. The Wasm Runner handles module loading, memory allocation, and I/O bindings, strictly enforcing sandbox policies that isolate Wasm workloads from host resources, thereby enhancing security.
Kubernetes Runtime Interface (KRI) serves as the crucial translation layer that bridges Kubernetes pod specifications into Wasm-specific runtime instructions. KRI interprets abstractions such as containers, volumes, and ports into Wasm-compatible constructs, while facilitating the telemetry feedback loop by emitting appropriate status and event updates back to the Kubernetes API server.
The distinctive modular architecture of Krustlet manifests in its layered design, which prioritizes security, extensibility, and maintainability. Security is systematically architected by enforcing strict sandbox boundaries around Wasm modules. Unlike container runtimes, which share kernel namespaces and capabilities, Wasm's virtual machine model restricts untrusted code execution to a confined, deterministic environment with explicit imports and exports. Krustlet further hardens node security by minimizing its host footprint and exposing a minimal surface area to workloads.
Extensibility is achieved through a plugin-oriented approach, particularly in the Wasm Runner and Node Provider components, allowing seamless integration with emergent Wasm runtimes and underlying infrastructure providers. For example, custom Node Providers can bind Krustlet nodes to diverse environments, including edge devices or specialized hardware. The Wasm Runner's pluggable interface facilitates adoption of new Wasm standards and emerging capability models without invasive rewrites.
Maintainability is sustained by embracing idiomatic Rust programming patterns and comprehensive API abstractions. Krustlet's codebase is strongly typed, asynchronous, and leverages traits to encapsulate behavior flexibly. This design choice allows independent evolution of core subsystems and simplifies testing. Additionally, robust telemetry and logging subsystems are integrated, providing fine-grained observability critical for debugging distributed Wasm workloads within Kubernetes clusters.
The lifecycle of scheduling a Wasm workload on a Krustlet node begins with a standard pod manifest submission containing a container image annotated or replaced by a Wasm module reference. The Kubernetes scheduler dispatches the pod to a Krustlet node, which interprets the specification via the Kubernetes Runtime Interface to identify Wasm-specific parameters such as entrypoint functions and capability requirements. The Wasm Runner then initiates the Wasm module within its securely sandboxed runtime. Through continuous monitoring and event reporting, Krustlet maintains synchronized state with the Kubernetes control plane, ensuring consistent cluster operations.
Distinguishing Krustlet from a traditional kubelet are several key factors:
- Runtime Environment: Traditional kubelets interface with container runtimes executing OS-level virtualization, whereas Krustlet operates on a Wasm virtual machine abstraction providing stronger isolation and lightweight startup.
- Workload Model: Krustlet inherently supports portable, polyglot Wasm modules that encapsulate application logic and dependencies more compactly than typical container images.
- Security Posture: Leveraging the Wasm VM's enforced memory safety and capability-based security model reduces attack surface relative to container privilege escalations.
- Resource Model Adaptation: Kubernetes resource specifications are adapted by Krustlet to Wasm runtime limits and capability gates rather than traditional cgroups and namespace isolation.
Collectively, these differentiators position Krustlet as a critical enabler of next-generation Kubernetes-native compute, fostering rapid adoption of Wasm for cloud, edge, and IoT scenarios. The architectural commitment to modularity and openness facilitates continuous innovation in Kubernetes scheduling while preserving the overall cluster's cohesion and security hygiene.
2.2 Krustlet as a Kubernetes Kubelet
Krustlet operates as an alternative implementation of the Kubernetes kubelet, the primary node agent responsible for managing the lifecycle of pods on Kubernetes nodes. While conventional kubelets interact with container runtimes adhering to Open Container Initiative (OCI) standards, Krustlet innovatively extends this paradigm by acting as a kubelet that schedules and manages WebAssembly (Wasm) workloads. This section explores the architectural and operational mechanisms by which Krustlet fulfills the kubelet contract, detailing its approach to node lifecycle management, pod reconciliation, and translation of Kubernetes pod specifications into Wasm runtime operations. Furthermore, it examines the challenges inherent in reconciling the differences between OCI and Wasm workload models and how Krustlet achieves transparent behavior from a Kubernetes control plane perspective.
At the core of Kubernetes operation lies the control loop, which continuously reconciles the desired cluster state with the actual state. The kubelet's role within this loop involves monitoring assigned pods, reporting status, and ensuring that containers are running as specified. Krustlet implements this control loop contract by subscribing to pod state changes from the Kubernetes API server, retrieving pod specifications that target its designated node. These specifications, known as PodSpecs, define containers with image references, commands, environment variables, resource limits, and volume mounts.
The principal distinction arises in the runtime environment: Krustlet does not launch traditional OCI containers but instead launches Wasm modules inside a Wasm runtime environment such as wasmtime or wasmer. To reconcile this, Krustlet employs a translation layer that interprets Kubernetes PodSpec semantics into configurations compatible with Wasm runtimes. For example, container images in the PodSpec-which typically reference OCI images in a registry-are converted into Wasm modules. These modules are retrieved using OCI distribution specifications adapted for Wasm images, ensuring compatibility ...
