feat: device-apiserver design doc

docs/design/device-apiserver.md

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+# Technical Specification: device-apiserver
+
+## Overview
+The Device API server validates and configures data for the hardware api objects which include GPUs. The API Server services gRPC operations and provides the frontend to a node’s shared hardware resource state through which all other local hardware-aware components interact.
+
+---
+
+## Architecture 
+The server implements the Kubernetes API provider pattern but is optimized for node-local footprint.
+
+### Storage Stack
+To avoid the overhead of a full Etcd cluster, the data layer utilizes:
+- **Kine**: An Etcd shim that translates Etcd v3 API calls into SQL queries.
+- **SQLite**: The database engine.
+  - **Default**: In-memory for ephemeral runtime state.
+  - **Optional**: Persistent file-based storage for state that must survive restarts.
+
+### State Semantics
+- **ResourceVersion (RV)**: Managed by Kine/SQLite. Increments on every write to provide Optimistic Concurrency Control.
+- **Generation**: Managed by the server. Increments only when `.Spec` is modified, signaling desired state changes.
+
+---
+
+## gRPC Interface Definition
+The server exposes the standard CRUD+UpdateStatus+Patch+Watch interface for hardware resources.
+
+| Method | Target | Scope |
+| :--- | :--- | :--- |
+| **CreateGpu** | Full Object | Spec/Metadata |
+| **UpdateGpu** | **Spec Only** | Spec/Metadata |
+| **PatchGpu** | Partial Object | Spec/Metadata |
+| **UpdateGpuStatus** | **Status Only** | Status |
+| **GetGpu** | Read-only | Single Resource |
+| **ListGpus** | Read-only | All Resources |
+| **WatchGpus** | Stream Events | All Resources |
+
+---
+
+## Resource Schema
+All API objects follow the Kubernetes Resource Model (KRM) other than the following exceptions:
+
+### Metadata
+- `ObjectMeta`: A subset of `k8s.io/apimachinery/pkg/apis/meta/v1.ObjectMeta`.
+  - `Name`
+  - `ResourceVersion`
+  - `Namespace`
+  - `UID`
+  - `Generation`
+  - `CreationTimestamp`
+
+- `ListMeta`: A subset of `k8s.io/apimachinery/pkg/apis/meta/v1.ListMeta`.
+  - `ResourceVersion`
+
+### Options
+- `CreateOptions`: Not supported.
+
+- `UpdateOptions`: Not supported.
+
+- `DeleteOptions`: Not supported.
+
+- `ListOptions`: A subset of `k8s.io/apimachinery/pkg/apis/meta/v1.ListOptions`.
+  - `ResourceVersion`
+
+---
+
+## Validation & Concurrency
+- **Immutability**: The server rejects updates attempting to change `metadata.name`, `metadata.uid` or `metadata.namespace`.
+- **Optimistic Concurrency**: If `incoming.ResourceVersion` does not match `current.ResourceVersion`, the server returns a `storage.Conflict` error, forcing the client to re-read and try again.
+- **No-Op**: Updates where the `incoming.Spec` matches `current.Spec` result in a successful return without a database write.
+
+---
+
+## Internal Mechanics
+
+### Bootstrapping & Persistence
+The server's lifecycle is tied to the Kine-managed SQLite instance:
+- **In-Memory**: The SQLite database exists purely in memory. The `device-apiserver` starts with a blank slate. Another component (e.g., the device plugin) is responsible for re-discovering and re-registering the GPUs on every start.
+- **On-Disk**: The SQLite database exists in a single ordinary disk file (e.g., `/var/lib/device-apiserver/state.db`). The `device-apiserver` starts from the last successfully persisted state.
+
+### API Discovery & Registration
+The `device-apiserver` uses a decentralized registration pattern to manage its API surface. During startup, available APIs are automatically discovered and registered with both the storage backend and gRPC server.
+
+---
+
+## Reliability
+- **Database Integrity**: SQLite's WAL (Write-Ahead Logging) mode is enabled by default to allow multiple concurrent readers and a single writer.
+
+---
+
+## Observability
+The server's observability stack is designed for production-grade monitoring.
+
+### Prometheus Metrics
+- **Build Metadata (`device_apiserver_build_info`)**: A constant `Gauge` containing labels for `version`, `revision`, `build_date`, and `goversion`, `compiler`, and `platform`.
+- **Service Availability (`device_apiserver_service_status`)**: A `GuageVec` that tracks the serving state of internal sub-services (`1`: Serving / Ready, `2`: Not Serving / Storage Backend Disconnected).
+- **gRPC Performance (`grpc_server_*`)**: Standard `Histogram`s and `Counter`s via the `grpcprom` provider.
+- **Storage Backend (`kine_*`)**:
+  - **`kine_sql_total`**: A `CounterVec` tracking the total number of SQL operations, labeled by `error_code`.
+  - **`kine_sql_time_seconds`**: A `HistogramVec` providing the distribution of SQL execution times.
+  - **`kine_compact_total`**: A `CounterVec` recording successful and failed history compactions.
+  - **`kine_insert_errors_total`**: A `CounterVec` tracking retries due to unique constraint violations
+
+### Admin & Reflection
+- **gRPC Reflection**: Dynamic discovery of API schema.
+- **Health Checks**: Standard `grpc.health.v1` for liveness and readiness probes.
+- **Channelz**: Low-level socket and connection-level statistics.
+
+---
+
+## Security
+// TODO
+
+---
+
+## Performance
+// TODO
+
+---