feat: add rest api server proposal

Signed-off-by: Arsen Gumin <60981750+aagumin@users.noreply.github.com>
Signed-off-by: Arsen Gumin <gumin@live.ru>

proposals/000-rest-submit-server/proposal.md

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+# KEP-XXXX: Add REST API Support for SparkApplication CRD
+
+<!--
+A lightweight REST API proxy for SparkApplication CRDs in the Spark Operator.
+-->
+
+## Summary
+
+Expose a RESTful HTTP interface alongside the Spark Operator to streamline the creation, retrieval, update, listing, and deletion of `SparkApplication` Custom Resources. By bundling a minimal Go-based HTTP server that proxies JSON payloads directly to the Kubernetes API (using `client-go`), users and external systems (CI/CD pipelines, web UIs, custom dashboards) can manage Spark jobs without requiring `kubectl` or deep Kubernetes expertise.
+
+## Motivation
+
+Currently, submitting Spark jobs via the Spark Operator demands crafting and applying Kubernetes manifests with `kubectl` or invoking client libraries. This creates friction for non-Kubernetes-native workflows and requires boilerplate integration code in external tools.
+
+### Goals
+
+- Provide HTTP endpoints for CRUD operations on `SparkApplication` CRs.
+- Allow cluster administrators to configure and integrate the authentication and authorization mechanisms of their choice.
+- Package the REST proxy as a container alongside the Spark Operator in Helm charts or manifests.
+- Ensure minimal resource overhead and operational complexity.
+
+### Non-Goals
+
+- Replacing general-purpose CLI tools like `kubectl` for arbitrary resources.
+- Implementing extensive admission logic or API aggregation capabilities beyond basic proxying.
+- Managing non-Spark CRDs or core Kubernetes objects in this phase.
+
+## Proposal
+
+Deploy a companion HTTP server with the Spark Operator that:
+
+1. **Listens** on a configurable port (default 8080) inside the same pod or as a sidecar.
+2. **Maps HTTP routes** to Kubernetes operations using `client-go`, operating only within a configured namespace scope:
+    - `POST /sparkapplications` → Create
+    - `GET /sparkapplications/{namespace}/{name}` → Get
+    - `PUT /sparkapplications/{namespace}/{name}` → Update
+    - `DELETE /sparkapplications/{namespace}/{name}` → Delete
+    - `GET /sparkapplications?namespace={ns}` → List
+3. **Accepts and returns** only JSON representations of the CRD, ensuring that manifests applied via `kubectl` or submitted via this REST API behave identically with no difference in outcomes.
+4. **Leverages in-cluster config** for authentication, mounting a namespaced ServiceAccount token bound to a Role granting access to `sparkapplications.sparkoperator.k8s.io` within that namespace.
+5. **Supports TLS termination** via mounted certificates (cert-manager or manual).
+6. **Emits** structured logs and exposes Prometheus metrics for request counts and latencies.
+
+### User Stories (Optional)
+
+#### Story 1
+As a data engineer, I want to submit Spark jobs by sending a single HTTP request from my CI pipeline, so I don’t need to install or configure `kubectl` on my build agents.
+
+#### Story 2
+As a platform operator, I want to integrate Spark job submission into our internal web portal using REST calls, so that users can launch jobs without learning Kubernetes details.
+
+#### Story 3
+As a user without Kubernetes expertise, I want to use a familiar HTTP API to submit Spark jobs, so I don’t need direct cluster access or knowledge of `kubectl` commands.
+
+### Notes/Constraints/Caveats (Optional)
+
+- This proxy does not implement Kubernetes API aggregation; it is a user-space proxy translating HTTP to Kubernetes API calls.
+- All CRD validation and defaulting is still handled by the CRD’s OpenAPI schema and the Spark Operator admission logic.
+- TLS and authentication configurations must be explicitly managed by the cluster administrator.
+
+### Risks and Mitigations
+
+| Risk                                    | Mitigation                                                    |
+|-----------------------------------------|---------------------------------------------------------------|
+| Exposed HTTP endpoint could be abused   | Enforce RBAC, require ServiceAccount tokens, support TLS.     |
+| Additional component to maintain        | Keep proxy logic minimal, reuse `client-go`, align with Operator releases. |
+| Single point of failure for submissions | Deploy as a sidecar or with HA replica sets.                  |
+
+## Design Details
+
+- **Server implementation**: Go HTTP server using Gorilla Mux or standard `net/http`, calling Kubernetes API via `client-go`.
+- **Deployment**: Update the Spark Operator Helm chart to include a new Deployment (or sidecar) for the REST proxy, with ServiceAccount and RBAC definitions limited to a namespace.
+- **Configuration**: Helm values for port, TLS cert paths, namespace scope filter, resource limits.
+
+### Test Plan
+
+- **Unit Tests**: Mock `client-go` interactions to verify request-to-API mappings and error handling.
+- **Integration Tests**: Deploy in a test cluster; execute CRUD operations via HTTP and assert correct CRD states.
+- **E2E Tests**: Use the existing Spark Operator E2E framework to submit jobs via the proxy and verify job completion.
+
+## Graduation Criteria
+
+- Alpha: Basic CRUD endpoints implemented, tested in one real cluster, enabled by a feature flag in Helm.
+- Beta: TLS support, metrics, and documentation completed; rolling upgrades tested.
+- Stable: No feature flag; production-grade documentation and test coverage ≥ 90%; promoted in Spark Operator release notes.
+
+## Implementation History
+
+- 2025-04-27: KEP created (provisional).
+
+## Drawbacks
+
+- Introduces an extra deployment and potential attack surface.
+- May duplicate future Kubernetes API aggregation capabilities.
+- Slight increase in operational complexity for cluster administrators.
+
+## Alternatives
+
+- **Standalone Spark Cluster**: Deploy Spark in standalone mode, which natively includes a REST submission server, eliminating the need for an additional proxy component and leveraging Spark’s built-in submission API.
+