# Surface.Validation Design (Epic: SURFACE-SHARING)

> **Status:** Draft v1.0 — aligns with tasks `SURFACE-VAL-01..05`, `LANG-SURFACE-01..03`, `ENTRYTRACE-SURFACE-01..02`, `ZASTAVA-SURFACE-02`, `SCANNER-SECRETS-01..03`.
>
> **Audience:** Engineers integrating Surface Env/FS/Secrets, QA guild, Security guild.

## 1. Objectives

Surface.Validation provides a shared validator framework to ensure all surface consumers meet configuration and data preconditions before performing work. It prevents subtle runtime errors by failing fast with actionable diagnostics.

## 2. Core Interfaces

```csharp
public interface ISurfaceValidator
{
    ValueTask<ValidationResult> ValidateAsync(SurfaceValidationContext context, CancellationToken ct = default);
}

public sealed record SurfaceValidationContext
(
    SurfaceEnvironmentSettings Environment,
    IServiceProvider Services,
    string ComponentName
);

public sealed record ValidationResult
(
    bool IsSuccess,
    IReadOnlyCollection<SurfaceValidationIssue> Issues
);

public sealed record SurfaceValidationIssue
(
    string Code,
    string Message,
    SurfaceValidationSeverity Severity,
    string? Hint = null
);
```

Validators register with DI (`services.AddSurfaceValidation()`). Hosts call `ISurfaceValidatorRunner.RunAllAsync()` during startup and periodically (optional) to re-check configuration.

## 3. Built-in Validators

| Code | Severity | Description |
|------|----------|-------------|
| `SURFACE_ENV_MISSING_ENDPOINT` | Error | Raised when `SurfaceFsEndpoint` absent. |
| `SURFACE_ENV_CACHE_DIR_UNWRITABLE` | Error | Cache root not writable or disk full. |
| `SURFACE_SECRET_MISSING` | Error | Secret provider cannot locate required secret type. |
| `SURFACE_SECRET_STALE` | Warning | Secret older than rotation window. |
| `SURFACE_FS_ENDPOINT_REACHABILITY` | Error | HEAD request to Surface.FS endpoint failed. |
| `SURFACE_FS_BUCKET_MISMATCH` | Error | Provided bucket does not exist / lacks permissions. |
| `SURFACE_FEATURE_UNKNOWN` | Warning | Feature flag not recognised. |
| `SURFACE_TENANT_MISMATCH` | Error | Tenant from environment differs from Authority token tenant. |

Validation pipeline stops on the first error (severity `Error`) unless `Surface:Validation:ContinueOnError=true` is set (useful for diagnostics mode).

## 4. Extensibility

Consumers can register custom validators:

```csharp
services.AddSurfaceValidation(builder =>
    builder.AddValidator<RegistryCredentialsValidator>()
           .AddValidator<RuntimeCacheWarmupValidator>());
```

Validators can access DI services (e.g., HttpClient, Authority token provider) through the context. To avoid long-running checks, recommended max validation time is 500ms per validator.

## 5. Reporting & Observability

- Results exposed via `ISurfaceValidationReporter` (default logs structured JSON to `Validation` category).
- Metrics: `surface_validation_issues_total{code,severity}`.
- Optional debug endpoint `/internal/surface/validation` (Scanner WebService) returns last validation run.

## 6. Integration Guidelines

- **Scanner Worker/WebService**: fail startup if any error-level issue occurs; log warnings but continue running.
- **Zastava Webhook**: treat validation errors as fatal (webhook should not enforce policies when surface preconditions fail). Display validation error summary in `/readyz` response to aid debugging.
- **Analysers**: call `SurfaceValidation.Ensure()` before executing heavy work to catch misconfiguration during integration tests.

## 7. Testing Strategy

- Unit tests for built-in validators using in-memory providers.
- Integration tests in Scanner/Zastava verifying validators run during startup and produce expected outcomes.
- Negative tests simulating missing secrets, unreachable endpoints, or mismatched tenants.

## 8. Error Handling & Remediation

- Each issue includes a hint describing remediation steps (e.g., “Verify `SCANNER_SURFACE_FS_ENDPOINT` is reachable from worker nodes”).
- DevOps runbooks should reference issue codes in troubleshooting sections.
- `surface_validation.json` file stored alongside application logs summarises the last run for offline support.

## 9. References

- `docs/modules/scanner/design/surface-env.md`
- `docs/modules/scanner/design/surface-fs.md`
- `docs/modules/scanner/design/surface-secrets.md`
- `docs/modules/devops/runbooks/zastava-deployment.md`