feat(api): Add concurrent subscription update conflict resolution wit… (Smartdevs17#685)

* feat(api): Add concurrent subscription update conflict resolution with OCC

Implements an optimistic concurrency control (OCC) mechanism using version numbers to prevent silent data loss during concurrent updates.

- Adds a 'version' field to key entities.
- Introduces a backend OptimisticLockService for version checking.
- Returns 409 Conflict on version mismatch.
- Implements a client-side service with automatic retries and exponential backoff.
- Prepares for manual conflict resolution via a Zustand store if retries fail.

Closes Smartdevs17#613

* fix(lint): resolve prettier, unused var, and path errors

Clears out multiple workspace errors:
- Fixes Prettier layout/syntax issues in crdt.ts and .storybook/main.js.
- Resolves import/export name collisions in the design-system barrel file.
- Corrects broken relative import paths in test files.
- Removes numerous unused variables and imports flagged by the linter.

---------

Co-authored-by: whitezaddy <austinihueze@gmail.com>

Author: Tekprecious

.storybook/main.js

@@ -1,15 +1,12 @@
 /**
  * Storybook Configuration for SubTrackr Design System
- * 
+ *
  * Location: .storybook/main.js
  * Run: npm run storybook
  */
 
 module.exports = {
-  stories: [
-    '../src/design-system/stories/**/*.stories.{ts,tsx}',
-    '../src/**/*.stories.{ts,tsx}',
-  ],
+  stories: ['../src/design-system/stories/**/*.stories.{ts,tsx}', '../src/**/*.stories.{ts,tsx}'],
   addons: [
     '@storybook/addon-essentials',
     '@storybook/addon-ondevice-actions',
@@ -30,7 +27,7 @@ module.exports = {
     reactDocgenTypescriptOptions: {
       shouldExtractLiteralValuesAsTypes: true,
       shouldRemoveUndefinedFromOptional: true,
-      propFilter: (prop: any) => {
+      propFilter: (prop) => {
         if (prop.parent) {
           return !prop.parent.fileName.includes('node_modules');
         }

backend/services/shared/apiResponse.ts

@@ -55,6 +55,8 @@ export interface ApiError {
   message: string;
   /** Optional field-level validation details. */
   details?: Record<string, string>;
+  /** For OCC conflicts, the current version of the resource on the server. */
+  version?: number;
 }
 
 /** Successful response envelope. */
@@ -94,6 +96,8 @@ export type ErrorCode =
   | 'UNAUTHORIZED'
   | 'FORBIDDEN'
   | 'CONFLICT'
+  /** Optimistic Concurrency Control failure. */
+  | 'CONFLICT_VERSION_MISMATCH'
   | 'BAD_REQUEST'
   | 'SERVICE_UNAVAILABLE'
   // ── Rate limiting ─────────────────────────────────────────────────────────
@@ -168,6 +172,7 @@ export const ERROR_HTTP_STATUS_MAP: Record<ErrorCode, number> = {
   UNAUTHORIZED: 401,
   FORBIDDEN: 403,
   CONFLICT: 409,
+  CONFLICT_VERSION_MISMATCH: 409,
   BAD_REQUEST: 400,
   SERVICE_UNAVAILABLE: 503,
   // Rate limiting
@@ -279,11 +284,12 @@ export function fail(
   code: ErrorCode,
   message: string,
   requestId?: string,
-  details?: Record<string, string>,
+  details?: Record<string, string> | { version?: number },
 ): ApiErrorResponse {
+  const errorPayload: ApiError = { code, message, ...details };
   return {
     success: false,
-    error: { code, message, ...(details ? { details } : {}) },
+    error: errorPayload,
     meta: buildMeta(requestId),
   };
 }

backend/services/shared/occ/OptimisticLockService.ts

@@ -0,0 +1,103 @@
+/**
+ * @file OptimisticLockService.ts
+ * @description Issue #613 - Service for Optimistic Concurrency Control (OCC).
+ *
+ * This service provides helpers to handle version-based optimistic locking.
+ * It ensures that concurrent updates do not silently overwrite each other.
+ */
+
+import { fail, fromError, ok, ApiResponse } from '../apiResponse';
+import { getLogger } from '../../../utils/logger';
+
+const logger = getLogger('OptimisticLockService');
+
+export interface VersionedEntity {
+  id: string | number;
+  version: number;
+}
+
+export interface UpdateOptions<T extends VersionedEntity> {
+  /** The entity state from the client, including the version they think they are updating. */
+  clientEntity: T;
+  /** The current entity state from the database. */
+  dbEntity: T;
+  /** The user or process making the request. */
+  actor: { id: string; type: 'user' | 'system' };
+  /** The unique request ID for logging. */
+  requestId?: string;
+  /** If true, bypasses the version check (for admin overrides). */
+  force?: boolean;
+}
+
+/**
+ * Checks if an update operation can proceed by comparing client and database entity versions.
+ *
+ * @returns A successful ApiResponse if the update is allowed, or a 409 Conflict error response if not.
+ */
+export function checkVersion<T extends VersionedEntity>(
+  options: UpdateOptions<T>,
+): ApiResponse<void> {
+  const { clientEntity, dbEntity, actor, requestId, force = false } = options;
+
+  if (force) {
+    logger.warn(
+      {
+        actor,
+        entityId: dbEntity.id,
+        clientVersion: clientEntity.version,
+        dbVersion: dbEntity.version,
+        requestId,
+      },
+      'OCC check bypassed with force=true',
+    );
+    return ok(undefined, requestId);
+  }
+
+  if (clientEntity.version !== dbEntity.version) {
+    logger.warn(
+      {
+        actor,
+        entityId: dbEntity.id,
+        clientVersion: clientEntity.version,
+        dbVersion: dbEntity.version,
+        requestId,
+      },
+      'OCC conflict detected: version mismatch',
+    );
+    return fail(
+      'CONFLICT_VERSION_MISMATCH',
+      `The resource was updated by another process. Please refresh and try again.`,
+      requestId,
+      { version: dbEntity.version },
+    );
+  }
+
+  return ok(undefined, requestId);
+}
+
+/**
+ * Executes a version-checked update.
+ *
+ * @param updateFn A function that performs the database update. It receives the new version number.
+ *                 It should return the updated entity or null/undefined if the update fails.
+ * @returns The result of the update function, or a conflict error.
+ */
+export async function withOptimisticLock<T extends VersionedEntity, R>(
+  options: UpdateOptions<T>,
+  updateFn: (newVersion: number) => Promise<R | null>,
+): Promise<ApiResponse<R>> {
+  const versionCheckResult = checkVersion(options);
+  if (!versionCheckResult.success) {
+    return versionCheckResult;
+  }
+
+  const newVersion = options.dbEntity.version + 1;
+
+  try {
+    const result = await updateFn(newVersion);
+    // Assuming the update function returns null if the DB update fails (e.g., row count 0)
+    return result ? ok(result, options.requestId) : fromError(new Error('Update failed'), options.requestId);
+  } catch (err) {
+    return fromError(err, options.requestId);
+  }
+}

backend/services/shared/occ/__tests__/OptimisticLockService.test.ts

@@ -0,0 +1,88 @@
+import { checkVersion, VersionedEntity } from '../OptimisticLockService';
+
+describe('OptimisticLockService', () => {
+  const actor = { id: 'user-123', type: 'user' as const };
+
+  describe('checkVersion', () => {
+    it('should succeed if versions match', () => {
+      const clientEntity: VersionedEntity = { id: 'sub-1', version: 2 };
+      const dbEntity: VersionedEntity = { id: 'sub-1', version: 2 };
+
+      const result = checkVersion({ clientEntity, dbEntity, actor });
+
+      expect(result.success).toBe(true);
+    });
+
+    it('should fail with 409 conflict if versions mismatch', () => {
+      const clientEntity: VersionedEntity = { id: 'sub-1', version: 1 };
+      const dbEntity: VersionedEntity = { id: 'sub-1', version: 2 };
+
+      const result = checkVersion({ clientEntity, dbEntity, actor });
+
+      expect(result.success).toBe(false);
+      if (!result.success) {
+        expect(result.error.code).toBe('CONFLICT_VERSION_MISMATCH');
+        expect(result.error.message).toContain('The resource was updated by another process.');
+        expect(result.error.version).toBe(2);
+        expect(result.meta.apiVersion).toBe(1);
+      }
+    });
+
+    it('should succeed if force=true is used, even with version mismatch', () => {
+      const clientEntity: VersionedEntity = { id: 'sub-1', version: 1 };
+      const dbEntity: VersionedEntity = { id: 'sub-1', version: 2 };
+
+      const result = checkVersion({ clientEntity, dbEntity, actor, force: true });
+
+      expect(result.success).toBe(true);
+    });
+
+    it('should include requestId in meta for both success and failure', () => {
+      const requestId = 'test-request-id';
+
+      // Success case
+      const successResult = checkVersion({
+        clientEntity: { id: 'sub-1', version: 1 },
+        dbEntity: { id: 'sub-1', version: 1 },
+        actor,
+        requestId,
+      });
+      expect(successResult.meta.requestId).toBe(requestId);
+
+      // Failure case
+      const failureResult = checkVersion({
+        clientEntity: { id: 'sub-1', version: 1 },
+        dbEntity: { id: 'sub-1', version: 2 },
+        actor,
+        requestId,
+      });
+      expect(failureResult.meta.requestId).toBe(requestId);
+    });
+
+    it('should handle a complex entity type', () => {
+      interface Subscription extends VersionedEntity {
+        name: string;
+        status: 'active' | 'paused';
+      }
+
+      const clientEntity: Subscription = {
+        id: 'sub-1',
+        version: 3,
+        name: 'New Name',
+        status: 'paused',
+      };
+      const dbEntity: Subscription = {
+        id: 'sub-1',
+        version: 4,
+        name: 'Old Name',
+        status: 'active',
+      };
+
+      const result = checkVersion({ clientEntity, dbEntity, actor });
+      expect(result.success).toBe(false);
+      if (!result.success) {
+        expect(result.error.version).toBe(4);
+      }
+    });
+  });
+});

mobile/app/services/conflictResolutionService.ts

@@ -0,0 +1,124 @@
+/**
+ * @file conflictResolutionService.ts
+ * @description Issue #613 - Client-side service for handling OCC conflicts.
+ *
+ * This service provides a wrapper for API mutation functions to automatically
+ * handle 409 version conflicts with a retry mechanism.
+ */
+
+import { create } from 'zustand';
+import { ApiErrorResponse } from '../../../backend/services/shared/apiResponse';
+
+
+export interface ConflictState<T> {
+  entityId: string | number;
+  /** The user's attempted changes that were rejected. */
+  localState: T;
+  /** The state of the entity on the server that caused the conflict. */
+  remoteState: T;
+  /** The error response from the server. */
+  error: ApiErrorResponse;
+}
+
+interface ConflictStore<T> {
+  conflict: ConflictState<T> | null;
+  resolve: (conflict: ConflictState<T> | null) => void;
+}
+
+// A generic Zustand store for managing a single, active conflict.
+// In a real app, you might want a map of conflicts by entityId.
+export const useConflictStore = create<ConflictStore<object>>((set) => ({
+  conflict: null,
+  resolve: (conflict) => set({ conflict }),
+}));
+
+export interface RetryOptions<T extends { id: string | number; version: number }> {
+  /** The mutation function to wrap. It must accept the entity to save. */
+  mutationFn: (entity: T) => Promise<ApiErrorResponse | { success: true; data: T }>;
+  /** A function to fetch the latest version of the entity from the server. */
+  fetchLatestFn: (id: string | number) => Promise<T>;
+  /** The initial entity state being submitted by the user. */
+  entity: T & { id: string | number; version: number };
+  /** Maximum number of retry attempts. Defaults to 3. */
+  maxRetries?: number;
+  /** Initial backoff delay in ms. Defaults to 100. */
+  initialBackoffMs?: number;
+  /** Optional callback for when retries are exhausted and manual resolution is required. */
+  onConflictResolved?: (conflict: ConflictState<T>) => void;
+}
+
+/**
+ * Wraps a mutation function with automatic retry logic for OCC conflicts.
+ * If all retries fail, it populates the conflict store for manual resolution.
+ */
+export async function withConflictResolution<T extends { id: string | number; version: number }>(
+  options: RetryOptions<T>,
+): Promise<ApiErrorResponse | { success: true; data: T }> {
+  const {
+    mutationFn,
+    fetchLatestFn,
+    entity,
+    maxRetries = 3,
+    initialBackoffMs = 100,
+    onConflictResolved,
+  } = options;
+
+  let lastError: ApiErrorResponse | null = null;
+  let currentEntity = entity;
+
+  for (let attempt = 0; attempt < maxRetries; attempt++) {
+    const response = await mutationFn(currentEntity);
+
+    if (response.success) {
+      return response;
+    }
+
+    lastError = response;
+
+    // Check if it's a version conflict error
+    if (response.error.code === 'CONFLICT_VERSION_MISMATCH' && response.error.version !== undefined) {
+      // It's a conflict, try to fetch the latest version and retry
+      console.log(`Attempt ${attempt + 1}: Conflict detected. Retrying...`);
+
+      // Exponential backoff
+      if (attempt > 0) {
+        const backoff = initialBackoffMs * Math.pow(2, attempt);
+        await new Promise((resolve) => setTimeout(resolve, backoff));
+      }
+
+      try {
+        const latestEntity = await fetchLatestFn(entity.id);
+        // Merge user's changes onto the new base version
+        currentEntity = { ...latestEntity, ...entity, version: latestEntity.version };
+        continue; // Retry the loop
+      } catch (fetchError) {
+        console.error('Failed to fetch latest entity for conflict resolution:', fetchError);
+        // If fetching the latest fails, we can't proceed automatically.
+        break;
+      }
+    } else {
+      // Not a conflict error, so fail immediately
+      return response;
+    }
+  }
+
+  // If all retries are exhausted, set the conflict state for the UI to handle
+  if (lastError && lastError.error.code === 'CONFLICT_VERSION_MISMATCH') {
+    try {
+      const remoteState = await fetchLatestFn(entity.id);
+      const conflict: ConflictState<T> = {
+        entityId: entity.id,
+        localState: entity,
+        remoteState: remoteState,
+        error: lastError,
+      };
+      // Use the callback if provided, otherwise fall back to the global store
+      onConflictResolved ? onConflictResolved(conflict) : useConflictStore.getState().resolve(conflict);
+    } catch (fetchError) {
+      console.error('Failed to fetch latest entity for manual conflict resolution:', fetchError);
+      // Return the original error as we cannot construct the full conflict state
+    }
+  }
+
+  return lastError!;
+}

sandbox/services/usageTrackingService.ts

@@ -1,4 +1,4 @@
-import { UsageMetrics, HourlyUsage, DailyUsage } from '../types/sandbox';
+import { UsageMetrics } from '../types/sandbox';
 
 export class UsageTrackingService {
   private usageData: Map<string, UsageMetrics> = new Map();