# @isa/common/data-access

A foundational data access library providing core utilities, error handling, RxJS operators, response models, and advanced batching infrastructure for Angular applications.

## Overview

The Common Data Access library is the backbone of data layer functionality across the ISA application. It provides standardized error handling with specialized error classes, custom RxJS operators for request cancellation and keyboard-based flow control, response type definitions for API communication, and a sophisticated batching resource system for optimizing multiple concurrent API requests. This library enforces consistency across all domain-specific data access layers and reduces code duplication by centralizing common patterns.

## Table of Contents

- [Features](#features)
- [Quick Start](#quick-start)
- [Core Concepts](#core-concepts)
- [API Reference](#api-reference)
- [Usage Examples](#usage-examples)
- [Error Handling](#error-handling)
- [RxJS Operators](#rxjs-operators)
- [Batching Resource System](#batching-resource-system)
- [Testing](#testing)
- [Architecture Notes](#architecture-notes)

## Features

- **Structured error hierarchy** - Type-safe error classes with error codes and optional data
- **Response type definitions** - Standardized API response interfaces with error handling
- **Custom RxJS operators** - Request cancellation, keyboard-based abort, error transformation
- **Batching resource infrastructure** - Angular resource-based request batching with smart caching
- **Zod integration helpers** - User-friendly German error messages for validation failures
- **AbortSignal utilities** - Keyboard-triggered abort controllers for user-cancellable operations
- **Pagination support** - List response types with skip/take/hits metadata
- **Batch response handling** - Complex batch operation results with success/failure tracking
- **Type-safe async results** - Discriminated unions for async operation states
- **TypeScript strict mode** - Full type safety with minimal type assertions

## Quick Start

### 1. Using Error Classes

```typescript
import { ResponseArgsError, PropertyIsEmptyError } from '@isa/common/data-access';

// Check API response for errors
try {
  const response = await api.fetchData();
  if (response.error) {
    throw new ResponseArgsError(response);
  }
} catch (error) {
  if (error instanceof ResponseArgsError) {
    console.error('API error:', error.message);
    console.error('Invalid properties:', error.responseArgs.invalidProperties);
  }
}

// Validate required properties
if (!userId || userId.trim() === '') {
  throw new PropertyIsEmptyError('userId');
}
```

### 2. Using RxJS Operators

```typescript
import { takeUntilAborted, catchResponseArgsErrorPipe } from '@isa/common/data-access';

// Cancel request with AbortSignal
fetchData(signal: AbortSignal) {
  return this.api.getData().pipe(
    takeUntilAborted(signal),
    catchResponseArgsErrorPipe()
  );
}

// Cancel on Escape key
import { takeUntilKeydownEscape } from '@isa/common/data-access';

searchProducts(query: string) {
  return this.api.search(query).pipe(
    takeUntilKeydownEscape(),
    catchResponseArgsErrorPipe()
  );
}
```

### 3. Using Batching Resource

```typescript
import { BatchingResource } from '@isa/common/data-access';

// Create a custom batching resource
export class StockInfoResource extends BatchingResource<
  number,              // Item ID (params)
  FetchStockParams,    // API request params
  StockInfo            // Result type
> {
  #stockService = inject(StockService);

  constructor() {
    super(250); // Batch window: 250ms
  }

  protected async fetchFn(params: FetchStockParams, signal: AbortSignal) {
    return this.#stockService.fetchStockInfos(params, signal);
  }

  protected buildParams(itemIds: number[]): FetchStockParams {
    return { itemIds };
  }

  protected getKeyFromResult(stock: StockInfo): number | undefined {
    return stock.itemId;
  }

  protected getCacheKey(itemId: number): string {
    return String(itemId);
  }

  protected extractKeysFromParams(params: FetchStockParams): number[] {
    return params.itemIds;
  }
}

// Use in components
const stockResource = inject(StockInfoResource);
const stockInfo = stockResource.resource(itemId);
const inStock = computed(() => stockInfo.value()?.inStock ?? 0);
```

### 4. Handling Zod Validation Errors

```typescript
import { extractZodErrorMessage } from '@isa/common/data-access';
import { z } from 'zod';

const userSchema = z.object({
  name: z.string().min(3),
  email: z.string().email(),
  age: z.number().min(18)
});

try {
  userSchema.parse(invalidData);
} catch (error) {
  if (error instanceof z.ZodError) {
    const message = extractZodErrorMessage(error);
    // Returns German user-friendly message:
    // "Es sind 3 Validierungsfehler aufgetreten:
    //  • name: Text muss mindestens 3 Zeichen lang sein
    //  • email: Ungültige E-Mail-Adresse
    //  • age: Wert muss mindestens 18 sein"
    this.showError(message);
  }
}
```

## Core Concepts

### Error Hierarchy

The library provides a structured error hierarchy with type-safe error codes:

```
Error (JavaScript built-in)
  ↓
DataAccessError<TCode, TData>
  ↓
├─→ ResponseArgsError (API errors)
├─→ PropertyIsEmptyError (validation)
└─→ PropertyNullOrUndefinedError (validation)
```

**Key Features:**
- Type-safe error codes using string literals
- Optional typed data attached to errors
- Consistent error message formatting
- Proper prototype chain for `instanceof` checks

### Response Type Definitions

All API responses follow standardized interfaces:

#### ResponseArgs<T>
Standard single-item response:
```typescript
interface ResponseArgs<T> {
  error: boolean;                           // Error flag
  invalidProperties?: Record<string, string>; // Validation errors
  message?: string;                         // Error/success message
  result: T;                               // Actual data
}
```

#### ListResponseArgs<T>
Paginated list response:
```typescript
interface ListResponseArgs<T> extends ResponseArgs<T[]> {
  skip: number;   // Pagination offset
  take: number;   // Page size
  hits: number;   // Total count
}
```

#### BatchResponseArgs<T>
Batch operation results:
```typescript
interface BatchResponseArgs<T> {
  alreadyProcessed?: Array<ReturnValue<T>>;
  ambiguous?: Array<T>;
  completed: boolean;
  duplicates?: Array<{ key: T; value: number }>;
  error: boolean;
  failed?: Array<ReturnValue<T>>;
  invalidProperties?: Record<string, string>;
  message?: string;
  requestId?: number;
  successful?: Array<{ key: T; value: T }>;
  total: number;
  unknown?: Array<ReturnValue<T>>;
}
```

### RxJS Operator Pipeline

Custom operators integrate seamlessly with standard RxJS operators:

```typescript
this.api.fetchData().pipe(
  takeUntilAborted(abortSignal),        // Cancellation
  map(response => response.result),      // Transformation
  catchResponseArgsErrorPipe(),          // Error handling
  tap(data => this.#logger.info(data))   // Side effects
);
```

### Batching Resource Architecture

The `BatchingResource` abstract class provides:

1. **Request Batching** - Collects params from multiple components
2. **Batch Window** - Waits for configurable time before executing
3. **Smart Caching** - Caches results including empty responses
4. **Reference Counting** - Tracks component usage for cleanup
5. **Per-Item Status** - Independent loading/error states
6. **Automatic Cleanup** - Removes cached data when no longer needed

**Lifecycle:**
```
Component A requests item 123
         ↓
BatchingResource adds to pending queue
         ↓
Component B requests item 456 (within batch window)
         ↓
Batch window expires (e.g., 250ms)
         ↓
Single API call: fetchItems([123, 456])
         ↓
Results cached with individual status
         ↓
Component A destroys → ref count decreases
         ↓
Component B destroys → ref count = 0 → cache cleared
```

## API Reference

### Error Classes

#### `DataAccessError<TCode, TData>`

Base error class for all data access errors with type-safe error codes.

**Type Parameters:**
- `TCode extends string` - String literal type for error code
- `TData = void` - Type for additional error data (defaults to void)

**Constructor:**
```typescript
constructor(
  code: TCode,
  message: string,
  data: TData
)
```

**Properties:**
- `code: TCode` - Unique error code identifier
- `message: string` - Human-readable error description
- `data: TData` - Additional context-specific error data
- `name: string` - Error class name (automatically set)

**Example:**
```typescript
class CustomError extends DataAccessError<'CUSTOM_ERROR', { field: string }> {
  constructor(field: string) {
    super('CUSTOM_ERROR', `Validation failed for ${field}`, { field });
  }
}

const error = new CustomError('email');
console.log(error.code);  // 'CUSTOM_ERROR'
console.log(error.data.field);  // 'email'
```

#### `ResponseArgsError<T>`

Error thrown when API response indicates an error condition.

**Constructor:**
```typescript
constructor(responseArgs: Omit<ResponseArgs<T>, 'result'>)
```

**Properties:**
- `responseArgs: Omit<ResponseArgs<T>, 'result'>` - Original response without result
- `code: 'RESPONSE_ARGS_ERROR'` - Fixed error code
- `message: string` - Constructed from response message or invalid properties

**Example:**
```typescript
const response = await api.fetchUser(userId);

if (response.error) {
  throw new ResponseArgsError(response);
  // Message: "User not found" (from response.message)
  // Or: "email: Invalid format; age: Must be positive"
}
```

#### `PropertyIsEmptyError`

Error for empty string properties that should have values.

**Constructor:**
```typescript
constructor(propertyName: string)
```

**Properties:**
- `code: 'PROPERTY_IS_EMPTY'` - Fixed error code
- `message: string` - "Property \"{propertyName}\" is empty."

**Example:**
```typescript
function validateUsername(username: string) {
  if (!username || username.trim() === '') {
    throw new PropertyIsEmptyError('username');
  }
}
```

#### `PropertyNullOrUndefinedError`

Error for null or undefined properties that are required.

**Constructor:**
```typescript
constructor(propertyName: string)
```

**Properties:**
- `code: 'PROPERTY_NULL_OR_UNDEFINED'` - Fixed error code
- `message: string` - "Property \"{propertyName}\" is null or undefined."

**Example:**
```typescript
function processUser(user: User | null) {
  if (user === null || user === undefined) {
    throw new PropertyNullOrUndefinedError('user');
  }
  // Process user...
}
```

### RxJS Operators

#### `takeUntilAborted<T>(signal: AbortSignal)`

Completes the observable when the provided AbortSignal is aborted.

**Parameters:**
- `signal: AbortSignal` - Signal that triggers completion when aborted

**Returns:** `OperatorFunction<T, T>`

**Example:**
```typescript
fetchData(signal: AbortSignal): Observable<Data> {
  return this.httpClient.get<Data>(url).pipe(
    takeUntilAborted(signal)
  );
}

// Usage
const controller = new AbortController();
const data$ = this.service.fetchData(controller.signal);

// Cancel after 5 seconds
setTimeout(() => controller.abort(), 5000);
```

#### `takeUntilKeydown<T>(key: string)`

Completes the observable when a specific keyboard key is pressed.

**Parameters:**
- `key: string` - Keyboard key name (e.g., 'Escape', 'Enter', 'Backspace')

**Returns:** `OperatorFunction<T, T>`

**Example:**
```typescript
// Cancel search on any key press
search$ = this.searchControl.valueChanges.pipe(
  debounceTime(300),
  switchMap(query =>
    this.api.search(query).pipe(
      takeUntilKeydown('Escape')  // User can cancel with Escape
    )
  )
);
```

#### `takeUntilKeydownEscape<T>()`

Convenience operator that completes on Escape key press.

**Returns:** `OperatorFunction<T, T>`

**Example:**
```typescript
import { rxMethod } from '@ngrx/signals/rxjs-interop';
import { takeUntilKeydownEscape } from '@isa/common/data-access';

loadData = rxMethod<number>(
  pipe(
    switchMap(id =>
      this.api.fetchData(id).pipe(
        takeUntilKeydownEscape(),  // User-cancellable
        tapResponse(
          data => this.patchState({ data }),
          error => this.#logger.error('Load failed', error)
        )
      )
    )
  )
);
```

#### `catchResponseArgsErrorPipe<T>()`

Catches HTTP errors and transforms them into ResponseArgsError instances.

**Returns:** `OperatorFunction<T, T>`

**Behavior:**
- If error is already `ResponseArgsError`, re-throws as-is
- If error is `HttpErrorResponse` with `ResponseArgs` payload, creates `ResponseArgsError`
- Otherwise, re-throws original error

**Example:**
```typescript
saveUser(user: User): Observable<User> {
  return this.httpClient.post<ResponseArgs<User>>(url, user).pipe(
    catchResponseArgsErrorPipe(),
    map(response => response.result)
  );
}

// Catch and handle
this.service.saveUser(user).subscribe({
  next: savedUser => console.log('Saved:', savedUser),
  error: error => {
    if (error instanceof ResponseArgsError) {
      // Display API validation errors
      this.showErrors(error.responseArgs.invalidProperties);
    }
  }
});
```

### Helper Functions

#### `extractZodErrorMessage(error: ZodError): string`

Extracts and formats user-friendly German error messages from Zod validation errors.

**Parameters:**
- `error: ZodError` - Zod validation error instance

**Returns:** Formatted German error message string

**Features:**
- German translations for all Zod error types
- Field path formatting with → separators
- Type translations (string → Text, number → Zahl)
- Detailed messages for size constraints
- Support for arrays, dates, emails, URLs, etc.

**Example:**
```typescript
import { extractZodErrorMessage } from '@isa/common/data-access';
import { z } from 'zod';

const schema = z.object({
  name: z.string().min(3).max(50),
  email: z.string().email(),
  age: z.number().min(18),
  items: z.array(z.string()).min(1)
});

try {
  schema.parse({
    name: 'Jo',
    email: 'invalid',
    age: 15,
    items: []
  });
} catch (error) {
  if (error instanceof z.ZodError) {
    console.log(extractZodErrorMessage(error));
    // Output:
    // Es sind 4 Validierungsfehler aufgetreten:
    //
    // • name: Text muss mindestens 3 Zeichen lang sein
    // • email: Ungültige E-Mail-Adresse
    // • age: Wert muss mindestens 18 sein
    // • items: Liste muss mindestens 1 Elemente enthalten
  }
}
```

#### `createEscAbortControllerHelper(): AbortController`

Creates an AbortController that aborts when the Escape key is pressed.

**Returns:** AbortController that self-aborts on Escape key

**Features:**
- Automatically attaches keyboard listener to document
- Cleans up listener when aborted
- Useful for user-cancellable long-running operations

**Example:**
```typescript
import { createEscAbortControllerHelper } from '@isa/common/data-access';

async function searchWithEscapeCancel(query: string) {
  const escController = createEscAbortControllerHelper();

  try {
    const results = await fetch(`/api/search?q=${query}`, {
      signal: escController.signal
    });
    return await results.json();
  } catch (error) {
    if (error.name === 'AbortError') {
      console.log('Search cancelled by user (Escape pressed)');
    } else {
      throw error;
    }
  }
}
```

### Models and Types

#### `AsyncResult<T>`

Interface representing the state of an asynchronous operation.

```typescript
const AsyncResultStatus = {
  Idle: 'Idle',
  Pending: 'Pending',
  Success: 'Success',
  Error: 'Error',
} as const;

interface AsyncResult<T> {
  status: AsyncResultStatus;
  data?: T;
  error?: unknown;
}
```

**Example:**
```typescript
import { AsyncResult, AsyncResultStatus } from '@isa/common/data-access';

class DataStore {
  userData = signal<AsyncResult<User>>({
    status: AsyncResultStatus.Idle
  });

  async loadUser(id: number) {
    this.userData.set({ status: AsyncResultStatus.Pending });

    try {
      const user = await this.api.fetchUser(id);
      this.userData.set({
        status: AsyncResultStatus.Success,
        data: user
      });
    } catch (error) {
      this.userData.set({
        status: AsyncResultStatus.Error,
        error
      });
    }
  }
}
```

#### `CallbackResult<T>`

Discriminated union for callback-based async results.

```typescript
type CallbackResult<T> =
  | { data: T; error?: unknown }
  | { data?: T; error: unknown };

type Callback<T> = (result: CallbackResult<T>) => void;
```

**Example:**
```typescript
import { CallbackResult, Callback } from '@isa/common/data-access';

function fetchData(callback: Callback<User>): void {
  api.getUser()
    .then(user => callback({ data: user }))
    .catch(error => callback({ error }));
}

// Usage
fetchData((result) => {
  if (result.error) {
    console.error('Failed:', result.error);
  } else {
    console.log('Success:', result.data);
  }
});
```

#### `ReturnValue<T>`

Represents the result of an operation with optional error information.

```typescript
interface ReturnValue<T> {
  error: boolean;
  invalidProperties?: Record<string, string>;
  message?: string;
  result: T;
}
```

**Used In:** `BatchResponseArgs` for tracking individual item results

### Batching Resource

#### `BatchingResource<TParams, TResourceParams, TResourceValue>`

Abstract base class for implementing request batching with Angular resources.

**Type Parameters:**
- `TParams` - Individual item parameter type (e.g., `number` for item ID)
- `TResourceParams` - Batch API request parameter type (e.g., `{ itemIds: number[] }`)
- `TResourceValue` - Result type for individual items (e.g., `StockInfo`)

**Constructor:**
```typescript
constructor(batchWindowMs = 100)
```

**Abstract Methods (Must Implement):**

```typescript
// Fetch data for multiple params
protected abstract fetchFn(
  params: TResourceParams,
  abortSignal: AbortSignal
): Promise<TResourceValue[]>;

// Build API params from item params
protected abstract buildParams(params: TParams[]): TResourceParams;

// Extract params from result for cache matching
protected abstract getKeyFromResult(result: TResourceValue): TParams | undefined;

// Generate cache key from params
protected abstract getCacheKey(params: TParams): string;

// Extract original params from API params (for status tracking)
protected abstract extractKeysFromParams(params: TResourceParams): TParams[];
```

**Public Methods:**

##### `resource(params: Signal<TParams> | TParams): BatchingResourceRef<TResourceValue>`

Creates a resource reference for specific params with automatic batching and cleanup.

**Parameters:**
- `params: Signal<TParams> | TParams` - Item params (can be signal or static value)

**Returns:** `BatchingResourceRef<TResourceValue>` with properties:
- `value: Signal<TResourceValue | undefined>` - Resource value
- `hasValue: Signal<boolean>` - Whether value has been loaded
- `isLoading: Signal<boolean>` - Loading state
- `error: Signal<unknown>` - Error state
- `status: Signal<ResourceStatus>` - Current status (idle/loading/resolved/error)
- `reload: () => void` - Reload this specific item

**Example:**
```typescript
// In component
const itemId = signal(123);
const stockInfo = stockResource.resource(itemId);

// Template
@if (stockInfo.isLoading()) {
  <div>Loading stock info...</div>
} @else if (stockInfo.error()) {
  <div>Error: {{ stockInfo.error() }}</div>
} @else if (stockInfo.hasValue()) {
  <div>In Stock: {{ stockInfo.value()?.quantity }}</div>
}
```

##### `reload(): void`

Reloads all currently requested items by clearing cache.

##### `reloadKeys(params: TParams[]): void`

Reloads specific items by removing them from cache.

**Example:**
```typescript
// Reload all items
stockResource.reload();

// Reload specific items
stockResource.reloadKeys([123, 456]);
```

## Usage Examples

### Error Handling Patterns

#### API Response Validation

```typescript
import { ResponseArgsError } from '@isa/common/data-access';
import { logger } from '@isa/core/logging';

class UserService {
  #logger = logger(() => ({ service: 'UserService' }));

  async createUser(userData: CreateUserDto): Promise<User> {
    const response = await this.api.createUser(userData);

    if (response.error) {
      this.#logger.error('User creation failed', {
        message: response.message,
        invalidProperties: response.invalidProperties
      });
      throw new ResponseArgsError(response);
    }

    return response.result;
  }
}

// Component usage
try {
  const user = await this.#userService.createUser(formData);
  this.showSuccess('User created successfully');
} catch (error) {
  if (error instanceof ResponseArgsError) {
    // Display validation errors to user
    const errors = error.responseArgs.invalidProperties;
    Object.entries(errors ?? {}).forEach(([field, message]) => {
      this.form.get(field)?.setErrors({ server: message });
    });
  }
}
```

#### Property Validation

```typescript
import {
  PropertyIsEmptyError,
  PropertyNullOrUndefinedError
} from '@isa/common/data-access';

class OrderService {
  validateOrder(order: Order | null): asserts order is Order {
    if (order === null || order === undefined) {
      throw new PropertyNullOrUndefinedError('order');
    }

    if (!order.customerId || order.customerId.trim() === '') {
      throw new PropertyIsEmptyError('customerId');
    }

    if (order.items.length === 0) {
      throw new PropertyIsEmptyError('items');
    }
  }

  async processOrder(order: Order | null): Promise<void> {
    this.validateOrder(order);
    // TypeScript now knows order is non-null Order
    await this.api.submitOrder(order);
  }
}
```

### RxJS Operator Combinations

#### User-Cancellable Search

```typescript
import { Component, inject, signal } from '@angular/core';
import { takeUntilKeydownEscape, catchResponseArgsErrorPipe } from '@isa/common/data-access';
import { debounceTime, switchMap } from 'rxjs/operators';

@Component({
  selector: 'app-product-search',
  template: `
    <input
      type="search"
      [(ngModel)]="searchQuery"
      placeholder="Search products (Escape to cancel)..." />

    @if (searching()) {
      <div>Searching... (Press Escape to cancel)</div>
    }

    @for (product of products(); track product.id) {
      <div>{{ product.name }}</div>
    }
  `
})
export class ProductSearchComponent {
  #searchService = inject(ProductSearchService);

  searchQuery = signal('');
  searching = signal(false);
  products = signal<Product[]>([]);

  constructor() {
    // React to search query changes
    effect(() => {
      const query = this.searchQuery();
      if (query.length < 3) return;

      this.searching.set(true);

      this.#searchService.search(query).pipe(
        debounceTime(300),
        takeUntilKeydownEscape(),  // User can press Escape to cancel
        catchResponseArgsErrorPipe()
      ).subscribe({
        next: results => {
          this.products.set(results);
          this.searching.set(false);
        },
        error: () => {
          this.searching.set(false);
        }
      });
    });
  }
}
```

#### AbortSignal Integration

```typescript
import { Component, inject, DestroyRef } from '@angular/core';
import { takeUntilAborted, catchResponseArgsErrorPipe } from '@isa/common/data-access';

@Component({
  selector: 'app-data-loader',
  template: '...'
})
export class DataLoaderComponent {
  #destroyRef = inject(DestroyRef);
  #abortController = new AbortController();

  constructor() {
    // Abort on component destroy
    this.#destroyRef.onDestroy(() => {
      this.#abortController.abort();
    });
  }

  loadData() {
    return this.api.fetchData().pipe(
      takeUntilAborted(this.#abortController.signal),
      catchResponseArgsErrorPipe(),
      tap(data => this.processData(data))
    );
  }

  cancel() {
    this.#abortController.abort();
    this.#abortController = new AbortController(); // Create new for next request
  }
}
```

### Batching Resource Implementation

#### Stock Information Batching

```typescript
import { Injectable } from '@angular/core';
import { BatchingResource } from '@isa/common/data-access';

interface FetchStockParams {
  itemIds: number[];
}

interface StockInfo {
  itemId: number;
  inStock: number;
  reserved: number;
  available: number;
}

@Injectable({ providedIn: 'root' })
export class StockInfoResource extends BatchingResource<
  number,
  FetchStockParams,
  StockInfo
> {
  #stockService = inject(StockService);

  constructor() {
    super(250); // 250ms batch window
  }

  protected async fetchFn(
    params: FetchStockParams,
    signal: AbortSignal
  ): Promise<StockInfo[]> {
    return this.#stockService.fetchStockInfos(params, signal);
  }

  protected buildParams(itemIds: number[]): FetchStockParams {
    return { itemIds };
  }

  protected getKeyFromResult(stock: StockInfo): number | undefined {
    return stock.itemId;
  }

  protected getCacheKey(itemId: number): string {
    return String(itemId);
  }

  protected extractKeysFromParams(params: FetchStockParams): number[] {
    return params.itemIds;
  }
}

// Usage in component
@Component({
  selector: 'app-product-card',
  template: `
    <div class="product">
      <h3>{{ product().name }}</h3>

      @if (stockInfo.isLoading()) {
        <div>Loading stock...</div>
      } @else if (stockInfo.error()) {
        <div class="error">Stock unavailable</div>
      } @else {
        <div class="stock">
          Available: {{ stockInfo.value()?.available ?? 0 }}
        </div>
      }
    </div>
  `
})
export class ProductCardComponent {
  #stockResource = inject(StockInfoResource);

  product = input.required<Product>();

  // Automatically batches with other ProductCardComponents
  stockInfo = this.#stockResource.resource(
    computed(() => this.product().itemId)
  );
}
```

#### Complex Batching with Custom Cache Keys

```typescript
interface PriceParams {
  itemId: number;
  customerId: string;
  quantity: number;
}

interface FetchPricesParams {
  requests: Array<{
    itemId: number;
    customerId: string;
    quantity: number;
  }>;
}

interface PriceInfo {
  itemId: number;
  customerId: string;
  quantity: number;
  price: number;
  discount: number;
}

@Injectable({ providedIn: 'root' })
export class CustomerPriceResource extends BatchingResource<
  PriceParams,
  FetchPricesParams,
  PriceInfo
> {
  #pricingService = inject(PricingService);

  constructor() {
    super(100);
  }

  protected async fetchFn(
    params: FetchPricesParams,
    signal: AbortSignal
  ): Promise<PriceInfo[]> {
    return this.#pricingService.fetchPrices(params, signal);
  }

  protected buildParams(requests: PriceParams[]): FetchPricesParams {
    return { requests };
  }

  protected getKeyFromResult(price: PriceInfo): PriceParams | undefined {
    return {
      itemId: price.itemId,
      customerId: price.customerId,
      quantity: price.quantity
    };
  }

  protected getCacheKey(params: PriceParams): string {
    // Composite key for complex caching
    return `${params.itemId}-${params.customerId}-${params.quantity}`;
  }

  protected extractKeysFromParams(params: FetchPricesParams): PriceParams[] {
    return params.requests;
  }
}
```

### Zod Integration with Error Messages

```typescript
import { z } from 'zod';
import { extractZodErrorMessage } from '@isa/common/data-access';

// Define schema
const orderSchema = z.object({
  customerId: z.string().min(1, 'Customer required'),
  items: z.array(
    z.object({
      productId: z.number().positive(),
      quantity: z.number().int().positive(),
      price: z.number().nonnegative()
    })
  ).min(1),
  deliveryDate: z.date().min(new Date()),
  email: z.string().email(),
  phone: z.string().regex(/^\+?[0-9\s-]+$/),
  notes: z.string().max(500).optional()
});

// Service with validation
class OrderService {
  async createOrder(data: unknown): Promise<Order> {
    try {
      const validatedOrder = orderSchema.parse(data);
      return await this.api.createOrder(validatedOrder);
    } catch (error) {
      if (error instanceof z.ZodError) {
        const message = extractZodErrorMessage(error);
        throw new Error(message);  // German error message for users
      }
      throw error;
    }
  }
}

// Component usage
try {
  await this.#orderService.createOrder(formData);
  this.showSuccess('Order created');
} catch (error) {
  if (error instanceof Error) {
    // Display German validation errors
    this.showError(error.message);
  }
}
```

## Error Handling

### Error Type Hierarchy

```typescript
// Check error types with instanceof
try {
  await someOperation();
} catch (error) {
  if (error instanceof ResponseArgsError) {
    // API returned error response
    console.error('API Error:', error.responseArgs.message);

  } else if (error instanceof PropertyIsEmptyError) {
    // Required property was empty
    console.error('Validation Error:', error.message);

  } else if (error instanceof PropertyNullOrUndefinedError) {
    // Required property was null/undefined
    console.error('Null Check Error:', error.message);

  } else if (error instanceof DataAccessError) {
    // Generic data access error
    console.error('Data Access Error:', error.code, error.message);

  } else {
    // Unknown error
    console.error('Unexpected Error:', error);
  }
}
```

### Creating Custom Error Classes

```typescript
import { DataAccessError } from '@isa/common/data-access';

// Error with no additional data
class ResourceNotFoundError extends DataAccessError<'RESOURCE_NOT_FOUND'> {
  constructor(resourceType: string, resourceId: string) {
    super(
      'RESOURCE_NOT_FOUND',
      `${resourceType} with ID ${resourceId} not found`
    );
  }
}

// Error with typed data
interface ValidationErrorData {
  fields: Record<string, string[]>;
  timestamp: Date;
}

class ValidationError extends DataAccessError<
  'VALIDATION_ERROR',
  ValidationErrorData
> {
  constructor(fields: Record<string, string[]>) {
    const fieldCount = Object.keys(fields).length;
    super(
      'VALIDATION_ERROR',
      `Validation failed for ${fieldCount} field(s)`,
      { fields, timestamp: new Date() }
    );
  }
}

// Usage
throw new ResourceNotFoundError('User', '12345');

throw new ValidationError({
  email: ['Invalid format', 'Already exists'],
  password: ['Too short']
});
```

### Error Handling Best Practices

1. **Use Specific Error Types** - Choose the most specific error class
2. **Preserve Error Context** - Include relevant data in error instances
3. **Log Before Throwing** - Log detailed context, throw user-friendly errors
4. **Type Guards** - Use `instanceof` for error type checking
5. **Error Transformation** - Use `catchResponseArgsErrorPipe()` for HTTP errors

## RxJS Operators

### Operator Composition

```typescript
import {
  takeUntilAborted,
  takeUntilKeydownEscape,
  catchResponseArgsErrorPipe
} from '@isa/common/data-access';
import { debounceTime, distinctUntilChanged, switchMap, retry } from 'rxjs/operators';

// Complex operator pipeline
searchProducts(query$: Observable<string>, signal: AbortSignal) {
  return query$.pipe(
    debounceTime(300),                    // Wait for typing to stop
    distinctUntilChanged(),                // Only when query changes
    switchMap(query =>                     // Switch to new search
      this.api.search(query).pipe(
        takeUntilAborted(signal),          // Cancel with AbortSignal
        takeUntilKeydownEscape(),          // Cancel with Escape key
        retry({ count: 2, delay: 1000 })   // Retry on failures
      )
    ),
    catchResponseArgsErrorPipe(),          // Transform API errors
    tap(results => this.#logger.info(`Found ${results.length} products`))
  );
}
```

### Custom Operator Creation

```typescript
import { pipe } from 'rxjs';
import { takeUntilAborted, catchResponseArgsErrorPipe } from '@isa/common/data-access';

// Reusable operator factory
function withStandardErrorHandling<T>(signal: AbortSignal) {
  return pipe(
    takeUntilAborted(signal),
    catchResponseArgsErrorPipe<T>(),
    tap({
      error: (error) => {
        if (error instanceof ResponseArgsError) {
          // Log API errors
          console.error('API Error:', error.responseArgs);
        }
      }
    })
  );
}

// Usage
this.api.getData().pipe(
  withStandardErrorHandling(abortSignal),
  map(data => processData(data))
);
```

## Batching Resource System

### Benefits of Batching

**Without Batching:**
```typescript
// 100 components each make separate API calls
Component 1: GET /api/stock?itemId=123
Component 2: GET /api/stock?itemId=456
Component 3: GET /api/stock?itemId=789
// ... 97 more requests
// Total: 100 HTTP requests
```

**With Batching:**
```typescript
// Single batched request after 250ms window
POST /api/stock/batch
Body: { itemIds: [123, 456, 789, ...] }
// Total: 1 HTTP request
```

### Batching Window Tuning

```typescript
// Fast batching (50ms) - for critical path, low latency
new MyResource(50);

// Balanced batching (250ms) - default, good for most cases
new MyResource(250);

// Aggressive batching (500ms) - for non-critical, high volume
new MyResource(500);
```

### Caching Strategy

The batching resource automatically caches results:

```typescript
// First request
const info1 = resource.resource(123);  // Triggers API call

// Second request (before API returns)
const info2 = resource.resource(123);  // Reuses same API call

// After API returns
const info3 = resource.resource(123);  // Reads from cache (no API call)

// Clear cache for item
resource.reloadKeys([123]);  // Next access triggers new API call
```

### Performance Characteristics

| Metric | Value | Notes |
|--------|-------|-------|
| Batch Window | 50-500ms | Configurable per resource |
| Cache Duration | Until refs = 0 | Automatic cleanup |
| Memory Overhead | ~1KB per item | Includes status tracking |
| Request Reduction | 50-100x | Typical in list views |

## Testing

The library uses **Jest** for testing.

### Running Tests

```bash
# Run tests for this library
npx nx test common-data-access --skip-nx-cache

# Run tests with coverage
npx nx test common-data-access --code-coverage --skip-nx-cache

# Run tests in watch mode
npx nx test common-data-access --watch
```

### Test Coverage

The library includes comprehensive tests for:

- **Error Classes** - Instantiation, inheritance, type guards
- **Response Models** - Type correctness, optional properties
- **RxJS Operators** - Completion triggers, error transformation
- **Batching Resource** - Request batching, caching, cleanup
- **Zod Helpers** - Error message formatting, German translations
- **AbortSignal Helpers** - Keyboard cancellation

### Example Tests

#### Error Class Testing

```typescript
import { describe, it, expect } from 'vitest';
import { DataAccessError, ResponseArgsError } from './errors';

describe('DataAccessError', () => {
  it('should create error with code and message', () => {
    class TestError extends DataAccessError<'TEST_ERROR'> {
      constructor() {
        super('TEST_ERROR', 'Test message');
      }
    }

    const error = new TestError();

    expect(error.code).toBe('TEST_ERROR');
    expect(error.message).toBe('Test message');
    expect(error.name).toBe('TestError');
  });

  it('should support instanceof checks', () => {
    const error = new ResponseArgsError({
      error: true,
      message: 'API error'
    });

    expect(error instanceof ResponseArgsError).toBe(true);
    expect(error instanceof DataAccessError).toBe(true);
    expect(error instanceof Error).toBe(true);
  });
});
```

#### Batching Resource Testing

```typescript
import { TestBed } from '@angular/core/testing';
import { describe, it, expect, beforeEach, vi } from 'vitest';
import { BatchingResource } from './batching-resource.base';

class TestResource extends BatchingResource<number, { ids: number[] }, { id: number; value: string }> {
  fetchFn = vi.fn();

  protected async fetchFn(params: { ids: number[] }, signal: AbortSignal) {
    return this.fetchFn(params, signal);
  }

  protected buildParams(ids: number[]) {
    return { ids };
  }

  protected getKeyFromResult(result: { id: number }) {
    return result.id;
  }

  protected getCacheKey(id: number) {
    return String(id);
  }

  protected extractKeysFromParams(params: { ids: number[] }) {
    return params.ids;
  }
}

describe('BatchingResource', () => {
  let resource: TestResource;

  beforeEach(() => {
    TestBed.configureTestingModule({
      providers: [TestResource]
    });
    resource = TestBed.inject(TestResource);
  });

  it('should batch multiple requests', async () => {
    resource.fetchFn.mockResolvedValue([
      { id: 1, value: 'one' },
      { id: 2, value: 'two' }
    ]);

    const ref1 = resource.resource(1);
    const ref2 = resource.resource(2);

    // Wait for batch window
    await new Promise(resolve => setTimeout(resolve, 150));

    expect(resource.fetchFn).toHaveBeenCalledTimes(1);
    expect(resource.fetchFn).toHaveBeenCalledWith(
      { ids: [1, 2] },
      expect.any(AbortSignal)
    );
  });

  it('should cache results', async () => {
    resource.fetchFn.mockResolvedValue([{ id: 1, value: 'one' }]);

    const ref1 = resource.resource(1);
    await new Promise(resolve => setTimeout(resolve, 150));

    const ref2 = resource.resource(1);  // Should use cache

    expect(resource.fetchFn).toHaveBeenCalledTimes(1);
  });
});
```

## Architecture Notes

### Current Architecture

```
Domain Services
       ↓
Common Data Access (this library)
       ↓
├─→ Error Classes (structured error handling)
├─→ RxJS Operators (request control)
├─→ Response Models (API contracts)
├─→ Batching Resources (request optimization)
└─→ Helper Functions (utilities)
       ↓
External Dependencies (RxJS, Zod, Angular)
```

### Design Patterns

#### 1. Error Hierarchy Pattern

Structured error types with discriminated unions:

```typescript
// Type-safe error handling
function handleError(error: unknown) {
  if (error instanceof ResponseArgsError) {
    // Compiler knows: error.responseArgs exists
  } else if (error instanceof PropertyIsEmptyError) {
    // Compiler knows: error.code === 'PROPERTY_IS_EMPTY'
  }
}
```

#### 2. Operator Composition Pattern

Composable RxJS operators for reusable logic:

```typescript
const withCancellation = (signal: AbortSignal) =>
  pipe(
    takeUntilAborted(signal),
    catchResponseArgsErrorPipe()
  );
```

#### 3. Abstract Resource Pattern

Template method pattern for batching resources:

```typescript
abstract class BatchingResource {
  // Template method
  resource(params) {
    this.addKeys(params);  // Common logic
    return this.createRef(params);  // Common logic
  }

  // Hook methods (implemented by subclasses)
  protected abstract fetchFn(...);
  protected abstract buildParams(...);
}
```

### Dependencies

#### Required Libraries

- `@angular/core` - Angular framework (signals, resources, DI)
- `rxjs` - Reactive programming
- `zod` - Runtime validation (peer dependency)

#### Path Alias

Import from: `@isa/common/data-access`

### Performance Considerations

1. **Batching Window Optimization** - Balance latency vs request reduction
2. **Cache Memory Management** - Reference counting prevents memory leaks
3. **Signal-Based Reactivity** - Minimal re-computation with Angular signals
4. **Operator Efficiency** - Custom operators avoid subscription overhead
5. **Error Object Reuse** - Error instances don't allocate large objects

### Future Enhancements

Potential improvements identified:

1. **Error Serialization** - Support for transmitting errors across workers
2. **Retry Strategies** - Built-in exponential backoff for transient failures
3. **Request Deduplication** - Prevent duplicate requests for same params
4. **Metrics Collection** - Track batch efficiency and cache hit rates
5. **Batch Priority Queues** - Prioritize critical requests over background
6. **Stale-While-Revalidate** - Serve cached data while fetching fresh data
7. **Offline Support** - Queue requests when network is unavailable

## License

Internal ISA Frontend library - not for external distribution.