Application Development Patterns

Overview

Common patterns for building real-world applications. These patterns solve recurring problems in application development.

---

CRUD Applications

Data Flow Pattern

┌─────────┐     ┌─────────┐     ┌─────────┐     ┌─────────┐
│  Form   │ ──→ │Validate │ ──→ │ Service │ ──→ │   DB    │
└─────────┘     └─────────┘     └─────────┘     └─────────┘
     ↑                                               │
     └───────────── Response ←───────────────────────┘

Form Handling Best Practices

// 1. Validation schema (shared frontend/backend)
const userSchema = z.object({
  email: z.string().email(),
  name: z.string().min(2).max(100),
  role: z.enum(['admin', 'user', 'guest'])
});

// 2. Server action with error handling
async function createUser(formData: FormData) {
  const result = userSchema.safeParse(Object.fromEntries(formData));

  if (!result.success) {
    return { error: result.error.flatten() };
  }

  try {
    const user = await db.user.create({ data: result.data });
    return { success: true, data: user };
  } catch (e) {
    if (e.code === 'P2002') {
      return { error: { email: 'Email already exists' } };
    }
    throw e;
  }
}

---

User Authentication

Authentication Flow

┌────────────────────────────────────────────────────────────┐
│                    Authentication Flows                     │
├────────────────────────────────────────────────────────────┤
│                                                            │
│  Email/Password:                                           │
│  Login → Validate → Create Session → Set Cookie → Redirect │
│                                                            │
│  OAuth (Social Login):                                     │
│  Redirect → Provider Auth → Callback → Upsert User → Done │
│                                                            │
│  Magic Link:                                               │
│  Email → Generate Token → Send Link → Verify → Login       │
│                                                            │
└────────────────────────────────────────────────────────────┘

Session Management

Strategy	Pros	Cons
JWT	Stateless, scalable	Can't revoke easily
Server Session	Revocable, secure	Requires session store
Hybrid	Best of both	More complex

Security Checklist

• Password hashing (bcrypt/argon2)
• Rate limiting on login
• CSRF protection
• Secure cookie settings (httpOnly, secure, sameSite)
• Account lockout after failed attempts
• Password reset token expiration

---

Admin Dashboards

Data Table Pattern

// Reusable data table with sorting, filtering, pagination
interface DataTableProps<T> {
  data: T[];
  columns: ColumnDef<T>[];
  pagination: { page: number; pageSize: number; total: number };
  sorting: { field: string; direction: 'asc' | 'desc' }[];
  filters: Record<string, unknown>;
  onStateChange: (state: TableState) => void;
}

// Server-side handling
async function getUsers(params: TableState) {
  const { page, pageSize, sorting, filters } = params;

  const query = {
    where: buildWhereClause(filters),
    orderBy: buildOrderBy(sorting),
    skip: (page - 1) * pageSize,
    take: pageSize,
  };

  const [users, total] = await Promise.all([
    db.user.findMany(query),
    db.user.count({ where: query.where })
  ]);

  return { data: users, total };
}

Bulk Operations

// Safe bulk delete with confirmation
async function bulkDelete(ids: string[]) {
  // 1. Validate permissions for each item
  const items = await db.item.findMany({
    where: { id: { in: ids } },
    select: { id: true, ownerId: true }
  });

  const authorized = items.filter(item =>
    canDelete(currentUser, item)
  );

  // 2. Soft delete or hard delete
  await db.item.updateMany({
    where: { id: { in: authorized.map(i => i.id) } },
    data: { deletedAt: new Date() }
  });

  return {
    deleted: authorized.length,
    skipped: ids.length - authorized.length
  };
}

---

File Management

Upload Strategies

Method	Use Case	Max Size
Direct to server	Small files	~10MB
Presigned URL	Large files	Unlimited
Chunked upload	Very large files	Unlimited
Resumable	Unreliable network	Unlimited

Presigned URL Flow

Client                    Server                    S3
   │                         │                       │
   │── Request upload URL ──→│                       │
   │                         │── Generate presigned ─→│
   │←── Return presigned URL─│                       │
   │                         │                       │
   │───────── Upload file directly ─────────────────→│
   │                         │                       │
   │── Confirm upload ──────→│                       │
   │                         │── Verify file exists ─→│
   │←── Success ─────────────│                       │

Image Processing Pipeline

async function processUpload(file: File) {
  // 1. Validate file type and size
  if (!ALLOWED_TYPES.includes(file.type)) {
    throw new Error('Invalid file type');
  }

  // 2. Generate variants
  const variants = await Promise.all([
    sharp(file.buffer).resize(100, 100).toBuffer(),   // thumbnail
    sharp(file.buffer).resize(800, 600).toBuffer(),   // medium
    sharp(file.buffer).resize(1920, 1080).toBuffer(), // large
  ]);

  // 3. Upload to CDN
  const urls = await uploadToS3(variants);

  // 4. Store metadata
  return db.image.create({
    data: {
      original: urls.original,
      thumbnail: urls.thumbnail,
      medium: urls.medium,
      large: urls.large,
      mimeType: file.type,
      size: file.size,
    }
  });
}

---

Search Implementation

Search Architecture

┌──────────────┐     ┌──────────────┐     ┌──────────────┐
│   Database   │ ──→ │    Sync      │ ──→ │   Search     │
│  (Primary)   │     │   Worker     │     │   Engine     │
└──────────────┘     └──────────────┘     └──────────────┘
                                                  ↑
                                                  │
┌──────────────┐     ┌──────────────┐             │
│    Client    │ ──→ │  Search API  │ ────────────┘
└──────────────┘     └──────────────┘

Search Features Checklist

• Full-text search
• Faceted filtering
• Autocomplete/suggestions
• Typo tolerance (fuzzy matching)
• Highlighting
• Synonyms
• Relevance tuning

---

Workflow Engines

State Machine Pattern

const orderStateMachine = {
  initial: 'pending',
  states: {
    pending: {
      on: {
        PAY: 'paid',
        CANCEL: 'cancelled'
      }
    },
    paid: {
      on: {
        SHIP: 'shipped',
        REFUND: 'refunded'
      }
    },
    shipped: {
      on: {
        DELIVER: 'delivered',
        RETURN: 'returned'
      }
    },
    delivered: { type: 'final' },
    cancelled: { type: 'final' },
    refunded: { type: 'final' },
    returned: {
      on: {
        REFUND: 'refunded'
      }
    }
  }
};

Approval Workflow

interface ApprovalStep {
  id: string;
  approvers: string[];        // User IDs or roles
  requiredApprovals: number;  // How many need to approve
  timeout?: Duration;         // Auto-escalate after
  escalateTo?: string;        // Next approver on timeout
}

async function processApproval(stepId: string, userId: string, decision: 'approve' | 'reject') {
  const step = await db.approvalStep.findUnique({ where: { id: stepId } });

  // Record decision
  await db.approval.create({
    data: { stepId, userId, decision, timestamp: new Date() }
  });

  // Check if complete
  const approvals = await db.approval.count({
    where: { stepId, decision: 'approve' }
  });

  if (approvals >= step.requiredApprovals) {
    await advanceToNextStep(step);
  }
}

---

Multi-language (i18n)

Translation Structure

locales/
├── en/
│   ├── common.json    # Shared strings
│   ├── auth.json      # Auth module
│   └── dashboard.json # Dashboard module
├── zh-TW/
│   ├── common.json
│   ├── auth.json
│   └── dashboard.json
└── ja/
    └── ...

Best Practices

1. Key Naming: Use namespaced keys

   {
     "auth.login.title": "Sign In",
     "auth.login.email": "Email Address",
     "auth.login.submit": "Sign In"
   }

2. Pluralization: Handle plural forms

   {
     "items": "{count, plural, =0 {No items} =1 {1 item} other {# items}}"
   }

3. Variables: Use interpolation

   {
     "welcome": "Welcome, {name}!"
   }

4. Date/Number Formatting: Use Intl APIs

   new Intl.DateTimeFormat(locale).format(date)
   new Intl.NumberFormat(locale, { style: 'currency', currency }).format(amount)

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Related Skills

• [[architecture-patterns]] - Overall system design
• [[frontend]] - UI implementation
• [[backend]] - Server implementation
• [[database]] - Data persistence
• [[security-practices]] - Security considerations