Write-Through Caching - Learning Module

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Use Cases

Choosing the Right Tool for the Job

Throughout this module, we've examined write-through caching from every angle—its mechanics, consistency guarantees, and performance trade-offs. Now comes the crucial question: When should you actually use it?

Write-through caching is not a universal solution. It excels in specific scenarios and fails spectacularly in others. The mark of an expert engineer is knowing not just how to use a tool, but when to reach for it.

This page provides a comprehensive catalog of use cases—both where write-through shines and where you should choose alternatives. You'll find decision frameworks, real-world examples, and implementation guidance for each scenario.

What You Will Learn

By the end of this page, you will be able to confidently identify when write-through caching is the optimal choice, understand the specific characteristics that make a use case suitable, and know how to adapt the pattern for different domains and requirements.

Ideal Use Case Characteristics

Before diving into specific scenarios, let's establish the characteristics that make a use case well-suited for write-through caching.

Suitability Indicators for Write-Through Caching
Characteristic	Ideal for Write-Through	Poor Fit for Write-Through
Read-to-Write Ratio	High (>10:1)	Low (<5:1)
Consistency Requirement	Strong, immediate	Eventual is acceptable
Data Criticality	High (financial, security)	Low (analytics, logs)
Write Latency Tolerance	50-100ms acceptable	Must be <10ms
Write Throughput Needs	Moderate (<10K/sec)	Very high (>100K/sec)
Data Durability	Zero loss tolerance	Some loss acceptable
Cache Hit Rate	High (>80%)	Low or unpredictable

The Decision Matrix:

Use this scoring system to evaluate whether write-through is appropriate:

Score each dimension 0-2:
  0 = Poor fit for write-through
  1 = Neutral
  2 = Excellent fit for write-through

Dimensions:
  □ Read-to-write ratio > 10:1              (0-2)
  □ Data must be immediately consistent     (0-2)
  □ Data is critical (financial, security)  (0-2)
  □ Write latency of 50ms is acceptable     (0-2)
  □ Write throughput < 10K/sec              (0-2)

Total Score:
  8-10: Write-through is strongly recommended
  5-7:  Write-through is viable, consider alternatives
  0-4:  Write-through is not recommended

The Golden Rule

Write-through caching is optimal when data consistency is more valuable than write performance. If your users or business logic require immediate visibility of changes, and your write volume is manageable, write-through is likely the right choice.

Use Case: Financial Services

Financial systems are the canonical use case for write-through caching. The cost of showing incorrect data—even briefly—can be measured in legal liability, regulatory fines, and lost customer trust.

Scenario: Account Balance Management

A retail banking application displays account balances to customers and processes transactions.

Requirements:

Customers must see accurate balances immediately after transactions
No overdrafts should be allowed due to stale cache data
Audit requirements mandate accurate historical records
Read-to-write ratio: ~50:1 (many balance checks, fewer transactions)

Why Write-Through Excels:

Immediate consistency prevents showing a $5,000 balance when the account has $50
Database-first writes ensure durability for audit trails
High read ratio means cache provides massive read performance benefits
50ms write latency is invisible to users pressing "Transfer" buttons

banking_write_through.ts
TypeScript
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class AccountService {
  async processTransaction(
    accountId: string,
    amount: number,
    type: 'credit' | 'debit'
  ): Promise<TransactionResult> {
    // Use database transaction for atomicity
    const result = await this.db.transaction(async (tx) => {
      // Lock the account row to prevent concurrent modifications
      const account = await tx.account.findUnique({
        where: { id: accountId },
        lock: true,  // FOR UPDATE
      });
 
      if (!account) {
        throw new AccountNotFoundError(accountId);
      }
 
      // Validate sufficient funds for debit
      const newBalance = type === 'credit' 
        ? account.balance + amount 
        : account.balance - amount;
 
      if (newBalance < 0 && !account.overdraftProtection) {
        throw new InsufficientFundsError(accountId, account.balance, amount);
      }
 
      // Update balance
      const updatedAccount = await tx.account.update({
        where: { id: accountId },
        data: { 
          balance: newBalance,
          lastTransactionAt: new Date(),
        },
      });
 
      // Create transaction record
      const transaction = await tx.transaction.create({
        data: {
          accountId,
          amount,
          type,
          balanceAfter: newBalance,
          timestamp: new Date(),
        },
      });
 
      return { account: updatedAccount, transaction };
    });
 
    // Write-through: Update cache after DB success
    await this.updateAccountCache(result.account);
 
    return {
      success: true,
      transactionId: result.transaction.id,
      newBalance: result.account.balance,
    };
  }
 
  private async updateAccountCache(account: Account): Promise<void> {
    const cacheKey = `account:${account.id}`;
    
    // Store versioned data for consistency verification
    const cacheData = {
      ...account,
      cachedAt: Date.now(),
      version: account.updatedAt.getTime(),
    };
 
    await this.cache.set(cacheKey, cacheData, { ttl: 3600 });
  }
 
  async getBalance(accountId: string): Promise<number> {
    const cacheKey = `account:${accountId}`;
    
    // Try cache first
    const cached = await this.cache.get<CachedAccount>(cacheKey);
    if (cached) {
      this.metrics.increment('balance_cache_hits');
      return cached.balance;
    }
 
    // Cache miss: load from database
    this.metrics.increment('balance_cache_misses');
    const account = await this.db.account.findUnique({
      where: { id: accountId },
    });
 
    if (!account) {
      throw new AccountNotFoundError(accountId);
    }
 
    // Populate cache for future reads
    await this.updateAccountCache(account);
    
    return account.balance;
  }
}

Other Financial Use Cases:

Portfolio Valuations — Investment portfolios must reflect current holdings after trades
Credit Limits — Credit card available credit must be accurate to prevent over-limit transactions
Trading Order Books — Order status must be immediately visible to traders
Payment Processing — Payment status changes must be immediately consistent across systems
Fraud Scores — Updated risk scores must be immediately visible for transaction decisions

Use Case: E-Commerce

E-commerce platforms have a mix of write-through and alternative caching needs. The key is identifying which data requires strong consistency.

Use Write-Through For

•Inventory counts — Prevent overselling
•Cart contents — Users expect immediate updates
•Order status — Critical for customer experience
•Price changes — Must be consistent across views
•Customer profile — Address changes, payment methods

Use Alternative For

•Product catalog — Updates can be eventually consistent
•Reviews/ratings — Slight delay acceptable
•Search indexes — Async updates via queues
•Recommendations — Precomputed, batch-updated
•Analytics/clickstream — High volume, no consistency needs

Scenario: Shopping Cart + Inventory

A customer adds an item to their cart. The system must:

Update the cart (write-through for immediate visibility)
Reserve inventory (write-through to prevent overselling)
Handle concurrent purchases of the same item

Implementation Approach:

cart_inventory.ts
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class CartService {
  async addToCart(
    userId: string,
    productId: string,
    quantity: number
  ): Promise<CartUpdateResult> {
    // Step 1: Check and reserve inventory
    const inventory = await this.inventoryService.reserve(productId, quantity);
    
    if (!inventory.success) {
      return {
        success: false,
        error: 'INSUFFICIENT_INVENTORY',
        available: inventory.available,
      };
    }
 
    try {
      // Step 2: Update cart in database
      const cart = await this.db.cartItem.upsert({
        where: { 
          userId_productId: { userId, productId } 
        },
        create: {
          userId,
          productId,
          quantity,
          reservationId: inventory.reservationId,
        },
        update: {
          quantity: { increment: quantity },
          reservationId: inventory.reservationId,
        },
      });
 
      // Step 3: Write-through to cache
      await this.syncCartCache(userId);
 
      return {
        success: true,
        cart: await this.getCart(userId),
        reservationId: inventory.reservationId,
      };
 
    } catch (error) {
      // Rollback inventory reservation on cart failure
      await this.inventoryService.releaseReservation(inventory.reservationId);
      throw error;
    }
  }
 
  private async syncCartCache(userId: string): Promise<void> {
    const cart = await this.db.cartItem.findMany({
      where: { userId },
      include: { product: true },
    });
 
    const cacheKey = `cart:${userId}`;
    const cacheData = {
      items: cart,
      itemCount: cart.reduce((sum, item) => sum + item.quantity, 0),
      subtotal: cart.reduce(
        (sum, item) => sum + item.quantity * item.product.price, 
        0
      ),
      updatedAt: Date.now(),
    };
 
    await this.cache.set(cacheKey, cacheData, { ttl: 3600 });
  }
}
 
class InventoryService {
  async reserve(
    productId: string,
    quantity: number
  ): Promise<ReservationResult> {
    // Use database transaction with row locking
    return this.db.transaction(async (tx) => {
      const inventory = await tx.inventory.findUnique({
        where: { productId },
        lock: true,
      });
 
      if (!inventory || inventory.available < quantity) {
        return {
          success: false,
          available: inventory?.available ?? 0,
        };
      }
 
      // Decrement available inventory
      const updated = await tx.inventory.update({
        where: { productId },
        data: {
          available: { decrement: quantity },
          reserved: { increment: quantity },
        },
      });
 
      // Create reservation record for tracking
      const reservation = await tx.inventoryReservation.create({
        data: {
          productId,
          quantity,
          expiresAt: new Date(Date.now() + 15 * 60 * 1000), // 15 min hold
        },
      });
 
      // Write-through: Update inventory cache
      await this.syncInventoryCache(productId, updated);
 
      return {
        success: true,
        reservationId: reservation.id,
        available: updated.available,
      };
    });
  }
 
  private async syncInventoryCache(
    productId: string, 
    inventory: Inventory
  ): Promise<void> {
    const cacheKey = `inventory:${productId}`;
    await this.cache.set(cacheKey, {
      available: inventory.available,
      reserved: inventory.reserved,
      updatedAt: Date.now(),
    }, { ttl: 300 }); // 5 minute TTL
  }
}

Flash Sales and High Concurrency

For flash sales or high-demand items, write-through alone isn't enough. You need distributed locks or optimistic concurrency control at the database level to prevent race conditions. Write-through ensures the cache reflects the truth; it doesn't prevent concurrent database conflicts.

Use Case: Session and Security

User sessions and security-related data are prime candidates for write-through caching. Security changes must propagate immediately to prevent vulnerabilities.

Scenario: Session Invalidation on Password Change

When a user changes their password, all existing sessions must be immediately invalidated to prevent unauthorized access from potentially compromised sessions.

The Problem with Eventual Consistency:

1. Attacker compromises user password
2. User notices and changes password
3. System updates password in database
4. Cache still has old session tokens (eventual consistency)
5. Attacker can continue using old sessions until cache expires
6. Security vulnerability window: minutes to hours

With Write-Through:

1. User changes password
2. System updates password in database
3. System invalidates all sessions in database
4. Write-through: Session invalidation stored in cache immediately
5. Attacker's session is rejected on next request
6. Security vulnerability window: milliseconds

session_security.ts
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class SecurityService {
  async changePassword(
    userId: string,
    currentPassword: string,
    newPassword: string
  ): Promise<PasswordChangeResult> {
    // Verify current password
    const user = await this.verifyCurrentPassword(userId, currentPassword);
    if (!user) {
      return { success: false, error: 'INVALID_CURRENT_PASSWORD' };
    }
 
    // Transaction: Update password and invalidate sessions
    await this.db.transaction(async (tx) => {
      // Update password
      await tx.user.update({
        where: { id: userId },
        data: {
          passwordHash: await this.hashPassword(newPassword),
          passwordChangedAt: new Date(),
        },
      });
 
      // Record password change for audit
      await tx.securityAudit.create({
        data: {
          userId,
          action: 'PASSWORD_CHANGE',
          timestamp: new Date(),
          ip: this.getClientIp(),
        },
      });
    });
 
    // Write-through: Invalidate all sessions immediately
    await this.invalidateAllSessions(userId);
 
    // Write-through: Update user cache
    await this.syncUserCache(userId);
 
    return { 
      success: true, 
      message: 'Password changed. All sessions have been logged out.',
    };
  }
 
  private async invalidateAllSessions(userId: string): Promise<void> {
    // Delete all sessions from database
    await this.db.session.deleteMany({
      where: { userId },
    });
 
    // Write-through: Update session invalidation marker in cache
    // This is checked on every authenticated request
    const invalidationKey = `session_invalidation:${userId}`;
    await this.cache.set(invalidationKey, {
      invalidatedAt: Date.now(),
      reason: 'PASSWORD_CHANGE',
    }, { ttl: 86400 }); // 24 hour TTL
 
    // Also delete individual session caches if they exist
    const sessionKeys = await this.cache.keys(`session:${userId}:*`);
    if (sessionKeys.length > 0) {
      await this.cache.del(...sessionKeys);
    }
  }
 
  // Called on every authenticated request
  async validateSession(sessionToken: string): Promise<SessionValidation> {
    // Check session in cache first
    const cachedSession = await this.cache.get<SessionData>(
      `session:${sessionToken}`
    );
 
    if (!cachedSession) {
      // Cache miss: validate against database
      return this.validateSessionFromDatabase(sessionToken);
    }
 
    // Check for session invalidation (write-through updated this immediately)
    const invalidation = await this.cache.get<InvalidationData>(
      `session_invalidation:${cachedSession.userId}`
    );
 
    if (invalidation && invalidation.invalidatedAt > cachedSession.createdAt) {
      // Session was created before invalidation
      return { valid: false, reason: 'SESSION_INVALIDATED' };
    }
 
    return { valid: true, session: cachedSession };
  }
}
 
class PermissionService {
  async revokePermission(
    userId: string,
    permission: string
  ): Promise<void> {
    // Database update
    await this.db.userPermission.delete({
      where: { userId_permission: { userId, permission } },
    });
 
    // Write-through: Update permission cache immediately
    // Critical for security - user should lose access instantly
    await this.syncPermissionCache(userId);
  }
 
  private async syncPermissionCache(userId: string): Promise<void> {
    const permissions = await this.db.userPermission.findMany({
      where: { userId },
    });
 
    const cacheKey = `permissions:${userId}`;
    await this.cache.set(cacheKey, {
      permissions: permissions.map(p => p.permission),
      updatedAt: Date.now(),
    }, { ttl: 300 });
  }
}

Other Security Use Cases:

MFA Enrollment/Revocation — Changes must be effective immediately
API Key Rotation — Old keys must stop working instantly
Account Lockout — Failed login thresholds must be enforced immediately
Role Changes — Privilege escalation/deescalation must be immediate
IP Blacklisting — Blocked IPs must be rejected instantly

Use Case: Real-Time Display Systems

Systems that display real-time information to users often benefit from write-through caching, especially when the information is critical for decision-making.

Real-Time Display Use Cases
System	Critical Data	Why Write-Through	Alternative Approaches
Dashboards	KPIs, metrics	Executives need current data for decisions	Streaming for sub-second updates
Leaderboards	Scores, rankings	Players expect immediate updates	Event sourcing for high-volume gaming
Status Pages	Service health	Incident response depends on accuracy	Push-based for real-time
Inventory Displays	Stock levels	Store staff need accurate counts	None—write-through is ideal
Booking Systems	Availability	Prevent double-booking	Locking for high contention

Scenario: Restaurant Reservation System

A restaurant booking platform shows available time slots. When a reservation is made, the slot must immediately show as unavailable to prevent double-booking.

reservation_system.ts
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class ReservationService {
  async makeReservation(
    restaurantId: string,
    date: Date,
    time: string,
    partySize: number,
    customerId: string
  ): Promise<ReservationResult> {
    const slotKey = `${restaurantId}:${date.toISOString().split('T')[0]}:${time}`;
 
    // Use distributed lock to prevent race conditions
    const lock = await this.locks.acquire(`reservation:${slotKey}`, 5000);
 
    try {
      // Check availability in database (source of truth)
      const availability = await this.db.availability.findUnique({
        where: { slotKey },
      });
 
      if (!availability || availability.remainingCapacity < partySize) {
        return {
          success: false,
          error: 'SLOT_UNAVAILABLE',
          remainingCapacity: availability?.remainingCapacity ?? 0,
        };
      }
 
      // Create reservation and update availability atomically
      const [reservation, updatedAvailability] = await this.db.transaction(
        async (tx) => {
          const reservation = await tx.reservation.create({
            data: {
              restaurantId,
              date,
              time,
              partySize,
              customerId,
              status: 'CONFIRMED',
              confirmationCode: this.generateConfirmationCode(),
            },
          });
 
          const updated = await tx.availability.update({
            where: { slotKey },
            data: {
              remainingCapacity: { decrement: partySize },
              reservationCount: { increment: 1 },
            },
          });
 
          return [reservation, updated];
        }
      );
 
      // Write-through: Update availability cache immediately
      await this.syncAvailabilityCache(slotKey, updatedAvailability);
 
      // Write-through: Update customer's reservations cache
      await this.syncCustomerReservationsCache(customerId);
 
      return {
        success: true,
        reservation,
        confirmationCode: reservation.confirmationCode,
      };
 
    } finally {
      await lock.release();
    }
  }
 
  async getAvailability(
    restaurantId: string,
    date: Date
  ): Promise<TimeSlot[]> {
    const cacheKey = `availability:${restaurantId}:${date.toISOString().split('T')[0]}`;
    
    // Check cache first
    const cached = await this.cache.get<TimeSlot[]>(cacheKey);
    if (cached) {
      return cached;
    }
 
    // Cache miss: load from database
    const slots = await this.db.availability.findMany({
      where: {
        restaurantId,
        date,
        remainingCapacity: { gt: 0 },
      },
    });
 
    // Cache for next request (but not too long - availability changes)
    await this.cache.set(cacheKey, slots, { ttl: 60 }); // 1 minute TTL
 
    return slots;
  }
 
  private async syncAvailabilityCache(
    slotKey: string,
    availability: Availability
  ): Promise<void> {
    // Update individual slot cache
    await this.cache.set(`slot:${slotKey}`, availability, { ttl: 300 });
 
    // Invalidate the date's availability list to force refresh
    const [restaurantId, date] = slotKey.split(':');
    await this.cache.del(`availability:${restaurantId}:${date}`);
  }
}

Anti-Patterns: When NOT to Use Write-Through

Equally important as knowing when to use write-through is knowing when to avoid it. Here are scenarios where write-through is the wrong choice.

Anti-Pattern 1: High-Volume Analytics/Logging

The Scenario:

Website tracking page views, clicks, and user behavior
Thousands of events per second per server
Data is written once and rarely read immediately

Why Write-Through Fails:

Write latency is unacceptable for event tracking (blocks page load)
Write volume overwhelms the database
Immediate consistency is unnecessary for analytics

Better Alternative: Write-back with batching, or write directly to a time-series database/message queue.

Anti-Pattern 2: High-Frequency Counters

The Scenario:

View counts, like counts, impression counters
Potentially millions of increments per minute
Exact real-time accuracy not critical

Why Write-Through Fails:

Each increment requires a database round-trip
Database becomes bottleneck instantly
Counter value is often approximate anyway

Better Alternative: Write-back with periodic flush, Redis atomic counters with periodic database sync, or HyperLogLog for approximate counting.

Anti-Pattern 3: Large Binary Data

The Scenario:

Uploading images, videos, documents
Files are megabytes in size
Stored in object storage (S3) with metadata in database

Why Write-Through Fails:

Caching large binary data is expensive (memory consumption)
Network transfer of large payloads is slow
Read patterns are often sequential, not random access

Better Alternative: Store metadata with write-through, but serve binary data directly from object storage with CDN caching.

Anti-Pattern 4: Append-Only Logs/Audit Trails

The Scenario:

System audit logs, compliance records
Write-once, read-rarely data
Historical queries, not real-time lookups

Why Write-Through Fails:

Data is rarely read (low cache hit rate)
High write volume strains cache as well as database
Caching adds no value for archival data

Better Alternative: Write-around (skip cache entirely), or write directly to append-only log storage.

The Red Flags

Avoid write-through when you see: (1) write-to-read ratio > 1:1, (2) data that's rarely accessed after writing, (3) approximate values being acceptable, (4) very high write throughput requirements (>100K/sec), or (5) large payloads (>1MB). Any of these is a signal to consider alternatives.

Implementation Decision Tree

Use this decision tree when evaluating whether to implement write-through caching for a specific data type or feature.

decision_tree.txt

Text

                   Start Here
                        │
                        ▼
        ┌─────────────────────────────────────┐
        │   Does data need to be cached at    │
        │   all? (High read volume, slow DB)  │
        └─────────────────────────────────────┘
                        │
              ┌─────────┼─────────┐
              │ NO      │         │ YES
              ▼         │         ▼
        ┌──────────┐    │   ┌─────────────────────────────────────┐
        │ No Cache │    │   │  Is immediate consistency required? │
        │ Needed   │    │   │  (Security, financial, user-facing) │
        └──────────┘    │   └─────────────────────────────────────┘
                        │                     │
                        │         ┌───────────┼───────────┐
                        │         │ NO        │           │ YES
                        │         ▼           │           ▼
                        │   ┌──────────────┐  │   ┌─────────────────────────────────────┐
                        │   │ Cache-Aside  │  │   │  Is write latency of 50ms+ okay?    │
                        │   │ (Lazy Load)  │  │   │  (Not real-time, user can wait)     │
                        │   └──────────────┘  │   └─────────────────────────────────────┘
                        │                     │                     │
                        │                     │         ┌───────────┼───────────┐
                        │                     │         │ NO        │           │ YES
                        │                     │         ▼           │           ▼
                        │                     │   ┌──────────────┐  │   ┌─────────────────────────────────────┐
                        │                     │   │ Write-Back   │  │   │  Is write volume manageable?        │
                        │                     │   │ (if data loss│  │   │  (<10K writes/sec)                  │
                        │                     │   │  acceptable) │  │   └─────────────────────────────────────┘
                        │                     │   └──────────────┘  │                     │
                        │                     │                     │         ┌───────────┼───────────┐
                        │                     │                     │         │ NO        │           │ YES
                        │                     │                     │         ▼           │           ▼
                        │                     │                     │   ┌──────────────┐  │   ┌──────────────┐
                        │                     │                     │   │ Shard DB +   │  │   │ WRITE-       │
                        │                     │                     │   │ Write-Through│  │   │ THROUGH      │
                        │                     │                     │   │ OR Evaluate  │  │   │ CACHING      │
                        │                     │                     │   │ Trade-offs   │  │   │ ✓            │
                        │                     │                     │   └──────────────┘  │   └──────────────┘
                        │                     │                     │                     │
                        └─────────────────────┴─────────────────────┴─────────────────────┘

Quick Reference Summary:

If you need...	Use...
Strong consistency + durability	Write-Through
Low write latency + some consistency	Write-Back
Simple implementation + read-heavy	Cache-Aside
Write-heavy + no immediate read	Write-Around
Proactive cache warming	Refresh-Ahead

Summary: Use Cases

We've explored the full spectrum of write-through caching use cases. Let's consolidate the key insights:

Key Takeaways

•Financial data is the canonical use case — Account balances, transactions, and credit limits require immediate consistency
•Security changes demand write-through — Session invalidation, permission changes, and password updates cannot tolerate staleness
•E-commerce has mixed needs — Inventory and carts need write-through; catalogs and reviews can use cache-aside
•Real-time displays benefit greatly — Reservation systems, dashboards, and status pages need current data
•Know the anti-patterns — High-volume analytics, counters, large files, and append-only logs should not use write-through
•Use the decision tree — Evaluate each data type systematically based on consistency needs, latency tolerance, and write volume

Module Complete:

Congratulations! You've completed the Write-Through Caching module. You now have a comprehensive understanding of:

How write-through works — The synchronous dual-write protocol
Coordination mechanics — How cache and database stay synchronized
Consistency benefits — Why write-through provides stronger guarantees
Performance trade-offs — The costs and how to optimize
Use cases — When to use write-through and when to choose alternatives

This knowledge prepares you to make informed decisions about caching strategies in your system designs.

Module Complete

You've mastered write-through caching—from theory to implementation to practical application. You can now confidently evaluate when this pattern is appropriate and implement it correctly for your specific requirements.

Use Cases

Choosing the Right Tool for the Job

What You Will Learn

Ideal Use Case Characteristics

Before diving into specific scenarios, let's establish the characteristics that make a use case well-suited for write-through caching.

Suitability Indicators for Write-Through Caching
Characteristic	Ideal for Write-Through	Poor Fit for Write-Through
Read-to-Write Ratio	High (>10:1)	Low (<5:1)
Consistency Requirement	Strong, immediate	Eventual is acceptable
Data Criticality	High (financial, security)	Low (analytics, logs)
Write Latency Tolerance	50-100ms acceptable	Must be <10ms
Write Throughput Needs	Moderate (<10K/sec)	Very high (>100K/sec)
Data Durability	Zero loss tolerance	Some loss acceptable
Cache Hit Rate	High (>80%)	Low or unpredictable

The Decision Matrix:

Use this scoring system to evaluate whether write-through is appropriate:

Score each dimension 0-2:
  0 = Poor fit for write-through
  1 = Neutral
  2 = Excellent fit for write-through

Dimensions:
  □ Read-to-write ratio > 10:1              (0-2)
  □ Data must be immediately consistent     (0-2)
  □ Data is critical (financial, security)  (0-2)
  □ Write latency of 50ms is acceptable     (0-2)
  □ Write throughput < 10K/sec              (0-2)

Total Score:
  8-10: Write-through is strongly recommended
  5-7:  Write-through is viable, consider alternatives
  0-4:  Write-through is not recommended

The Golden Rule

Use Case: Financial Services

Scenario: Account Balance Management

A retail banking application displays account balances to customers and processes transactions.

Requirements:

Customers must see accurate balances immediately after transactions
No overdrafts should be allowed due to stale cache data
Audit requirements mandate accurate historical records
Read-to-write ratio: ~50:1 (many balance checks, fewer transactions)

Why Write-Through Excels:

Immediate consistency prevents showing a $5,000 balance when the account has $50
Database-first writes ensure durability for audit trails
High read ratio means cache provides massive read performance benefits
50ms write latency is invisible to users pressing "Transfer" buttons

banking_write_through.ts
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class AccountService {
  async processTransaction(
    accountId: string,
    amount: number,
    type: 'credit' | 'debit'
  ): Promise<TransactionResult> {
    // Use database transaction for atomicity
    const result = await this.db.transaction(async (tx) => {
      // Lock the account row to prevent concurrent modifications
      const account = await tx.account.findUnique({
        where: { id: accountId },
        lock: true,  // FOR UPDATE
      });
 
      if (!account) {
        throw new AccountNotFoundError(accountId);
      }
 
      // Validate sufficient funds for debit
      const newBalance = type === 'credit' 
        ? account.balance + amount 
        : account.balance - amount;
 
      if (newBalance < 0 && !account.overdraftProtection) {
        throw new InsufficientFundsError(accountId, account.balance, amount);
      }
 
      // Update balance
      const updatedAccount = await tx.account.update({
        where: { id: accountId },
        data: { 
          balance: newBalance,
          lastTransactionAt: new Date(),
        },
      });
 
      // Create transaction record
      const transaction = await tx.transaction.create({
        data: {
          accountId,
          amount,
          type,
          balanceAfter: newBalance,
          timestamp: new Date(),
        },
      });
 
      return { account: updatedAccount, transaction };
    });
 
    // Write-through: Update cache after DB success
    await this.updateAccountCache(result.account);
 
    return {
      success: true,
      transactionId: result.transaction.id,
      newBalance: result.account.balance,
    };
  }
 
  private async updateAccountCache(account: Account): Promise<void> {
    const cacheKey = `account:${account.id}`;
    
    // Store versioned data for consistency verification
    const cacheData = {
      ...account,
      cachedAt: Date.now(),
      version: account.updatedAt.getTime(),
    };
 
    await this.cache.set(cacheKey, cacheData, { ttl: 3600 });
  }
 
  async getBalance(accountId: string): Promise<number> {
    const cacheKey = `account:${accountId}`;
    
    // Try cache first
    const cached = await this.cache.get<CachedAccount>(cacheKey);
    if (cached) {
      this.metrics.increment('balance_cache_hits');
      return cached.balance;
    }
 
    // Cache miss: load from database
    this.metrics.increment('balance_cache_misses');
    const account = await this.db.account.findUnique({
      where: { id: accountId },
    });
 
    if (!account) {
      throw new AccountNotFoundError(accountId);
    }
 
    // Populate cache for future reads
    await this.updateAccountCache(account);
    
    return account.balance;
  }
}

Other Financial Use Cases:

Portfolio Valuations — Investment portfolios must reflect current holdings after trades
Credit Limits — Credit card available credit must be accurate to prevent over-limit transactions
Trading Order Books — Order status must be immediately visible to traders
Payment Processing — Payment status changes must be immediately consistent across systems
Fraud Scores — Updated risk scores must be immediately visible for transaction decisions

Use Case: E-Commerce

E-commerce platforms have a mix of write-through and alternative caching needs. The key is identifying which data requires strong consistency.

Use Write-Through For

•Inventory counts — Prevent overselling
•Cart contents — Users expect immediate updates
•Order status — Critical for customer experience
•Price changes — Must be consistent across views
•Customer profile — Address changes, payment methods

Use Alternative For

•Product catalog — Updates can be eventually consistent
•Reviews/ratings — Slight delay acceptable
•Search indexes — Async updates via queues
•Recommendations — Precomputed, batch-updated
•Analytics/clickstream — High volume, no consistency needs

Scenario: Shopping Cart + Inventory

A customer adds an item to their cart. The system must:

Update the cart (write-through for immediate visibility)
Reserve inventory (write-through to prevent overselling)
Handle concurrent purchases of the same item

Implementation Approach:

cart_inventory.ts
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class CartService {
  async addToCart(
    userId: string,
    productId: string,
    quantity: number
  ): Promise<CartUpdateResult> {
    // Step 1: Check and reserve inventory
    const inventory = await this.inventoryService.reserve(productId, quantity);
    
    if (!inventory.success) {
      return {
        success: false,
        error: 'INSUFFICIENT_INVENTORY',
        available: inventory.available,
      };
    }
 
    try {
      // Step 2: Update cart in database
      const cart = await this.db.cartItem.upsert({
        where: { 
          userId_productId: { userId, productId } 
        },
        create: {
          userId,
          productId,
          quantity,
          reservationId: inventory.reservationId,
        },
        update: {
          quantity: { increment: quantity },
          reservationId: inventory.reservationId,
        },
      });
 
      // Step 3: Write-through to cache
      await this.syncCartCache(userId);
 
      return {
        success: true,
        cart: await this.getCart(userId),
        reservationId: inventory.reservationId,
      };
 
    } catch (error) {
      // Rollback inventory reservation on cart failure
      await this.inventoryService.releaseReservation(inventory.reservationId);
      throw error;
    }
  }
 
  private async syncCartCache(userId: string): Promise<void> {
    const cart = await this.db.cartItem.findMany({
      where: { userId },
      include: { product: true },
    });
 
    const cacheKey = `cart:${userId}`;
    const cacheData = {
      items: cart,
      itemCount: cart.reduce((sum, item) => sum + item.quantity, 0),
      subtotal: cart.reduce(
        (sum, item) => sum + item.quantity * item.product.price, 
        0
      ),
      updatedAt: Date.now(),
    };
 
    await this.cache.set(cacheKey, cacheData, { ttl: 3600 });
  }
}
 
class InventoryService {
  async reserve(
    productId: string,
    quantity: number
  ): Promise<ReservationResult> {
    // Use database transaction with row locking
    return this.db.transaction(async (tx) => {
      const inventory = await tx.inventory.findUnique({
        where: { productId },
        lock: true,
      });
 
      if (!inventory || inventory.available < quantity) {
        return {
          success: false,
          available: inventory?.available ?? 0,
        };
      }
 
      // Decrement available inventory
      const updated = await tx.inventory.update({
        where: { productId },
        data: {
          available: { decrement: quantity },
          reserved: { increment: quantity },
        },
      });
 
      // Create reservation record for tracking
      const reservation = await tx.inventoryReservation.create({
        data: {
          productId,
          quantity,
          expiresAt: new Date(Date.now() + 15 * 60 * 1000), // 15 min hold
        },
      });
 
      // Write-through: Update inventory cache
      await this.syncInventoryCache(productId, updated);
 
      return {
        success: true,
        reservationId: reservation.id,
        available: updated.available,
      };
    });
  }
 
  private async syncInventoryCache(
    productId: string, 
    inventory: Inventory
  ): Promise<void> {
    const cacheKey = `inventory:${productId}`;
    await this.cache.set(cacheKey, {
      available: inventory.available,
      reserved: inventory.reserved,
      updatedAt: Date.now(),
    }, { ttl: 300 }); // 5 minute TTL
  }
}

Flash Sales and High Concurrency

Use Case: Session and Security

User sessions and security-related data are prime candidates for write-through caching. Security changes must propagate immediately to prevent vulnerabilities.

Scenario: Session Invalidation on Password Change

When a user changes their password, all existing sessions must be immediately invalidated to prevent unauthorized access from potentially compromised sessions.

The Problem with Eventual Consistency:

1. Attacker compromises user password
2. User notices and changes password
3. System updates password in database
4. Cache still has old session tokens (eventual consistency)
5. Attacker can continue using old sessions until cache expires
6. Security vulnerability window: minutes to hours

With Write-Through:

1. User changes password
2. System updates password in database
3. System invalidates all sessions in database
4. Write-through: Session invalidation stored in cache immediately
5. Attacker's session is rejected on next request
6. Security vulnerability window: milliseconds

session_security.ts
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class SecurityService {
  async changePassword(
    userId: string,
    currentPassword: string,
    newPassword: string
  ): Promise<PasswordChangeResult> {
    // Verify current password
    const user = await this.verifyCurrentPassword(userId, currentPassword);
    if (!user) {
      return { success: false, error: 'INVALID_CURRENT_PASSWORD' };
    }
 
    // Transaction: Update password and invalidate sessions
    await this.db.transaction(async (tx) => {
      // Update password
      await tx.user.update({
        where: { id: userId },
        data: {
          passwordHash: await this.hashPassword(newPassword),
          passwordChangedAt: new Date(),
        },
      });
 
      // Record password change for audit
      await tx.securityAudit.create({
        data: {
          userId,
          action: 'PASSWORD_CHANGE',
          timestamp: new Date(),
          ip: this.getClientIp(),
        },
      });
    });
 
    // Write-through: Invalidate all sessions immediately
    await this.invalidateAllSessions(userId);
 
    // Write-through: Update user cache
    await this.syncUserCache(userId);
 
    return { 
      success: true, 
      message: 'Password changed. All sessions have been logged out.',
    };
  }
 
  private async invalidateAllSessions(userId: string): Promise<void> {
    // Delete all sessions from database
    await this.db.session.deleteMany({
      where: { userId },
    });
 
    // Write-through: Update session invalidation marker in cache
    // This is checked on every authenticated request
    const invalidationKey = `session_invalidation:${userId}`;
    await this.cache.set(invalidationKey, {
      invalidatedAt: Date.now(),
      reason: 'PASSWORD_CHANGE',
    }, { ttl: 86400 }); // 24 hour TTL
 
    // Also delete individual session caches if they exist
    const sessionKeys = await this.cache.keys(`session:${userId}:*`);
    if (sessionKeys.length > 0) {
      await this.cache.del(...sessionKeys);
    }
  }
 
  // Called on every authenticated request
  async validateSession(sessionToken: string): Promise<SessionValidation> {
    // Check session in cache first
    const cachedSession = await this.cache.get<SessionData>(
      `session:${sessionToken}`
    );
 
    if (!cachedSession) {
      // Cache miss: validate against database
      return this.validateSessionFromDatabase(sessionToken);
    }
 
    // Check for session invalidation (write-through updated this immediately)
    const invalidation = await this.cache.get<InvalidationData>(
      `session_invalidation:${cachedSession.userId}`
    );
 
    if (invalidation && invalidation.invalidatedAt > cachedSession.createdAt) {
      // Session was created before invalidation
      return { valid: false, reason: 'SESSION_INVALIDATED' };
    }
 
    return { valid: true, session: cachedSession };
  }
}
 
class PermissionService {
  async revokePermission(
    userId: string,
    permission: string
  ): Promise<void> {
    // Database update
    await this.db.userPermission.delete({
      where: { userId_permission: { userId, permission } },
    });
 
    // Write-through: Update permission cache immediately
    // Critical for security - user should lose access instantly
    await this.syncPermissionCache(userId);
  }
 
  private async syncPermissionCache(userId: string): Promise<void> {
    const permissions = await this.db.userPermission.findMany({
      where: { userId },
    });
 
    const cacheKey = `permissions:${userId}`;
    await this.cache.set(cacheKey, {
      permissions: permissions.map(p => p.permission),
      updatedAt: Date.now(),
    }, { ttl: 300 });
  }
}

Other Security Use Cases:

MFA Enrollment/Revocation — Changes must be effective immediately
API Key Rotation — Old keys must stop working instantly
Account Lockout — Failed login thresholds must be enforced immediately
Role Changes — Privilege escalation/deescalation must be immediate
IP Blacklisting — Blocked IPs must be rejected instantly

Use Case: Real-Time Display Systems

Systems that display real-time information to users often benefit from write-through caching, especially when the information is critical for decision-making.

Real-Time Display Use Cases
System	Critical Data	Why Write-Through	Alternative Approaches
Dashboards	KPIs, metrics	Executives need current data for decisions	Streaming for sub-second updates
Leaderboards	Scores, rankings	Players expect immediate updates	Event sourcing for high-volume gaming
Status Pages	Service health	Incident response depends on accuracy	Push-based for real-time
Inventory Displays	Stock levels	Store staff need accurate counts	None—write-through is ideal
Booking Systems	Availability	Prevent double-booking	Locking for high contention

Scenario: Restaurant Reservation System

A restaurant booking platform shows available time slots. When a reservation is made, the slot must immediately show as unavailable to prevent double-booking.

reservation_system.ts
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class ReservationService {
  async makeReservation(
    restaurantId: string,
    date: Date,
    time: string,
    partySize: number,
    customerId: string
  ): Promise<ReservationResult> {
    const slotKey = `${restaurantId}:${date.toISOString().split('T')[0]}:${time}`;
 
    // Use distributed lock to prevent race conditions
    const lock = await this.locks.acquire(`reservation:${slotKey}`, 5000);
 
    try {
      // Check availability in database (source of truth)
      const availability = await this.db.availability.findUnique({
        where: { slotKey },
      });
 
      if (!availability || availability.remainingCapacity < partySize) {
        return {
          success: false,
          error: 'SLOT_UNAVAILABLE',
          remainingCapacity: availability?.remainingCapacity ?? 0,
        };
      }
 
      // Create reservation and update availability atomically
      const [reservation, updatedAvailability] = await this.db.transaction(
        async (tx) => {
          const reservation = await tx.reservation.create({
            data: {
              restaurantId,
              date,
              time,
              partySize,
              customerId,
              status: 'CONFIRMED',
              confirmationCode: this.generateConfirmationCode(),
            },
          });
 
          const updated = await tx.availability.update({
            where: { slotKey },
            data: {
              remainingCapacity: { decrement: partySize },
              reservationCount: { increment: 1 },
            },
          });
 
          return [reservation, updated];
        }
      );
 
      // Write-through: Update availability cache immediately
      await this.syncAvailabilityCache(slotKey, updatedAvailability);
 
      // Write-through: Update customer's reservations cache
      await this.syncCustomerReservationsCache(customerId);
 
      return {
        success: true,
        reservation,
        confirmationCode: reservation.confirmationCode,
      };
 
    } finally {
      await lock.release();
    }
  }
 
  async getAvailability(
    restaurantId: string,
    date: Date
  ): Promise<TimeSlot[]> {
    const cacheKey = `availability:${restaurantId}:${date.toISOString().split('T')[0]}`;
    
    // Check cache first
    const cached = await this.cache.get<TimeSlot[]>(cacheKey);
    if (cached) {
      return cached;
    }
 
    // Cache miss: load from database
    const slots = await this.db.availability.findMany({
      where: {
        restaurantId,
        date,
        remainingCapacity: { gt: 0 },
      },
    });
 
    // Cache for next request (but not too long - availability changes)
    await this.cache.set(cacheKey, slots, { ttl: 60 }); // 1 minute TTL
 
    return slots;
  }
 
  private async syncAvailabilityCache(
    slotKey: string,
    availability: Availability
  ): Promise<void> {
    // Update individual slot cache
    await this.cache.set(`slot:${slotKey}`, availability, { ttl: 300 });
 
    // Invalidate the date's availability list to force refresh
    const [restaurantId, date] = slotKey.split(':');
    await this.cache.del(`availability:${restaurantId}:${date}`);
  }
}

Anti-Patterns: When NOT to Use Write-Through

Equally important as knowing when to use write-through is knowing when to avoid it. Here are scenarios where write-through is the wrong choice.

Anti-Pattern 1: High-Volume Analytics/Logging

The Scenario:

Website tracking page views, clicks, and user behavior
Thousands of events per second per server
Data is written once and rarely read immediately

Why Write-Through Fails:

Write latency is unacceptable for event tracking (blocks page load)
Write volume overwhelms the database
Immediate consistency is unnecessary for analytics

Better Alternative: Write-back with batching, or write directly to a time-series database/message queue.

Anti-Pattern 2: High-Frequency Counters

The Scenario:

View counts, like counts, impression counters
Potentially millions of increments per minute
Exact real-time accuracy not critical

Why Write-Through Fails:

Each increment requires a database round-trip
Database becomes bottleneck instantly
Counter value is often approximate anyway

Better Alternative: Write-back with periodic flush, Redis atomic counters with periodic database sync, or HyperLogLog for approximate counting.

Anti-Pattern 3: Large Binary Data

The Scenario:

Uploading images, videos, documents
Files are megabytes in size
Stored in object storage (S3) with metadata in database

Why Write-Through Fails:

Caching large binary data is expensive (memory consumption)
Network transfer of large payloads is slow
Read patterns are often sequential, not random access

Better Alternative: Store metadata with write-through, but serve binary data directly from object storage with CDN caching.

Anti-Pattern 4: Append-Only Logs/Audit Trails

The Scenario:

System audit logs, compliance records
Write-once, read-rarely data
Historical queries, not real-time lookups

Why Write-Through Fails:

Data is rarely read (low cache hit rate)
High write volume strains cache as well as database
Caching adds no value for archival data

Better Alternative: Write-around (skip cache entirely), or write directly to append-only log storage.

The Red Flags

Implementation Decision Tree

Use this decision tree when evaluating whether to implement write-through caching for a specific data type or feature.

decision_tree.txt

Text

                   Start Here
                        │
                        ▼
        ┌─────────────────────────────────────┐
        │   Does data need to be cached at    │
        │   all? (High read volume, slow DB)  │
        └─────────────────────────────────────┘
                        │
              ┌─────────┼─────────┐
              │ NO      │         │ YES
              ▼         │         ▼
        ┌──────────┐    │   ┌─────────────────────────────────────┐
        │ No Cache │    │   │  Is immediate consistency required? │
        │ Needed   │    │   │  (Security, financial, user-facing) │
        └──────────┘    │   └─────────────────────────────────────┘
                        │                     │
                        │         ┌───────────┼───────────┐
                        │         │ NO        │           │ YES
                        │         ▼           │           ▼
                        │   ┌──────────────┐  │   ┌─────────────────────────────────────┐
                        │   │ Cache-Aside  │  │   │  Is write latency of 50ms+ okay?    │
                        │   │ (Lazy Load)  │  │   │  (Not real-time, user can wait)     │
                        │   └──────────────┘  │   └─────────────────────────────────────┘
                        │                     │                     │
                        │                     │         ┌───────────┼───────────┐
                        │                     │         │ NO        │           │ YES
                        │                     │         ▼           │           ▼
                        │                     │   ┌──────────────┐  │   ┌─────────────────────────────────────┐
                        │                     │   │ Write-Back   │  │   │  Is write volume manageable?        │
                        │                     │   │ (if data loss│  │   │  (<10K writes/sec)                  │
                        │                     │   │  acceptable) │  │   └─────────────────────────────────────┘
                        │                     │   └──────────────┘  │                     │
                        │                     │                     │         ┌───────────┼───────────┐
                        │                     │                     │         │ NO        │           │ YES
                        │                     │                     │         ▼           │           ▼
                        │                     │                     │   ┌──────────────┐  │   ┌──────────────┐
                        │                     │                     │   │ Shard DB +   │  │   │ WRITE-       │
                        │                     │                     │   │ Write-Through│  │   │ THROUGH      │
                        │                     │                     │   │ OR Evaluate  │  │   │ CACHING      │
                        │                     │                     │   │ Trade-offs   │  │   │ ✓            │
                        │                     │                     │   └──────────────┘  │   └──────────────┘
                        │                     │                     │                     │
                        └─────────────────────┴─────────────────────┴─────────────────────┘

Quick Reference Summary:

If you need...	Use...
Strong consistency + durability	Write-Through
Low write latency + some consistency	Write-Back
Simple implementation + read-heavy	Cache-Aside
Write-heavy + no immediate read	Write-Around
Proactive cache warming	Refresh-Ahead

Summary: Use Cases

We've explored the full spectrum of write-through caching use cases. Let's consolidate the key insights:

Key Takeaways

•Financial data is the canonical use case — Account balances, transactions, and credit limits require immediate consistency
•Security changes demand write-through — Session invalidation, permission changes, and password updates cannot tolerate staleness
•E-commerce has mixed needs — Inventory and carts need write-through; catalogs and reviews can use cache-aside
•Real-time displays benefit greatly — Reservation systems, dashboards, and status pages need current data
•Know the anti-patterns — High-volume analytics, counters, large files, and append-only logs should not use write-through
•Use the decision tree — Evaluate each data type systematically based on consistency needs, latency tolerance, and write volume

Module Complete:

Congratulations! You've completed the Write-Through Caching module. You now have a comprehensive understanding of:

How write-through works — The synchronous dual-write protocol
Coordination mechanics — How cache and database stay synchronized
Consistency benefits — Why write-through provides stronger guarantees
Performance trade-offs — The costs and how to optimize
Use cases — When to use write-through and when to choose alternatives

This knowledge prepares you to make informed decisions about caching strategies in your system designs.

Module Complete