What is Write-Through?

Write-Through is a caching pattern where writes go to both the cache and the database synchronously. The cache acts as the main interface for both reads and writes.

How It Works

Application                Cache                Database
     │                       │                     │
     │─── Write(key, data) ─►│                     │
     │                       │                     │
     │                       │─── Write ──────────►│
     │                       │                     │
     │                       │◄── OK ─────────────│
     │                       │                     │
     │◄── OK ────────────────│                     │

Both cache and database are updated before returning success.

Read Flow

Application                Cache                Database
     │                       │                     │
     │─── Read(key) ────────►│                     │
     │                       │                     │
     │◄── Data (if cached) ──│                     │
     │                       │                     │
     │        OR if miss:    │                     │
     │                       │─── Query ──────────►│
     │                       │◄── Data ───────────│
     │◄── Data ──────────────│                     │

Advantages

Benefit	Description
Cache always consistent	Writes update cache immediately
No stale data	Cache reflects DB state
Simple reads	Always read from cache
Data durability	DB always has latest

Disadvantages

Drawback	Description
Higher write latency	Must write to both
Cache may have unused data	Writes cached even if never read
Write amplification	Every write hits DB and cache

Use Cases

Good for:

Data that's frequently read after write
Strong consistency requirements
Applications where write latency is acceptable

Not ideal for:

Write-heavy workloads
Data rarely read after write
Latency-sensitive writes

Comparison

Aspect	Write-Through	Cache-Aside
Write latency	Higher	Lower
Cache consistency	Always consistent	May be stale briefly
Cache population	On every write	On read (lazy)
Complexity	Cache handles writes	App manages both

Interview Tips

Explain synchronous write to both cache and DB
Discuss latency trade-off vs consistency benefit
Compare with write-behind (async) pattern
Mention when write-through is appropriate

Write-Through

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