In a Write-Through cache, the application treats the cache as the main data store. Every time data is written, it is written to the cache and the database at the same time.

Think of a Double-Entry Accountant:

1. You give the Accountant a receipt for $100.
2. The Accountant has two books: a Pocket Ledger (The Cache) and a Heavy Safe (The Database).
3. The Accountant refuses to say "Done" until they have written the $100 in both books.
4. This takes a bit longer to write, but you are 100% sure that both books are always identical.

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How it works

1. Application wants to write data.
2. It writes to the Cache.
3. The Cache (or the App) immediately writes to the Database.
4. Only after both succeed does the Application continue.

Pros

• Consistency: The cache never has "stale" or old data.
• Reliability: If the system crashes, the data is already in the DB.
• Fast Reads: Since every write updates the cache, subsequent reads are almost always "Cache Hits."

Cons

• Write Latency: Writes are slow because you have to wait for two storage systems.
• Resource Heavy: You might be caching data that is never read again (Polluting the cache).

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Use Case

Best for systems where data consistency is critical but writes are relatively infrequent compared to reads.

Cache Invalidation?

Interestingly, Write-Through solves the invalidation problem because the cache is always updated as part of the write process.

Comparison

• Cache Aside: Good for "Read-Heavy, Lazy Loading."
• Write Through: Good for "Consistency-Heavy, High-Confidence." 🏛️