Write-through pattern in Redis - Time & Space Complexity

Time Complexity: Write-through pattern

O(n)

Understanding Time Complexity

We want to understand how the time it takes to save data changes as the amount of data grows when using the write-through pattern in Redis.

Specifically, how does writing to both cache and database affect performance as data size increases?

Scenario Under Consideration

Analyze the time complexity of the following Redis write-through code snippet.

-- Write-through cache update
redis.call('SET', key, value)  -- write to cache
database_write(key, value)     -- write to database
return 'OK'
    

This code writes data to the Redis cache first, then writes the same data to the database, ensuring both are updated immediately.

Identify Repeating Operations

In this snippet, there are no loops or recursion. The operations happen once per write.

• Primary operation: Two write commands: one to Redis cache, one to the database.
• How many times: Once per data write request.

How Execution Grows With Input

Each write involves two steps regardless of data size. So, if you write 10 items, you do 20 writes total; for 100 items, 200 writes; for 1000 items, 2000 writes.

Input Size (n)	Approx. Operations
10	20
100	200
1000	2000

Pattern observation: The number of operations grows directly with the number of writes; doubling the writes doubles the operations.

Final Time Complexity

Time Complexity: O(n)

This means the time to complete all writes grows linearly with the number of items you write.

Common Mistake

[X] Wrong: "Writing to cache and database at the same time means the time stays the same no matter how many writes happen."

[OK] Correct: Each write still happens one after another for each item, so more items mean more total work and longer time.

Interview Connect

Understanding how write-through caching affects time helps you explain trade-offs between speed and data consistency in real systems.

Self-Check

"What if we changed the write-through pattern to write-back caching? How would the time complexity change?"