Write-through pattern in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis write-through cache sequence?

Consider a Redis cache used with a write-through pattern. The application writes a key user:1 with value "Alice" to Redis and the database simultaneously. Then it reads the key user:1 from Redis.

What will the Redis GET command return?

Redis

SET user:1 "Alice"
GET user:1

A nil

B "Alice"

C SyntaxError

D "user:1"

💡 Hint

In write-through caching, data is written to cache and database at the same time.

Since the write-through pattern writes data to Redis immediately, the GET command returns the stored value "Alice".

🧠 Conceptual

intermediate

Why is write-through caching beneficial for data consistency?

Which of the following best explains why write-through caching helps keep data consistent between cache and database?

A Because data is written to cache and database simultaneously

B Because cache is updated only when data is read from database

C Because data is written to cache only after database write succeeds

D Because cache is cleared after every write to database

💡 Hint

Think about when the cache and database get updated in write-through.

Write-through caching writes data to both cache and database at the same time, ensuring they stay consistent.

📝 Syntax

advanced

Which Redis command sequence correctly implements a write-through cache update?

You want to update the user name for user:2 in Redis cache and the database using write-through. Which Redis command sequence below correctly updates the cache?

A

DEL user:2
SET user:2 "Bob"

B SETNX user:2 "Bob"

C SET user:2 "Bob"

D

GET user:2
SET user:2 "Bob"

💡 Hint

Write-through means writing the new value directly to cache.

Using SET directly updates the cache with the new value, which is required for write-through.

❓ optimization

advanced

How can write-through caching impact write latency and how to mitigate it?

Write-through caching writes data to both cache and database synchronously. What is a common impact on write latency and a typical way to reduce it?

A Increases latency; mitigate by batching writes asynchronously

B Decreases latency; mitigate by adding more cache servers

C No impact on latency; mitigate by using faster database

D Increases latency; mitigate by disabling cache writes

💡 Hint

Think about writing to two places at once and how to improve speed.

Write-through increases write latency because it waits for both cache and database writes. Batching writes asynchronously can reduce this latency.

🔧 Debug

expert

What error occurs if Redis SET command is used incorrectly in write-through caching?

Given this Redis command used in a write-through cache update:

SET user:3

What error will Redis return?

A "OK"

B (error) ERR syntax error

C nil

D (error) ERR wrong number of arguments for 'set' command

💡 Hint

Check the required arguments for the SET command.

The SET command requires a key and a value. Missing the value causes a wrong number of arguments error.

Practice

1. What is the main purpose of the write-through pattern in Redis caching?

easy

A. To delete data from the cache when the database is updated

B. To update only the cache and delay database writes

C. To read data only from the cache without updating the database

D. To update both the cache and the database at the same time

Solution

1. Step 1: Understand write-through pattern basics

   The write-through pattern means writing data to the cache and database simultaneously to keep them in sync.

2. Step 2: Identify the correct behavior in Redis context

   In Redis, this pattern ensures cache is updated immediately when the database changes, avoiding stale data.

3. Final Answer:

   To update both the cache and the database at the same time -> Option D

4. Quick Check:

   Write-through = update cache + database [OK]

Hint: Write-through updates cache and DB together [OK]

Common Mistakes:

• Thinking write-through updates only cache
• Confusing write-through with write-back pattern
• Assuming cache is updated after database asynchronously

2. Which Redis command sequence correctly implements a write-through cache update when saving a user profile with key user:123 and value {"name":"Alice"}?

easy

A. DEL user:123 then SET user:123 '{"name":"Alice"}'

B. GET user:123 then SET user:123 '{"name":"Alice"}'

C. SET user:123 '{"name":"Alice"}' followed by DB update

D. SET user:123 '{"name":"Alice"}' only, no DB update

Solution

1. Step 1: Identify write-through steps

   Write-through requires writing to cache (Redis) and database together.

2. Step 2: Match Redis commands to write-through

   Using SET updates Redis cache; followed by DB update ensures both are in sync.

3. Final Answer:

   SET user:123 '{"name":"Alice"}' followed by DB update -> Option C

4. Quick Check:

   SET + DB update = write-through [OK]

Hint: SET cache then update DB for write-through [OK]

Common Mistakes:

• Only updating cache without DB
• Deleting cache before setting without DB update
• Getting cache before setting without DB write

3. Consider this pseudo-code using write-through pattern:

cache.SET('item:1', 'value1')
database.UPDATE('item:1', 'value1')
result = cache.GET('item:1')

What will result contain after execution?

medium

A. null

B. 'value1'

C. Old cached value

D. Error due to missing database read

Solution

1. Step 1: Analyze cache write

   The cache is set to 'value1' for key 'item:1' before database update.

2. Step 2: Analyze cache read

   Reading from cache immediately after setting returns the new value 'value1'.

3. Final Answer:

   'value1' -> Option B

4. Quick Check:

   Cache GET after SET returns new value [OK]

Hint: GET after SET returns updated cache value [OK]

Common Mistakes:

• Assuming cache GET returns null if DB updated
• Thinking old cached value remains after SET
• Expecting error without explicit DB read

4. You implemented write-through caching but notice stale data in Redis after database updates. Which mistake likely causes this?

medium

A. Updating database but forgetting to update Redis cache

B. Updating Redis cache but not the database

C. Using GET instead of SET to update cache

D. Deleting database entries without clearing cache

Solution

1. Step 1: Identify cause of stale cache

   If database updates but cache is not updated, cache holds old data causing staleness.

2. Step 2: Match mistake to write-through pattern

   Write-through requires cache update on every DB write; missing cache update breaks this.

3. Final Answer:

   Updating database but forgetting to update Redis cache -> Option A

4. Quick Check:

   Missing cache update causes stale data [OK]

Hint: Always update cache when DB changes [OK]

Common Mistakes:

• Updating cache only without DB
• Using GET to update cache instead of SET
• Not clearing cache on DB deletes

5. You want to implement a write-through cache in Redis for a product catalog. Which approach best ensures data consistency and minimal stale reads?

hard

A. On product update, update both Redis cache and database synchronously in the same operation

B. On product update, update the database first, then delete the Redis cache entry

C. On product update, update only the database and rely on cache expiration to refresh

D. On product update, write to Redis cache first, then update the database asynchronously

Solution

1. Step 1: Understand write-through consistency needs

   Write-through requires cache and database to be updated together to avoid stale reads.

2. Step 2: Evaluate options for consistency

   Updating both synchronously ensures cache always reflects latest DB state, minimizing stale data.

3. Final Answer:

   On product update, update both Redis cache and database synchronously in the same operation -> Option A

4. Quick Check:

   Synchronous cache + DB update = consistent write-through [OK]

Hint: Update cache and DB together synchronously [OK]

Common Mistakes:

• Updating cache before DB asynchronously causing race conditions
• Deleting cache instead of updating it
• Relying only on cache expiration for freshness