Write-through pattern in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a key-value pair in Redis cache.

Redis

redis_client.[1]('user:1', 'Alice')

Drag options to blanks, or click blank then click option'

Aset

Bget

Cdelete

Dexists

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' instead of 'set' to store data.

The set command stores a key-value pair in Redis.

2fill in blank

medium

Complete the code to update the database after setting the cache in write-through pattern.

Redis

database.[1]('UPDATE users SET name = %s WHERE id = %s', ('Alice', 1))

Drag options to blanks, or click blank then click option'

Acommit

Bfetch

Cexecute

Dselect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'fetch' which is for retrieving data, not updating.

The execute method runs the SQL update command on the database.

3fill in blank

hard

Fix the error in the write-through update code by completing the missing Redis command.

Redis

def write_through_update(key, value):
    redis_client.[1](key, value)
    database.execute('UPDATE users SET name = %s WHERE id = %s', (value, 1))
    database.commit()

Drag options to blanks, or click blank then click option'

Aset

Bget

Cdelete

Dexists

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' which only reads data, not updates.

To update the cache in write-through, use set to write the new value.

4fill in blank

hard

Fill both blanks to complete the write-through pattern: update cache and then update database.

Redis

def write_through_update(key, value):
    redis_client.[1](key, value)
    database.[2]('UPDATE users SET name = %s WHERE id = %s', (value, 1))
    database.commit()

Drag options to blanks, or click blank then click option'

Aset

Bexecute

Cget

Dfetch

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping 'set' and 'execute' commands.

First, use set to update Redis cache, then execute to update the database.

5fill in blank

hard

Fill all three blanks to implement a write-through update: set cache key, execute DB update, and commit transaction.

Redis

def write_through_update(key, value):
    redis_client.[1](key, value)
    database.[2]('UPDATE users SET name = %s WHERE id = %s', (value, 1))
    database.[3]()

Drag options to blanks, or click blank then click option'

Aget

Bexecute

Ccommit

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to commit the database transaction.

Use set to update cache, execute to run the SQL update, and commit to save changes in the database.

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