Cache-aside pattern in Azure - Step-by-Step Execution

Process Flow - Cache-aside pattern

Request data

↓

Check cache

↓

Store in cache

↓

Return data

This flow shows how the cache-aside pattern works: first check cache, if data is missing, get it from database, store it in cache, then return it.

Execution Sample

Azure

def get_data(key):
    data = cache.get(key)
    if data is None:
        data = database.get(key)
        cache.set(key, data)
    return data

This code tries to get data from cache first; if not found, it fetches from database, caches it, then returns.

Process Table

Step	Action	Cache State	Database Access	Returned Data
1	Request data with key 'user123'	{}	No	None
2	Check cache for 'user123'	{}	No	None
3	Cache miss, fetch from database	{}	Yes	UserData123
4	Store 'UserData123' in cache	{'user123': 'UserData123'}	No	UserData123
5	Return data 'UserData123'	{'user123': 'UserData123'}	No	UserData123
6	Next request for 'user123'	{'user123': 'UserData123'}	No	UserData123
7	Check cache for 'user123'	{'user123': 'UserData123'}	No	UserData123
8	Cache hit, return cached data	{'user123': 'UserData123'}	No	UserData123

💡 Execution stops after returning cached data on second request.

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 8
cache	{}	{}	{'user123': 'UserData123'}	{'user123': 'UserData123'}
data	None	UserData123	UserData123	UserData123

Key Moments - 3 Insights

Why do we check the cache before the database?

Because checking the cache first avoids unnecessary database calls, improving speed and reducing load, as shown in steps 2 and 7 where cache is checked first.

What happens when the data is not in the cache?

When data is missing in cache (step 3), the system fetches it from the database and then stores it in cache (step 4) for future requests.

Why do we store data in cache after fetching from database?

Storing data in cache after fetching ensures next requests get faster responses without hitting the database again, as seen in step 4 and step 8.

Visual Quiz - 3 Questions

Look at the execution table, what is the cache state after step 4?

A {'user123': 'UserData123'}

B None

C {}

D {'user123': None}

💡 Hint

Check the 'Cache State' column in row for step 4.

At which step does the system access the database?

A Step 2

B Step 3

C Step 6

D Step 7

💡 Hint

Look at the 'Database Access' column to find 'Yes'.

If the cache already contains the data, what changes in the execution table?

A Data is fetched twice from database

B Cache is cleared

C Database access is skipped

D Returned data is None

💡 Hint

Refer to steps 6-8 where cache hit avoids database access.

Concept Snapshot

Cache-aside pattern:
- Check cache first for data
- If missing, fetch from database
- Store fetched data in cache
- Return data to requester
Improves performance by reducing database load.

Full Transcript

The cache-aside pattern works by first checking if the requested data is in the cache. If it is, the data is returned immediately, avoiding a database call. If the data is not in the cache, the system fetches it from the database, stores it in the cache for future use, and then returns it. This pattern helps improve application speed and reduces database load by caching frequently accessed data.

Practice

1. What is the main purpose of the cache-aside pattern in Azure applications?

easy

A. To load data into cache only when it is requested

B. To preload all data into cache at application start

C. To automatically update cache without application control

D. To store data only in the database without caching

Solution

1. Step 1: Understand cache-aside pattern behavior

   The cache-aside pattern loads data into cache only when the application requests it and the data is not already cached.

2. Step 2: Compare options with this behavior

   To load data into cache only when it is requested matches this behavior. Options B and C describe other caching strategies, and A ignores caching.

3. Final Answer:

   To load data into cache only when it is requested -> Option A

4. Quick Check:

   Cache-aside loads on demand = D [OK]

Hint: Cache-aside means load cache only when needed [OK]

Common Mistakes:

• Thinking cache preloads all data
• Assuming cache updates automatically
• Confusing cache-aside with write-through caching

2. Which of the following is the correct sequence when using the cache-aside pattern in Azure?

easy

A. Return data -> Check cache -> Read database -> Store in cache

B. Read database -> Store in cache -> Return data -> Check cache

C. Store in cache -> Check cache -> Read database -> Return data

D. Check cache -> If miss, read database -> Store in cache -> Return data

Solution

1. Step 1: Recall cache-aside pattern steps

   The application first checks the cache. If data is missing (cache miss), it reads from the database, stores the data in cache, then returns it.

2. Step 2: Match the sequence with options

   Check cache -> If miss, read database -> Store in cache -> Return data correctly shows this sequence. Other options have steps in wrong order.

3. Final Answer:

   Check cache -> If miss, read database -> Store in cache -> Return data -> Option D

4. Quick Check:

   Cache check first, then DB read if miss = A [OK]

Hint: Cache check first, then DB read if miss [OK]

Common Mistakes:

• Reading database before checking cache
• Storing data in cache before reading database
• Returning data before checking cache

3. Consider this Azure cache-aside pseudocode:

data = cache.get('user123')
if data is None:
    data = database.read('user123')
    cache.set('user123', data)
return data

What happens if the cache contains stale data for 'user123'?

medium

A. The application reads fresh data from the database every time

B. The application returns the stale data without checking the database

C. The cache automatically updates stale data before returning

D. The application throws an error due to stale cache

Solution

1. Step 1: Analyze cache-aside code behavior

   The code returns cached data if present, without verifying freshness. It reads database only if cache miss.

2. Step 2: Understand stale data impact

   If cache has stale data, the application returns it directly, ignoring database updates until cache expires or is invalidated.

3. Final Answer:

   The application returns the stale data without checking the database -> Option B

4. Quick Check:

   Cache returns stale data if present = A [OK]

Hint: Cache returns data if present, even if stale [OK]

Common Mistakes:

• Assuming cache auto-refreshes stale data
• Thinking database is read every time
• Believing stale cache causes errors

4. You implemented cache-aside in Azure but notice your cache never updates after data changes in the database. What is the most likely cause?

medium

A. The application does not invalidate or update cache after database writes

B. The cache service is down and cannot store data

C. The database is not reachable for reads

D. The cache is set to expire data too frequently

Solution

1. Step 1: Understand cache-aside update responsibility

   In cache-aside, the application must update or invalidate cache after database changes to keep cache fresh.

2. Step 2: Identify cause of stale cache

   If cache never updates, likely the app is not managing cache invalidation after writes, causing stale data to persist.

3. Final Answer:

   The application does not invalidate or update cache after database writes -> Option A

4. Quick Check:

   App must update cache after DB changes = C [OK]

Hint: App must update cache after DB changes [OK]

Common Mistakes:

• Blaming cache service downtime without checking app logic
• Assuming database read issues cause stale cache
• Thinking frequent expiry causes no updates

5. You want to optimize an Azure app using cache-aside pattern for user profiles. Which approach best ensures cache consistency when profiles update frequently?

hard

A. Only update cache when the profile is requested next time

B. Set cache expiration to 24 hours and never update manually

C. After updating the database, immediately update or remove the cached profile

D. Preload all profiles into cache at app startup

Solution

1. Step 1: Consider cache consistency needs

   For frequently updated profiles, cache must reflect changes quickly to avoid stale data.

2. Step 2: Evaluate options for freshness

   After updating the database, immediately update or remove the cached profile updates or removes cache immediately after DB update, ensuring fresh data. Only update cache when the profile is requested next time delays update causing stale reads. Set cache expiration to 24 hours and never update manually relies on expiry only, which is slow. Preload all profiles into cache at app startup wastes resources and may cause stale data.

3. Final Answer:

   After updating the database, immediately update or remove the cached profile -> Option C

4. Quick Check:

   Immediate cache update after DB write = B [OK]

Hint: Update cache right after DB changes for freshness [OK]

Common Mistakes:

• Relying only on cache expiration for updates
• Delaying cache update until next read
• Preloading cache wastes memory and risks staleness