Connection pooling in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the maximum number of connections in the pool.

HLD

max_connections = [1]

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

Amax_pool_size

Bpool_max

Cconnection_limit

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined variable names instead of an integer.

Setting the pool size to zero or negative numbers.

The maximum number of connections is set to a fixed integer value like 100 to limit resource usage.

2fill in blank

medium

Complete the code to acquire a connection from the pool.

HLD

connection = pool.[1]()

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

Arelease

Bacquire

Cclose

Dopen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'release' which is for returning a connection.

Using 'close' which terminates a connection.

To get a connection from the pool, you call the acquire method.

3fill in blank

hard

Fix the error in releasing a connection back to the pool.

HLD

pool.[1](connection)

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

Arelease

Bget

Cacquire

Dconnect

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'acquire' instead of 'release'.

Using 'connect' which is unrelated.

To return a connection to the pool, use the release method.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps connection IDs to their status if the connection is active.

HLD

status_map = {conn.id: conn.[1] for conn in connections if conn.[2]

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

Astatus

Bis_active

Cconnected

Dopen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'connected' or 'open' which may not be boolean properties.

Mixing up the order of properties.

The dictionary maps connection IDs to their status only if the connection is_active.

5fill in blank

hard

Fill all three blanks to filter connections with load less than 50 and map their IDs to load values.

HLD

filtered_loads = {conn.[1]: conn.[2] for conn in pool if conn.[3] < 50}

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

Aid

Bload

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'status' instead of 'load' for filtering.

Mixing keys and values in the dictionary comprehension.

The dictionary comprehension maps connection id to its load only if the load is less than 50.

Practice

1. What is the main purpose of connection pooling in system design?

easy

A. To reuse existing connections and reduce the overhead of creating new ones

B. To create a new connection for every request to ensure freshness

C. To store user data temporarily for faster access

D. To encrypt all data sent between client and server

Solution

1. Step 1: Understand connection creation cost

   Creating new connections for every request is slow and resource-heavy.

2. Step 2: Role of connection pooling

   Connection pooling keeps connections open and reuses them to save time and resources.

3. Final Answer:

   To reuse existing connections and reduce the overhead of creating new ones -> Option A

4. Quick Check:

   Connection pooling = reuse connections [OK]

Hint: Pooling means reuse connections to save time [OK]

Common Mistakes:

• Thinking pooling creates new connections each time
• Confusing pooling with caching user data
• Assuming pooling encrypts data

2. Which of the following is the correct way to configure a connection pool size in a system design?

easy

A. Set pool size to the maximum integer value to allow unlimited connections

B. Set pool size to zero to disable pooling

C. Set pool size to a fixed number based on expected load, e.g., 50

D. Set pool size to negative number to auto-scale connections

Solution

1. Step 1: Understand pool size meaning

   Pool size controls how many connections can be reused simultaneously.

2. Step 2: Choose appropriate pool size

   Setting a fixed number based on expected load balances resource use and performance.

3. Final Answer:

   Set pool size to a fixed number based on expected load, e.g., 50 -> Option C

4. Quick Check:

   Pool size = fixed number for load [OK]

Hint: Pool size should match expected load, not zero or negative [OK]

Common Mistakes:

• Setting pool size to zero disables pooling
• Using negative or unlimited values causes errors
• Ignoring load when setting pool size

3. Consider this pseudocode for connection pooling:

pool = createPool(size=3)
conn1 = pool.getConnection()
conn2 = pool.getConnection()
conn3 = pool.getConnection()
conn4 = pool.getConnection()

What happens when conn4 is requested?

medium

A. conn4 closes one existing connection and reuses it

B. conn4 creates a new connection ignoring the pool size

C. conn4 returns null immediately because pool is full

D. conn4 waits until one connection is released back to the pool

Solution

1. Step 1: Analyze pool size and connections taken

   The pool size is 3, and 3 connections are already taken (conn1, conn2, conn3).

2. Step 2: Behavior on extra connection request

   Requesting conn4 must wait until a connection is released because pool is full.

3. Final Answer:

   conn4 waits until one connection is released back to the pool -> Option D

4. Quick Check:

   Pool full = wait for release [OK]

Hint: Extra requests wait if pool is full [OK]

Common Mistakes:

• Assuming new connections are created beyond pool size
• Expecting null return immediately
• Thinking pool forcibly closes connections

4. A system uses connection pooling but experiences slow responses under high load. Which of the following is the most likely cause?

medium

A. Connections are not reused and closed immediately

B. Pool size is too small, causing requests to wait for connections

C. Pool size is too large, causing too many open connections

D. Timeout settings are too high, causing connections to stay open too long

Solution

1. Step 1: Identify impact of small pool size

   A small pool size means limited connections, so requests wait longer.

2. Step 2: Understand effect on response time

   Waiting for connections causes slow responses under high load.

3. Final Answer:

   Pool size is too small, causing requests to wait for connections -> Option B

4. Quick Check:

   Small pool = slow responses [OK]

Hint: Small pool size causes waiting delays [OK]

Common Mistakes:

• Blaming large pool size for slowness
• Ignoring connection reuse behavior
• Confusing timeout with pool size

5. You design a connection pool for a web service expected to handle 1000 concurrent users. Which combination of settings best balances performance and resource use?

hard

A. Pool size = 100, Timeout = 30 seconds, Max wait time = 10 seconds

B. Pool size = 1000, Timeout = 5 minutes, No max wait time

C. Pool size = 10, Timeout = 1 hour, Max wait time = 1 second

D. Pool size = 5000, Timeout = 10 seconds, No max wait time

Solution

1. Step 1: Evaluate pool size for concurrency

   Pool size of 100 allows many connections but avoids resource overload compared to 1000 or 5000.

2. Step 2: Assess timeout and wait time

   Timeout of 30 seconds frees unused connections quickly; max wait time of 10 seconds prevents long waits.

3. Step 3: Compare other options

   Too large pool sizes waste resources; too small pool or short wait times cause failures.

4. Final Answer:

   Pool size = 100, Timeout = 30 seconds, Max wait time = 10 seconds -> Option A

5. Quick Check:

   Balanced pool size and timeouts = Pool size = 100, Timeout = 30 seconds, Max wait time = 10 seconds [OK]

Hint: Balance pool size and timeouts for best performance [OK]

Common Mistakes:

• Choosing pool size equal to max users wastes resources
• Ignoring max wait time causes request failures
• Setting too long timeout holds connections unnecessarily