Shard key selection in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the shard key based on user ID.

HLD

shard_key = [1]

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

Asession_id

Btimestamp

Cuser_id

Drequest_type

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a non-unique or rapidly changing attribute like timestamp.

Using request_type which may not distribute data evenly.

The shard key should be a stable and evenly distributed attribute like user_id to ensure balanced data distribution.

2fill in blank

medium

Complete the code to ensure shard key supports range queries.

HLD

if shard_key == [1]:
    enable_range_queries = True

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

Arequest_type

Buser_id

Csession_id

Dtimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Using user_id which is not ordered.

Choosing session_id which may not be sequential.

timestamp is suitable for range queries because it represents a continuous timeline.

3fill in blank

hard

Fix the error in shard key selection to avoid hotspotting.

HLD

shard_key = [1]  # Avoid using this key to prevent hotspots

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

Afixed_value

Btimestamp

Cregion_code

Duser_id

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing fixed_value which leads to all data in one shard.

Using region_code which might cause uneven distribution if skewed.

Using a fixed_value as shard key causes all data to go to one shard, creating a hotspot.

4fill in blank

hard

Fill both blanks to create a shard key that balances distribution and supports queries.

HLD

shard_key = ([1], [2])  # Composite shard key

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

Auser_id

Btimestamp

Cregion_code

Dsession_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using two non-unique or non-distributed keys.

Choosing keys that do not support query patterns.

Combining user_id and timestamp helps balance data and supports range queries.

5fill in blank

hard

Fill all three blanks to define shard key selection logic with validation.

HLD

def select_shard_key(record):
    key = record.get([1])
    if key is None or not isinstance(key, [2]):
        raise ValueError('Invalid shard key')
    return key if key [3] 0 else None

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

A'user_id'

Bint

C>

Dstr

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong key name or type.

Missing validation for key presence or type.

Using incorrect comparison operator.

The function gets 'user_id' from the record, checks it is an int, and ensures it is positive with > 0.

Practice

1. What is the main purpose of selecting a shard key in a distributed database system?

easy

A. To compress data to save storage space

B. To encrypt data for security purposes

C. To backup data automatically

D. To split data across servers for better load distribution and faster access

Solution

1. Step 1: Understand shard key role

   The shard key determines how data is divided across multiple servers or shards.

2. Step 2: Identify main benefit

   Splitting data helps balance load and speeds up data retrieval by parallelizing queries.

3. Final Answer:

   To split data across servers for better load distribution and faster access -> Option D

4. Quick Check:

   Shard key purpose = split data load [OK]

Hint: Shard key splits data to spread load and speed up access [OK]

Common Mistakes:

• Confusing shard key with encryption key
• Thinking shard key compresses data
• Assuming shard key is for backup

2. Which of the following is a good characteristic of a shard key?

easy

A. Has very few unique values

B. Has many unique values and distributes data evenly

C. Is always a timestamp

D. Is a large text field

Solution

1. Step 1: Identify shard key traits

   A good shard key should have many unique values to avoid data concentration.

2. Step 2: Understand distribution importance

   Even distribution prevents hotspots and balances load across shards.

3. Final Answer:

   Has many unique values and distributes data evenly -> Option B

4. Quick Check:

   Good shard key = unique + even distribution [OK]

Hint: Choose shard keys with many unique values and even spread [OK]

Common Mistakes:

• Picking keys with few unique values causing hotspots
• Assuming timestamp is always best shard key
• Using large text fields that slow down indexing

3. Consider a user database sharded by country_code which has only 5 unique values. What is the likely problem with this shard key?

medium

A. Data will be evenly distributed across shards

B. It will encrypt user data automatically

C. Some shards will be overloaded due to uneven data distribution

D. Queries will be faster because of fewer shards

Solution

1. Step 1: Analyze unique values count

   Only 5 unique values means few shards and many users per shard.

2. Step 2: Understand load distribution

   Popular countries will have more users, causing some shards to be overloaded.

3. Final Answer:

   Some shards will be overloaded due to uneven data distribution -> Option C

4. Quick Check:

   Few unique keys = uneven shard load [OK]

Hint: Few unique shard keys cause uneven load and hotspots [OK]

Common Mistakes:

• Assuming fewer shards always improve speed
• Thinking shard key encrypts data
• Ignoring uneven data distribution effects

4. A system uses user_id % 10 as a shard key but notices uneven shard sizes. What is the likely cause?

medium

A. User IDs are not uniformly distributed modulo 10

B. Shard key is too unique

C. Shard key is a string instead of number

D. System is encrypting shard keys

Solution

1. Step 1: Understand modulo shard key

   Modulo operation expects uniform distribution of user IDs.

2. Step 2: Identify distribution problem

   If user IDs cluster in certain ranges, modulo results cluster causing uneven shards.

3. Final Answer:

   User IDs are not uniformly distributed modulo 10 -> Option A

4. Quick Check:

   Non-uniform IDs cause uneven modulo shard keys [OK]

Hint: Modulo shard keys need uniform input distribution [OK]

Common Mistakes:

• Assuming modulo always balances shards
• Confusing shard key type with encryption
• Ignoring data distribution patterns

5. You design a sharded system for an e-commerce app. Users query orders mostly by user_id and order_date. Which shard key choice best supports fast queries and balanced load?

hard

A. Shard by a composite key of user_id and order_date

B. Shard by user_id only

C. Shard by order_date only

D. Shard by a random UUID

Solution

1. Step 1: Match shard key to query patterns

   Users query by both user_id and order_date, so shard key should support both.

2. Step 2: Evaluate shard key options

   Sharding by user_id alone may cause hotspots; order_date alone may cause uneven load by time.

3. Step 3: Use composite key benefits

   Composite key balances load and supports queries filtering by either field efficiently.

4. Final Answer:

   Shard by a composite key of user_id and order_date -> Option A

5. Quick Check:

   Composite shard key matches queries and balances load [OK]

Hint: Composite keys balance load and match query patterns best [OK]

Common Mistakes:

• Choosing single field shard keys ignoring query patterns
• Using random UUID causing inefficient queries
• Ignoring load balancing in shard key choice