Consistent hashing in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is consistent hashing?

Consistent hashing is a technique to distribute data across multiple servers so that when servers are added or removed, only a small portion of data needs to be moved. It helps systems scale smoothly.

intermediate

Why is consistent hashing better than simple modulo hashing for distributed systems?

Simple modulo hashing moves almost all data when the number of servers changes. Consistent hashing moves only a small part, reducing data reshuffling and downtime.

beginner

What is a hash ring in consistent hashing?

A hash ring is a circular space where both servers and data keys are placed using a hash function. Data is assigned to the next server clockwise on the ring.

intermediate

How do virtual nodes improve consistent hashing?

Virtual nodes are multiple points per server on the hash ring. They help balance load better by spreading data more evenly across servers.

beginner

What happens when a server is removed in consistent hashing?

Only the data assigned to that server moves to the next server on the ring. Other data stays on their servers, minimizing data movement.

What does consistent hashing minimize when servers change?

A Number of servers

B Amount of data moved

C Hash function complexity

D Network latency

Consistent hashing minimizes the amount of data that needs to be moved when servers are added or removed.

In consistent hashing, where is data assigned on the hash ring?

A To the server at the next clockwise position

B To the server at the previous counterclockwise position

C To a random server

D To all servers equally

Data is assigned to the server at the next clockwise position on the hash ring.

What is the purpose of virtual nodes in consistent hashing?

A To increase hash collisions

B To encrypt data

C To reduce the number of servers

D To improve load balancing

Virtual nodes help distribute data more evenly, improving load balancing.

Which problem does consistent hashing solve in distributed systems?

A User authentication

B Slow database queries

C Data reshuffling on server changes

D Network packet loss

Consistent hashing reduces data reshuffling when servers are added or removed.

What shape is used to visualize consistent hashing?

A Circle (hash ring)

B Square grid

C Tree diagram

D Line graph

Consistent hashing is visualized as a circle called a hash ring.

Explain how consistent hashing helps when adding or removing servers in a distributed system.

Think about how data is distributed and what happens when servers change.

Describe the role of virtual nodes in consistent hashing and why they are useful.

Consider how virtual nodes affect data balance across servers.

Practice

1. What is the main advantage of consistent hashing in distributed systems?

easy

A. It stores data only on one server

B. It guarantees equal data size on all servers

C. It requires no hashing function

D. It minimizes data movement when servers are added or removed

Solution

1. Step 1: Understand consistent hashing purpose

   Consistent hashing is designed to reduce the amount of data that needs to be moved when servers join or leave the system.

2. Step 2: Compare options with consistent hashing benefits

   Only It minimizes data movement when servers are added or removed correctly states this key advantage. Other options are incorrect or unrelated.

3. Final Answer:

   It minimizes data movement when servers are added or removed -> Option D

4. Quick Check:

   Minimize data movement = D [OK]

Hint: Remember: consistent hashing reduces data reshuffling [OK]

Common Mistakes:

• Thinking consistent hashing balances data perfectly
• Assuming no hashing function is needed
• Believing data is stored on only one server

2. Which of the following is a correct step in implementing consistent hashing?

easy

A. Assign each server multiple points on the hash ring (virtual nodes)

B. Store all data on a single server to avoid hashing

C. Use a linear list instead of a circular ring

D. Hash data keys directly to server IP addresses without hashing servers

Solution

1. Step 1: Recall consistent hashing implementation details

   Consistent hashing uses a circular ring and assigns multiple virtual nodes per server to balance load.

2. Step 2: Evaluate each option

   Assign each server multiple points on the hash ring (virtual nodes) correctly describes virtual nodes. Options A, C, and D contradict consistent hashing principles.

3. Final Answer:

   Assign each server multiple points on the hash ring (virtual nodes) -> Option A

4. Quick Check:

   Virtual nodes = B [OK]

Hint: Virtual nodes improve load balance in consistent hashing [OK]

Common Mistakes:

• Ignoring virtual nodes and using single points
• Using linear lists instead of circular rings
• Skipping hashing of servers

3. Given a consistent hashing ring with servers at hash points 10, 30, and 50, where will the data key with hash 35 be stored?

medium

A. Server at 50

B. Server at 30

C. Server at 10

D. Server at 35

Solution

1. Step 1: Understand consistent hashing key assignment

   Data is assigned to the next server clockwise on the ring after the key's hash.

2. Step 2: Find the next server after hash 35

   Servers are at 10, 30, and 50. After 35 clockwise is 50.

3. Final Answer:

   Server at 50 -> Option A

4. Quick Check:

   Next clockwise server after 35 = 50 [OK]

Hint: Assign data to next clockwise server on ring [OK]

Common Mistakes:

• Assigning data to previous server instead of next
• Choosing server at 30 because it's closest numerically
• Assuming server at 35 exists

4. In a consistent hashing system, a developer notices that when a new server is added, almost all data moves to the new server. What is the likely cause?

medium

A. The data keys are not hashed before assignment

B. The new server has fewer virtual nodes than others

C. The hash function is not distributing servers evenly on the ring

D. The ring is implemented as a linear list

Solution

1. Step 1: Analyze the problem of data movement on server addition

   If almost all data moves to the new server, it means the new server's position dominates the ring.

2. Step 2: Identify cause related to server distribution

   This happens if the hash function places servers unevenly, clustering many keys near the new server.

3. Final Answer:

   The hash function is not distributing servers evenly on the ring -> Option C

4. Quick Check:

   Uneven server distribution = A [OK]

Hint: Check server hash distribution to avoid data skew [OK]

Common Mistakes:

• Blaming virtual nodes count without checking hash function
• Ignoring hashing of data keys
• Confusing ring structure with linear list

5. You are designing a distributed cache using consistent hashing. To handle server failures gracefully, which approach is best?

hard

A. Assign each data key to only one server without replication

B. Use virtual nodes and replicate data on the next N servers clockwise

C. Use a fixed number of servers without adding or removing nodes

D. Hash data keys to servers randomly each time

Solution

1. Step 1: Understand fault tolerance in consistent hashing

   To handle failures, data should be replicated to multiple servers to avoid data loss.

2. Step 2: Identify best practice for replication

   Using virtual nodes improves load balance, and replicating data on next N servers clockwise ensures availability.

3. Final Answer:

   Use virtual nodes and replicate data on the next N servers clockwise -> Option B

4. Quick Check:

   Replication + virtual nodes = A [OK]

Hint: Replicate data on next servers for fault tolerance [OK]

Common Mistakes:

• Not replicating data causing data loss on failure
• Avoiding virtual nodes and causing imbalance
• Randomly hashing keys each time causing inconsistency