Cluster, node, and shard architecture in Elasticsearch - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Elasticsearch cluster?

An Elasticsearch cluster is a group of one or more nodes (servers) that together hold your entire data and provide indexing and search capabilities. It acts like a team working together to handle data.

beginner

Define a node in Elasticsearch.

A node is a single server that is part of the Elasticsearch cluster. It stores data and participates in the cluster's indexing and search operations.

beginner

What is a shard in Elasticsearch?

A shard is a smaller piece of an index. Elasticsearch splits large indexes into shards to distribute data across nodes, making search and storage faster and more efficient.

intermediate

Why does Elasticsearch use shards?

Shards help spread data across multiple nodes, allowing Elasticsearch to handle large amounts of data and perform searches quickly by working in parallel.

intermediate

Explain the difference between primary and replica shards.

Primary shards hold the original data, while replica shards are copies of primary shards. Replicas provide fault tolerance and improve search speed by sharing the load.

What does an Elasticsearch cluster consist of?

A Only shards without nodes

B A single node only

C Multiple nodes working together

D A database server

An Elasticsearch cluster is made up of multiple nodes that work together to store and search data.

What is the role of a node in Elasticsearch?

A It stores data and participates in search and indexing

B It only manages user access

C It is a backup server outside the cluster

D It only handles network traffic

A node stores data and helps with indexing and searching within the cluster.

Why are shards important in Elasticsearch?

A They backup data to external storage

B They encrypt data for security

C They compress data to save space

D They split data to improve speed and scalability

Shards split large indexes into smaller parts to distribute data and speed up search and storage.

What is a replica shard?

A A shard that holds user credentials

B A copy of a primary shard for fault tolerance

C A shard that is deleted after indexing

D A shard that stores metadata only

Replica shards are copies of primary shards that help with fault tolerance and load balancing.

How does Elasticsearch improve search speed?

A By distributing shards across nodes to search in parallel

B By storing all data on a single node

C By disabling replicas

D By limiting the number of nodes

Elasticsearch spreads shards across nodes so searches can happen at the same time on different parts of data.

Describe how clusters, nodes, and shards work together in Elasticsearch.

Think of a cluster as a team, nodes as team members, and shards as tasks shared among them.

Explain why Elasticsearch uses replica shards and how they help the system.

Consider how having backups and helpers can keep a system running smoothly.

Practice

1. What is a node in Elasticsearch cluster architecture?

easy

A. A single machine that is part of the cluster

B. A group of machines working together

C. A piece of data stored in the index

D. A backup copy of data

Solution

1. Step 1: Understand the definition of a node

   A node is one machine or server that joins the Elasticsearch cluster to share work.

2. Step 2: Differentiate node from cluster and shard

   A cluster is many nodes working together; shards are parts of data inside nodes.

3. Final Answer:

   A single machine that is part of the cluster -> Option A

4. Quick Check:

   Node = single machine [OK]

Hint: Node means one machine in the cluster [OK]

Common Mistakes:

• Confusing node with cluster
• Thinking node is a data piece
• Mixing node with shard

2. Which of the following is the correct way to describe a shard in Elasticsearch?

easy

A. A part of an index that stores data

B. A node in the cluster

C. A full copy of the entire index

D. A cluster of nodes

Solution

1. Step 1: Define what a shard is

   A shard is a smaller piece or partition of an index that holds part of the data.

2. Step 2: Compare with other options

   It is not a full copy (that's a replica), not a node or cluster.

3. Final Answer:

   A part of an index that stores data -> Option A

4. Quick Check:

   Shard = index part [OK]

Hint: Shard means a data part of an index [OK]

Common Mistakes:

• Thinking shard is a full index copy
• Confusing shard with node
• Mixing shard with cluster

3. Consider an Elasticsearch cluster with 3 nodes and an index configured with 5 primary shards and 1 replica per shard. How many total shards will be in the cluster?

medium

A. 5 shards

B. 10 shards

C. 6 shards

D. 15 shards

Solution

1. Step 1: Calculate total shards per primary and replica

   Each primary shard has 1 replica, so total shards = primary shards + replicas = 5 + 5 = 10 shards.

2. Step 2: Understand shard distribution across nodes

   Since there are 3 nodes, shards are distributed but total shards remain 10. However, the question asks total shards in cluster, including replicas.

3. Step 3: Re-check options

   15 shards says 15 shards, which is incorrect. The correct total is 10 shards.

4. Final Answer:

   10 shards -> Option B

5. Quick Check:

   Total shards = primary + replica = 5 + 5 = 10 [OK]

Hint: Total shards = primary shards x (1 + replicas) [OK]

Common Mistakes:

• Counting only primary shards
• Adding nodes to shard count incorrectly
• Confusing replicas with nodes

4. You have an Elasticsearch cluster with 2 nodes and an index with 4 primary shards and 1 replica. You try to assign 2 replicas per shard. What is the problem with this configuration?

medium

A. Replicas must be zero for cluster to work

B. Primary shards must be equal to replica shards

C. Number of nodes must be equal to number of shards

D. You cannot have more replicas than nodes minus one

Solution

1. Step 1: Understand replica shard limits

   Replicas cannot exceed the number of nodes minus one because replicas cannot be on the same node as primary.

2. Step 2: Apply to given cluster

   With 2 nodes, max replicas per shard = 1 (2 - 1). Trying 2 replicas is invalid.

3. Final Answer:

   You cannot have more replicas than nodes minus one -> Option D

4. Quick Check:

   Replicas ≤ nodes - 1 [OK]

Hint: Replicas max = nodes minus one [OK]

Common Mistakes:

• Setting replicas more than nodes - 1
• Confusing primary and replica counts
• Assuming replicas can be zero always

5. You want to design an Elasticsearch cluster for high availability with 4 nodes. You plan to create an index with 6 primary shards and 2 replicas per shard. What is the best shard and replica configuration to avoid shard allocation errors?

hard

A. 6 primary shards and 3 replicas per shard, total 24 shards

B. 6 primary shards and 2 replicas per shard, total 18 shards

C. 6 primary shards and 1 replica per shard, total 12 shards

D. 4 primary shards and 2 replicas per shard, total 12 shards

Solution

1. Step 1: Check replica limit based on nodes

   With 4 nodes, max replicas per shard = nodes - 1 = 3. So 2 replicas is allowed.

2. Step 2: Consider shard allocation rules

   Each shard copy must be on a different node. 6 primary shards + 2 replicas = 18 shards total, but 18 shards on 4 nodes means some nodes hold multiple shards.

3. Step 3: Avoid allocation errors by balancing shards

   Too many shards per node can cause performance issues. 1 replica (total 12 shards) is safer and avoids errors.

4. Final Answer:

   6 primary shards and 1 replica per shard, total 12 shards -> Option C

5. Quick Check:

   Replicas ≤ nodes - 1 and balanced shards [OK]

Hint: Replicas max = nodes - 1; balance shards per node [OK]

Common Mistakes:

• Setting replicas more than nodes - 1
• Ignoring shard distribution limits
• Choosing fewer shards than needed