Cassandra – What it is, How it works and What it’s used for

Before we jump into it, if you are trying to visualize your Cassandra data, take a look at our Cassandra Analytics page. You can also set up a call with a our team to see if Knowi is a good BI solution for your use case.

Introduction

Cassandra is a NoSQL database designed for handling large amounts of data across many commodity servers, providing high availability without sacrificing performance. Cassandra was initially developed at Facebook by Avinash Lakshman and Prashant Malik to power their Inbox Search feature. It was inspired by Amazon’s DynamoDB and Google’s Bigtable. Later, it was released as an open-source project under the Apache Foundation. While Cassandra is available as an open-source project, commercial support is offered by companies like DataStax, which provides additional features and support for Cassandra deployments.

Cassandra Query Language (CQL)

Cassandra utilizes the Cassandra Query Language (CQL), which supports SQL-like commands. This extends to SQL-based operations found in databases like MySQL and Oracle, where foundational SQL standards, such as SQL-92, serve as the basis for interactions. Operations like “SELECT *”, “INSERT INTO”, and other common SQL commands are supported in Cassandra, except with some minor differences. While there are distinctions in theoretical and architectural aspects between Cassandra and these other systems, the practical experience of using CQL for data manipulation and queries feel familiar, making it easier for developers to learn.

Architecture

Cassandra’s architecture is fundamentally designed to achieve scalability, fault tolerance, and high availability, making it an excellent choice for applications requiring distributed data across many nodes with no single point of failure. Here’s a breakdown of its core architectural components and how they contribute to its robustness.

Basic Terminology:

1. Nodes: Node is the basic component in Cassandra. It is the place where data is stored. For Example: As shown in the diagram, node which has IP address 11.0.0.5 contain data (keyspace which contain one or more tables). 

2. Data center: Data Center is a collection of nodes. 

3. Cluster: It is the collection of many data centers. 

Decentralized, Peer-to-Peer Model

Unlike traditional databases that use a master-slave architecture, Cassandra operates on a peer-to-peer model. This setup means that all nodes in a Cassandra cluster are identical, with no master nodes. Each node communicates with the other nodes directly, which ensures there are no bottlenecks or single points of failure.

Data Distribution and Replication

• Partitioning: Cassandra distributes data across the cluster using partitioning. It hashes the partition key of a row with a consistent hashing algorithm to determine which node will store that row. Each node is responsible for a range of data determined by its position on the hash ring.
• Replication: To ensure data availability and fault tolerance, Cassandra replicates partitions across multiple nodes. The replication factor, which can be configured per keyspace, defines how many copies of the data exist across the cluster. This replication strategy ensures that even in the event of node failures, the data is still accessible from replica nodes.

Consistency Levels: Tunable Consistency

Cassandra allows users to choose the consistency level for their read and write operations, balancing between consistency and availability. Higher consistency levels ensure that more nodes agree on the data’s current state but might reduce availability in case of node failures. Lower consistency levels increase availability but with a risk of reading outdated data.

Data Storage Mechanism

• Commit Log: Every write operation in Cassandra is first written to a commit log, a durable write-ahead log on disk. This mechanism ensures data durability and provides a recovery point in case of a crash.
• Memtable: After writing to the commit log, data is stored in a memtable, an in-memory data structure. Once the memtable reaches a certain size or after a specific time, it is flushed to disk.
• SSTables: When data from a memtable is flushed to disk, it is stored in an SSTable (Sorted String Table), an immutable data file. Cassandra merges and compacts SSTables periodically to optimize storage and query efficiency.

Read and Write Paths

• Writes: Cassandra’s write path is designed for high performance. Writes are first logged in the commit log for durability and then written to the memtable. This process ensures rapid write operations with minimal latency.
• Reads: Reading data in Cassandra involves checking both the memtable and SSTables. To optimize read performance, Cassandra uses bloom filters to quickly determine if an SSTable contains the requested data, minimizing unnecessary disk reads.

Gossip Protocol

• Node Discovery and Communication: Cassandra uses the Gossip protocol for inter-node communication. This protocol ensures nodes within the cluster exchange information about themselves and other nodes, maintaining a consistent and updated view of the cluster’s state. Gossip allows Cassandra to monitor the health of nodes and manage the cluster’s topology dynamically.

Cassandra’s architecture, characterized by its decentralized model, efficient data distribution, replication strategies, and tunable consistency levels, is tailored to provide a highly available, scalable, and fault-tolerant distributed database system. This architecture makes Cassandra an ideal choice for applications that require reliable performance across large-scale, distributed environments.

Use Cases

Cassandra is particularly well-suited for applications that require high availability, scalable performance, and can tolerate eventual consistency. Common use cases include:

• High-Throughput Applications: Its ability to handle large volumes of writes makes it ideal for logging, event streaming, and real-time analytics.
• Internet of Things (IoT): Perfect for storing data from sensors and devices due to its write efficiency and scalability.
• Web Activity Tracking: Capable of managing vast amounts of user interaction data in real-time.
• Time-Series Data: Efficiently stores and retrieves time-stamped data for metrics, monitoring, and analytics.

Advantages

• Scalability: Easily scales horizontally, allowing more nodes to be added without downtime.
• Performance: Exceptional at handling write-heavy workloads due to its efficient write path.
• Fault Tolerance: Designed to handle failures gracefully, ensuring data is always accessible.
• Flexibility: Supports various data formats and structures, accommodating a wide range of applications.

Disadvantages

• Complexity in Data Modeling: Requires careful planning of data models to ensure efficient queries.
• Consistency Trade-Off: While consistency can be tuned, achieving strong consistency across all operations can be challenging.
• Operational Complexity: Managing and tuning a Cassandra cluster for optimal performance requires expertise.

Cassandra’s architecture, designed for distributed, scalable, and high-performance workloads, makes it a prime choice for modern applications dealing with large datasets and requiring high availability. By understanding its core principles, advantages, and limitations, developers can leverage Cassandra to build robust, scalable applications capable of handling the demands of today’s data-intensive environments.