Cassandra’s Indexing Limitations in Real-Time Analytics: A Deep Dive

Introduction

Cassandra, a highly-scalable NoSQL database, is renowned for its write-optimized architecture and resilience. With its capacity to handle vast volumes of data across distributed systems, many consider it a go-to choice for various applications. However, like all databases, it has its strengths and weaknesses. One such area where Cassandra may not shine as brightly is in the realm of real-time analytics, particularly when leveraging its indexing mechanisms. Let’s delve into why Cassandra’s indexes might not be the optimal choice for analytical queries.

Cassandra indexes, especially secondary indexes, come with several characteristics and limitations that make them inefficient for real-time analytics:

  1. Nature of Cassandra:
    • Cassandra is primarily designed as a write-optimized system, using structures like Log-Structured Merge Trees (LSM Trees). While this facilitates high-speed data ingestion, it might not be the most efficient for read-heavy operations, especially complex analytical queries.
  2. Secondary Indexes Overhead:
    • In Cassandra, when you create a secondary index, the index data isn’t stored separately like in traditional RDBMS. Instead, each node maintains its local secondary index. So, when a query uses a secondary index, it might need to contact multiple nodes to resolve the query, leading to higher latencies.
  3. Non-global Secondary Indexes:
    • Secondary indexes in Cassandra are local to each node. This means that when you query against a secondary index, many nodes might be contacted, even if only a few have the data you’re interested in. This is inefficient in terms of network communication and overall response time.
  4. High Cardinality Columns:
    • Secondary indexes are particularly inefficient for columns with high cardinality (columns that have a large number of unique values). This is because the index will have too many entries, leading to more extensive searches and potentially querying many nodes to satisfy a single query.
  5. Lack of Compound Indexes:
    • In traditional RDBMS, you can create compound indexes that include multiple columns, optimized for specific query patterns. In contrast, Cassandra does not support these out of the box, limiting the efficiency of querying multiple columns simultaneously.
  6. Write Amplification:
    • Every write in Cassandra that involves a column with a secondary index means an additional write to update the index. This amplifies the amount of write activity, which can be particularly problematic when ingesting data at high rates.
  7. Stale Indexes:
    • In a distributed system like Cassandra, there’s a possibility of nodes failing or becoming temporarily unreachable. During such scenarios, secondary indexes might not get updated in real-time or might become inconsistent, leading to incorrect query results.
  8. Limited Query Expressiveness:
    • Analytical queries often involve aggregations, joins, and subqueries. While Cassandra has made strides in its querying capabilities with CQL, it’s still not as expressive as SQL in traditional RDBMS, limiting its efficiency for real-time analytics.

To sum up, while Cassandra offers tremendous scalability and is excellent for specific use cases like time-series data or high-speed data ingestion, its indexing mechanisms are not optimized for complex analytical queries. For real-time analytics, especially at scale ClickHouse will be more suitable.

Conclusion

While Cassandra provides robust scalability and fault tolerance, its indexing capabilities are not cut out for real-time analytics. The inherent design of secondary indexes, coupled with the nuances of distributed data systems, introduces challenges in query efficiency and consistency. Businesses and data architects should be mindful of these limitations when considering Cassandra for analytics-heavy applications. In situations demanding intricate analytical queries, it might be wise to explore specialized analytical databases or use Cassandra in tandem with other complementary big data tools.

About Shiv Iyer 206 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.