Is Apache Kafka a Database? With ksqlDB, Most Definitely

Apache Kafka can provide database-like capabilities, including durable event storage, replication, ordering, transactions, and potentially indefinite retention, but it is generally not a direct replacement for specialized systems such as relational databases, document stores, search engines, or data lakes. The choice between Kafka alone and Kafka plus another database depends on requirements for retention, data structure, query complexity, transaction guarantees, and lookup performance. A conventional customer-profile CRUD architecture may write events to Kafka while using an external database for low-latency reads and updates, but this duplicates data, complicates resiliency and source-of-truth management, and adds operational cost. Kafka topics cannot efficiently perform arbitrary key lookups because a key can occur at multiple offsets, yet ksqlDB addresses this limitation by materializing the latest keyed records in RocksDB-backed state stores and exposing them through materialized views. These state stores support fast key-based reads, optimized primary-key range scans, updates through new events, and deletions through null-valued tombstone records, while changelog topics enable state recovery. This Kafka-and-ksqlDB design can simplify certain CRUD workloads and reduce dependence on an external database, although it has limitations involving complex queries, storage availability and capacity, recovery requirements, and potential single-node failure risks.

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