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For developers, who are building real-time data-driven applications, Redis is the preferred, fastest, and most feature-rich cache, data structure server, and document and vector query engine.

Apache Kafka is an open-source distributed event streaming platform used by thousands of companies for high-performance data pipelines, streaming analytics, data integration, and mission-critical applications.

Dragonfly is an in-memory data store built for modern application workloads.

NSQ is a realtime distributed messaging platform designed to operate at scale, handling billions of messages per day.

Open source RabbitMQ: core server and tier 1 (built-in) plugins

Whether you're preparing for an interview or you want to design a distributed/microservice oriented application, this list will definitely help you achieve that.

RQ (Redis Queue) is a simple Python library for queueing jobs and processing them in the background with workers. It is backed by Redis/Valkey and is designed to have a low barrier to entry while scaling incredibly well for large applications.

PicoMQ is durable, real-time streams over HTTP and Kafka, built on S3-compatible object storage.