From my understanding (don't have much backend experience) you need those only for specific workloads. First learn difference between OLTP and OLAP. Traditional

From my understanding (don't have much backend experience) you need those only for specific workloads. First learn difference between OLTP and OLAP. Traditional DBs are usually designed for OLTP, and new DBs are designed for OLAP and some for mega scale (Petabytes). I recommend you learn: * ES - for text search * Clickhouse - simplest OLAP * Cassandra (Petabytes of data, columnar store) * Learn some about timeseries DBs (analytics) * Graph DBs RabittMQ, Kafka or Pulsar are used for message bus/que implementations. Simple case: producing message takes 1 time unit but processing 5 units, so you want to implement kind of threading without coupling to specific hosts, so you use queue and subscribe to quue with readers. Read ZeroMQ docs on all communication patterns to learn typical cases.