Databases and Queues are crucial for application scalability. Often SQL query optimization leads to significant performance improvement, hence it is important to know how to work with the data. Common relational databases include PostgeSQL, MySQL or Oracle. Unstructured data usually requires NoSQL database knowledge, such as MongoDB, CouchDB and Riak. Advanced skill translates into partitioning, failover and sharding knowledge. Servers often communicate with each other and with clients via messages, so it is relevant to understand messaging queues, including RabbitMQ, ZeroMQ or Kafka.