Universe

A YugabyteDB universe, is a group of nodes (VMs, physical machines or containers) that collectively function as a highly available and resilient database.

The universe can be deployed in a variety of configurations depending on business requirements, and latency considerations. Some examples:

• Single availability zone (AZ/rack/failure domain)
• Multiple AZs in a region
• Multiple regions (with synchronous and asynchronous replication choices)

Organization of user data

A YugabyteDB universe can consist of one or more namespaces. Each of these namespaces can contain one or more user tables.

Yugabyte automatically shards, replicates and load-balances these tables across the nodes in the universe, while respecting user-intent such as cross-AZ or region placement requirements, desired replication factor, and so on. Yugabyte automatically handles failures (e.g., node, process, AZ or region failures), and re-distributes and re-replicates data back to desired levels across the remaining available nodes while still respecting any data placement requirements.

YSQL

Namespaces in YSQL are referred to as databases and are logically the same as in other RDBMS databases (such as PostgreSQL).

YCQL

A namespace in YCQL is referred to as a keyspace and is logically the same as a keyspace in Apache Cassandra's CQL.

Processes and services

A universe comprises of two sets of processes, YB-TServer and YB-Master. The YB-TServer and YB-Master processes form two respective distributed services using Raft  as a building block. High Availability (HA) of both these services is achieved by the failure-detection, leader election and data replication mechanisms in the Raft implementation.

These serve different purposes as described below.

YB-TServer process

The YB-TServer (aka the YugabyteDB Tablet Server) processes are responsible for hosting/serving user data (e.g, tables). They deal with all the user queries.

You can read more about YB-TServers.

YB-Master process

The YB-Master (aka the YugabyteDB Master Server) processes are responsible for keeping system metadata, coordinating system-wide operations such as create/alter/drop tables, and initiating maintenance operations such as load-balancing.

You can read more about YB-TServers.

Below is an illustration of a simple 4-node Yugabyte universe:

Universe vs cluster

A YugabyteDB universe can comprise of one or more clusters. Each cluster is a logical group of nodes running YB-TServers that are either performing one of the following replication modes:

• Synchronous replication
• Asynchronous replication

The set of nodes that are performing strong replication are referred to as the Primary cluster and other groups are called Read Replica clusters.

Note that:

• There is always one primary cluster in a universe
• There can be zero or more read replica clusters in that universe.

You can find more information about read replicas here.