In DWS, instances are a group of database processes running in the memory. An instance can manage one or more databases that form a cluster. A cluster is an area in the storage disk. This area is initialized during installation and composed of a directory. The directory, called data directory, stores all data and is created by initdb. Theoretically, one server can start multiple instances on different ports, but DWS manages only one instance at a time. The start and stop of an instance rely on the specific data directory. For compatibility purposes, the concept of instance name may be introduced.
Databases manage various data objects and are isolated from each other. While creating a database, you can specify a tablespace. If you do not specify it, the object will be saved to the PG_DEFAULT by default. Objects managed by a database can be distributed to multiple tablespaces.
DWS schemas logically separate databases. All database objects are created under certain schemas. In DWS, schemas and users are loosely bound. When you create a user, a schema with the same name as the user will be created automatically. You can also create a schema or specify another schema.
DWS uses users and roles to control the access to databases. A role can be a database user or a group of database users, depending on role settings. In DWS, the difference between roles and users is that a role does not have the LOGIN permission by default. In DWS, one user can have only one role, but you can put a user's role under a parent role to grant multiple permissions to the user.
In DWS, transactions are managed by multi-version concurrency control (MVCC) and two-phase locking (2PL). It enables smooth data reads and writes. In DWS, MVCC saves historical version data together with the current tuple version. DWS uses the VACUUM process instead of rollback segments to routinely delete historical version data. Unless in performance optimization, you do not need to pay attention to the VACUUM process. Transactions are automatically submitted in DWS.