Architecture

Virtuoso is designed to take advantage of operating system threading support and multiple CPUs. It consists of a single process with an adjustable pool of threads shared between clients. Multiple threads may work on a single index tree with minimal interference with each other. One cache of database pages is shared among all threads and old dirty pages are written back to disk as a background process.

The database has at all times a clean checkpoint state and a delta of committed or uncommitted changes to this checkpointed state. This makes it possible to do a clean backup of the checkpoint state while transactions proceed on the commit state.

A transaction log file records all transactions since the last checkpoint. Transaction log files may be preserved and archived for an indefinite time, providing a full, recoverable history of the database.

A single set of files is used for storing all tables. A separate set of files is used for all temporary data. The maximum size of a file set is 32 terabytes, for 4G × 8K pages.

Locking

Virtuoso provides dynamic locking, starting with row level locks and escalating to page level locks when a cursor holds a large percentage of a page's rows or when it has a history of locking entire pages. Lock escalation only happens when no other transactions hold locks on the same page, hence it never deadlocks. Virtuoso SQL provides means for exclusive read and for setting transaction isolation.

Transactions

All four levels of isolation are supported: Dirty read, read committed, repeatable read and serializable. The level of isolation may be specified operation by operation within a single transaction. Virtuoso can also act as a resource manager and/or transaction coordinator under Microsoft's Distributed Transaction Coordinator (MS DTC) or the XA standard.

Data integrity

Virtuoso ORDBMS database supports entity integrity and referential integrity. Virtuoso ensures that relationships between records in related tables are valid by enforcing referential integrity. Integrity constraints include:

• NOT NULL – Within the definition of a table, Virtuoso allows data to contain a NULL value. This NULL value is not really a value at all and is considered an absence of value. The constraint of NOT NULL forces a value to be given to a column.
• Unique key – Uniqueness for a column or set of columns means that the values in that column or set of columns must be different from all other columns or set of columns in that table. A unique key may contain NULL values since they are by definition a unique non-valued value.
• Primary key – Primary key are much like unique keys except that they are designed to uniquely identify a row in a table. They can consist of a single column or multiple columns. The primary key cannot contain a NULL value.
• CHECK Constraint – Virtuoso provides on a column an integrity constraint that requires certain conditions to be met before the data is inserted or modified. If the checks are not satisfied then the transaction cannot be completed.

Data dictionary

Virtuoso stores all its information about all user objects in the database in the system catalog tables designated by db.dba*.

Components

Virtuoso is made up of client and server components. These components typically communicate with a local or remote Virtuoso server, which include:

• Virtuoso Drivers for ODBC, JDBC, ADO.NET and OLE DB
• Conductor, a web-based database administration user interface
• ISQL (Interactive SQL) and ISQO Utilities
• Documentation and Tutorials
• Samples

Installations come with two databases: a default and a demo database.