Rel

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Rel is a free, open-source, true relational database management system with an advanced query language called Tutorial D. Intended for educational purposes and written in Java, it can run on any OS with a Java VM. It uses Oracle’s Berkeley DB as a storage engine, and allows the user to interact with the database via either the Tutorial D language or a visual query language implemented in the Rel application. While it currently does not support the entirety of the Tutorial D language, it has features for a large subset of the language. Using the JDBC driver, it can be incorporated into any application that uses Java, as long as the application doesn’t specify SQL syntax.

History

CJ Date and Hugh Darwen’s The Third Manifesto outlines a set of programming language conventions and restrictions that lead to a truly relational database language that also incorporates benefits commonly associated with object-oriented languages. Tutorial D is a query language developed by Date and Darwen that meets those specifications, developed for education. Seeing the lack of complete open source Tutorial D language implementations, Dave Voorhis released the first version of the Rel database management system in 2004 in order to provide an implementation of Date & Darwen’s specification for instructional purposes.

Concurrency Control

Multi-version Concurrency Control (MVCC) Two-Phase Locking (Deadlock Prevention)

Concurrency control is handled by the storage engine, so the concurrency control protocols in Rel are those that are implemented by BerkeleyDB -- namely, MVCC and 2-phase locking.

System Architecture

Shared-Memory

Rel consists of two parts: a Rel database server and DBrowser, the client application. The role of DBrowser is to send Tutorial D expressions to the server, while the server itself does the actual query processing. Multiple clients can be connected to the same server, but the server itself is a single node.

Isolation Levels

Serializable Snapshot Isolation

Isolation levels are handled by the storage engine, so the isolation levels available to Rel are serializable (when BerkeleyDB uses pessimistic concurrency control) and snapshot isolation (when it uses MVCC).

Data Model

Relational

While the backend storage engine that Rel uses, Oracle’s BerkeleyDB, uses the Key/Value and Document/XML models, Rel’s data model is truly relational, as specified by Date & Darwen’s Tutorial D syntax.

Storage Model

Custom

Rel’s storage engine is a key-value store. Due to the fact that key-value pairs point to the memory location of an object as well as its size, there is no bound on the length of a data item.

Joins

Nested Loop Join Sort-Merge Join

Currently, Rel does not have any optimizations for joins beyond the algorithms used by the storage engine. Thus, in BerkeleyDB, the possible join algorithms are those supported by SQLite, and even in SQLite, there isn’t support for sort-merge join over non-unique keys.

Query Execution

Tuple-at-a-Time Model

Rel’s storage engine, BerkeleyDB, uses SQLite to execute queries. SQLite uses the Tuple-at-a-Time execution model, so BerkeleyDB inherits that model, and Rel inherits it from BerkeleyDB.

Foreign Keys

Supported

Foreign keys are not specifically implemented in Rel, since it isn’t part of the official Tutorial D syntax. However, foreign keys are a more specific instance of Rel’s constraint system, so you can implement them as a specific type of constraint.

Storage Architecture

Disk-oriented

Since Rel uses the disk-oriented BerkeleyDB as a storage engine, it is also disk-oriented.

Query Interface

Custom API

In Rel, users can write queries in two ways. First, they can directly send queries in Tutorial D, where the the language interpreter will parse and process the queries. Second, they can use the Rel application’s visual query language. Queries made with this will get transformed into Tutorial D and will then be fed into the language processor.

Revision #6 | Updated 12/04/2018 9:11 p.m.