Polyhedra DBMS

Polyhedra is a family of relational database management systems offered by ENEA AB, a Swedish company. The original version of Polyhedra (now referred to as Polyhedra IMDB) was an in-memory database management system which could be used in high availability configurations; in 2006 Polyhedra Flash DBMS was introduced to allow databases to be stored in flash memory. All versions employ the client–server model to ensure the data are protected from misbehaving application software, and they use the same SQL, ODBC and type-4 JDBC interfaces. Polyhedra is targeted primarily for embedded use by Original Equipment Manufacturers (OEMs), and big-name customers include Ericsson, ABB, Emerson, Lockheed Martin, United Utilities and Siemens AG.

Company

Polyhedra development was started in 1991 by Perihelion Technology Ltd, a subsidiary of Perihelion Software Ltd (PSL); initially, the project had a working title the "Perihelion Application Toolkit", but was soon renamed Polyhedra (using a left-over trademark from another PSL project). There was a management buyout of PTL in 1994, and the company name changed to Polyhedra plc to match the name of the product. Polyhedra plc was in turn acquired by Enea AB in 2001. All development and support is still done in the English town of Shepton Mallet, where PSL was based.

Features

Tim King, the founder of Perihelion Software Ltd, developed a relational DBMS for historical data as part of his PhD work; Dave Stoneham, who set up PTL, had previously developed a SCADA system. Building on these experiences, Polyhedra was originally developed "to bring the benefits of relational technology to the embedded market". To this end, it had to be small footprint, very fast... and it had to avoid the need for polling, which is a performance killer. Consequently, it was designed from the start to:
* keep the working copy of the data in-memory (though there is now a variant that keeps the data in a flash-based file);
* use a client–server architecture to protect the data from corruption by rogue application code;
* have an 'active query' mechanism to update client applications when relevant database changes occur;
* have a very simple processing model where a transaction is either a schema change, a query, or a request for a set of inserts, updates and/or deletes - such alterations can either be expressed via SQL statements or by updating through the active queries with (in conjunction with active queries) an optimistic concurrency mechanism to handle clashing updates;
* have a table inheritance mechanism which, when combined with Database triggers (via the CL language, see below), allows the database designer to program the database in an object-oriented fashion. Table inheritance also avoids or reduces the need for supplementary tables whose primary key is a foreign key to another table, and thus can simplify many queries and updates.
* have a Historian module to allow large volumes of times-series data to be captured, stored, archived and queried in an efficient fashion.

Polyhedra IMDB achieves data persistence through the use of snapshots and journal logging; Polyhedra Flash DBMS uses shadow paging, with 2 levels of recursion. In addition, Polyhedra can be used in hot-standby configurations for improved availability. The transactional model used by all Polyhedra products ensures atomicity, consistency and isolation (as defined by the ACID properties); durability is enforced in Polyhedra Flash DBMS, while in Polyhedra IMDB clients can choose the durability model when they issue their transactions.

"The Polyhedra DBMS system is fundamentally different compared to other relational systems, because of its active behaviour. This is achieved through two mechanisms, active queries and by the control language (CL). An active query looks quite like a normal query where some data is retrieved and/or written, but instead the query stays in the database until explicitly aborted. When a change in the data occurs that would alter the result of the query, the application is notified. The CL, which is a fully object-oriented script language that supports encapsulation, information hiding and inheritance, can determine the behaviour of data in the database. This means that methods, private or public, can be associated with data performing operations on them without involving the application."

Polyhedra is not a general-purpose DBMS, as the restricted transactional model does not meet all needs, and its fault-tolerance model is based on the hot-standby approach (to minimise hardware costs) rather than clustering (which is better for load-sharing). However, its limitations are benefits in embedded use, where the emphasis in a deployed application is on performance and cost rather than handling continually varying usage patterns.

Most of the Polyhedra products are made available for purchase under a proprietary