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SimpleDBM Overview

Author: Dibyendu Majumdar
Contact: d dot majumdar at gmail dot com
Date: 27 July 2014
Version: 1.0.23
Copyright: Copyright by Dibyendu Majumdar, 2005-2016

Overview

Introduction

SimpleDBM is a transactional database engine, written in Java. It has a very small footprint and can be embedded in the address space of an application. It provides a simple Java application programming interface (API), which can be learned very quickly.

Features

SimpleDBM implements the following features:

  • Transactional - SimpleDBM fully supports ACID transactions. A STEAL and NO-FORCE buffer management strategy is used for transactions which is optimum for performance.
  • Multi-threaded - SimpleDBM is multi-threaded and supports concurrent reads and writes of data.
  • Write Ahead Log - SimpleDBM uses a write ahead log to ensure transaction recovery in the event of system crashes.
  • Lock based concurrency - SimpleDBM uses row-level shared, update and exclusive locks to manage concurrency.
  • Multiple Isolation Levels - SimpleDBM supports read committed, repeatable read, and serializable isolation levels.
  • B-Tree Indexes - SimpleDBM implements B-plus Tree indexes, that fully support concurrent reads, inserts and deletes. SimpleDBM B-Trees continually rebalance themselves, and do not suffer from fragmentation.
  • Tables - SimpleDBM supports tables, but for maximum flexibility, treats table rows as blobs of data. Table rows can have any internal structure as you like, and can span multiple disk pages. Standard table rows with multiple columns are supported via add-on modules.
  • Latches and Locks - SimpleDBM uses latches for internal consistency, and locks for concurrency. Latches are more efficient locking mechanisms that do not suffer from deadlocks.
  • Deadlock detection - SimpleDBM has support for deadlock detection. A background thread periodically checks the lock table for deadlocks and aborts transactions to resolve deadlocks.
  • Network API - From release 1.0.18 a network client server implementation is included that allows SimpleDBM servers to run standalone and remote clients to connect via TCP/IP. Only Java bindings available right now.

Non-Features

  • SimpleDBM is not an SQL engine.
  • There is no support for distributed transactions (XA).

Status

SimpleDBM is fully usable, and is available via Maven Central. If you discover a bug, please report it - I will do my best to fix any bugs. Enhancements are currently not being done as I have no time available.

Getting Started

See Getting Started for instructions on how to start using SimpleDBM in your application.

Architecture

The core database engine of SimpleDBM is the RSS (named in honor of the first IBM Relational Database prototype System-R Relational Storage System). The RSS provides the underlying storage structures for transactions, locking, b-trees etc. The functions of the RSS subsystem will be based upon the description of the System-R RSS component in [ASTRA-76].

The Relational Storage Interface (RSI) is an internal interface which handles access to single tuples of base relations. This interface and its supporting system, the Relational Storage System (RSS), is actually a complete storage subsystem in that it manages devices, space allocation, storage buffers, transaction consistency and locking, deadlock detection, backout, transaction recovery, and system recovery. Furthermore, it maintains indexes on selected fields of base relations, and pointer chains across relations.

The RSS API is however, somewhat low level for ordinary users. It is meant to be used by people interested in building their own Database Engines on top of RSS.

To provides users with a simplified API, three additional modules are available.

The first one is the SimpleDBM TypeSystem module, which adds support for typed data values and multi-attribute row objects.

The second module, the Database API. This module implements a high level Database API and uses the TypeSystem module on top of the RSS.

Finally there is the Network API that provides a Client / Server model.

Technology

SimpleDBM is written in Java and uses features available since version 5.0 of this language.

Third party libraries

SimpleDBM has no run-time dependency on external libraries as it uses only out of the box Java 1.6 functionality. For test cases there is a dependency on JUnit.

Using SimpleDBM

SimpleDBM is available in three levels of abstraction.

The Network API modules allow SimpleDBM to be deployed in a simple Client / server configuration.

The add-on modules SimpleDBM-Database and SimpleDBM Type-System provide a high level API wth support for data dictionary, and the ability to create tables with traditional row/column structure. Indexes can be associated with tables. Details of how to use this API can be found in the document SimpleDBM Database API.

The lower level RSS module works at the level of containers and arbitrary types. The document named RSS User Manual provides instructions on how to develop using the RSS. Note that this is for advanced users who want to implement their own type system and data dictionary.

Developing SimpleDBM

The instructions in this section are for those who wish to develop SimpleDBM.

Obtaining SimpleDBM

SimpleDBM source code can be obtained from the SimpleDBM Project site. Source code is maintained in a Mercurial repository, so you will need a Mercurial client on your computer.

The SimpleDBM SCM repository is organized as follows:

projects +--- simpledbm-rss          This is the core database engine - named RSS
         |                           after IBM's research prototype. The RSS offers
         |                           a low level API - most users will prefer to
         |                           use the higher level API offered by
         |                           simpledbm-database.
         |
         +--- simpledbm-common       This contains basic utilities that are
         |                           shared by all projects.
         |
         +--- simpledbm-typesystem   This contains a simple typesystem
         |                           that can be used with SimpleDBM.
         |
         +--- simpledbm-database     This contains a higher level DB
         |                           API that makes life easier for
         |                           users. It uses the typesystem
         |                           component.
         |
         +--- simpledbm-network-framework  implements an NIO server over TCP/IP.
         |
         +--- simpledbm-network-common     contains code that is common to client
         |                                 and server.
         |
         +--- simpledbm-network-server     contains the network server implementation.
         |
         +--- simpledbm-network-client     contains the network client implementation.
         |
         +--- simpledbm-samples      This contains some sample programs
         |                           that demonstrate how to use SimpleDBM.
         |
         +--- simpledbm-docs         Contains the documentation sources.

Build Instructions

Pre-requisites

SimpleDBM uses Maven for build management. You will need to obtain a copy of Maven 3. Install Maven and set up your PATH so that Maven can be executed by typing the following command.

mvn

SimpleDBM development is being done using Eclipse. You can use any IDE of your choice, but you may need to find ways of converting the maven projects to the format recognized by your IDE.

You will need a Git client in order to checkout the code for SimpleDBM.

SimpleDBM requires Java SE 1.6 or above.

Instructions for Eclipse

The following instructions are for the simpledbm-rss project. However, the same instructions apply for the other projects.

  1. Use the Mercurial command line tools to create a local clone of the SimpleDBM Repository:

    git clone https://github.com/dibyendumajumdar/simpledbm.git
    
  2. Import the SimpleDBM Maven projects into Eclipse. The parent pom file is in the build folder. This is a multi-module pom file and will generate sub projects below it.

Maven commands

You can also compile, test and do other operations using maven commands. The following maven commands are commonly used.

To run the test cases.

cd build
mvn test

To create the package and install it in the local repository.

mvn install

Please visit the SimpleDBM project Wiki pages for additional platform specific instructions.

Building releases

SimpleDBM releases are published to Maven Central. Please contribute your changes to SimpleDBM maintainer (admin@simpledbm.org) as releases can only be performed by the maintainer.

Coding and Design Principles

Modular design

SimpleDBM RSS is made up of several modules. Each module implements a particular component, and is contained in its own package.

Each module has a public API, which is specified via a set of Java interfaces. Classes are generally not used as part of the public API, though there are a few exceptional cases.

To make the modules reusable and as independent of each other as possible, the interface of a module is deliberately specified in general terms. Where possible, direct dependence between modules is avoided. The only permissible way for one module to interact with another is to go via the public interfaces of the other module. Modules are not allowed to depend upon implementation specifics of other modules.

A strict rule is that two modules cannot have cyclic dependency. Module dependencies are one-way only, higher level modules depend upon lower level modules. This is illustrated below.

images/component-model.png

SimpleDBM uses constructor based dependency injection to link modules. It is being designed in such a way that a third-party IoC (Inversion of Control) container may be used to manage the dependencies.

Java coding standards

Where possible, classes are made immutable. This helps in improving the robustness of the system. The serialization mechanism used by SimpleDBM is designed to work with immutable objects.

In the interest of concurrency, fine-grained locking is used as opposed to coarse-grained synchronization. This makes the code complex in some cases, as careful ordering of locks is required for deadlock avoidance. Also, the correctness of synchronization logic is of paramount importance.

Unchecked exceptions are used throughout. Due to the nature of unchecked exceptions, the code that throws the exception has the responsibility of logging an error message at the point where the exception is thrown. This ensures that even if the exception is not caught by the client, an error message will be logged to indicate the nature of the error.

All error messages are given unique error codes.

The code relies upon the efficiency of modern garbage collectors and does not attempt to manage memory. Rather than using object pools, SimpleDBM encourages the use of short-lived objects, on the basis that this aids the garbage collector in reclaiming space more quickly. The aim is to keep permanently occupied memory to a low level.

JUnit based test cases are being added constantly to improve the test coverage. Simple code coverage statistics are not a good indicator of the usefulness of test cases, due to the multi-threaded nature of most SimpleDBM components. Where possible, test cases are created to simulate specific thread interactions, covering common scenarios.

Particular attention is paid to cleaning up of resources. To ensure that resources are cleaned up during normal as well as exceptional circumstances, finally blocks are used.

Debug messages are used liberally - and are executed conditionally so that if debug is switched off, there is minimal impact on performance.

A special Trace module is used to capture runtime trace. This module is designed to be lock-free, and is very low overhead, so that trace can be collected with negligible overhead. This feature is still being implemented across modules; the intention is that when fatal errors occur, the last 5000 trace messages will be dumped to help debug the error condition.

Documentation

Most of the documentation for SimpleDBM is written in reStructuredText. HTML and PDF versions are generated from the source documents. There is a generous amount of comments in the source code as well.

Being an educational project, producing good documentation is high priority.

The design of most modules is based upon published research. References are provided in appropriate places, both in this document, and in the source code. This acts as another source of information.

Following documents are recommended as starting points:

For advanced stuff, read:

[ASTRA-76]M.M.Astrahan, M.W.Blasgen, D.D.Chamberlin, K.P.Eswaran, J.N.Gray, P.P.Griffiths, W.F.King, R.A.Lorie, P.R.McJones, J.W.Mehl, G.R.Putzolu, I.L.Traiger, B.W.Wade AND V.Watson. System R: Relational Approach to Database Management, ACM, Copyright 1976, ACM Transactions on Database Systems, Vol 1, No. 2, June 1976, Pages 97-137.