forchid / simpledb

SimpleDB is a multi-user transactional database server written in Java, which interacts with Java client programs via JDBC. The system is intended for pedagogical use only. The code is clean and compact. The APIs are straightforward.

THE SIMPLEDB DATABASE SYSTEM

General Information and Instructions

This document contains the following sections:

• Release Notes
• Server Installation
• Running the Server
• Running Client Programs
• SimpleDB Limitations
• The Organization of the Server Code

I. Release Notes:

This release of the SimpleDB system is Version 3.2, which was uploaded on August 25, 2019. This release is a thorough rewrite of the version 2 code.

SimpleDB is distributed in a WinZip-formatted file. This file contains four items:

* The folder simpledb, which contains the server-side Java code.
* The folder simpleclient, which contains some client-side code 
  for an example database.
* The folder derbyclient, which contains similar client-side code 
  For the Derby database, but with added features not supported
  by SimpleDB.
* This document.

The author welcomes all comments, including bug reports, suggestions for improvement, and anectodal experiences. His email address is [e-mail]

II. Installation Instructions:

1. Install the Java SDK(1.8+).

2. To install the SimpleDB engine, you must add the simpledb folder to your classpath. To do so using Eclipse, first create a new project; call it ÒSimpleDB EngineÓ. Then from the operating system, copy the simpledb folder to the src folder of the project. Finally, refresh the project from Eclipse, using the refresh command in the File menu.

3. The simpleclient folder contains example programs that call the SimpleDB engine. You should create a new project for them; call it ÒSimpleDB ClientsÓ. To ensure that the example programs can find the SimpleDB engine code, you should add the SimpleDB Engine project to the build path of SimpleDB Clients. Then use the operating system to copy the contents of simpleclient into the src directory of SimpleDB Clients.

4. The derbyclient folder contains example programs that call the Derby engine. This code illustrates features of JDBC that SimpleDB does not support, and is used in Chapter 2 of my "Database Design and Implementation" text.

III. Running the SimpleDB Server:

You run the server code on a host machine, where it will sit and wait for connections from clients. It is able to handle multiple simultaneous requests from clients, each on possibly different machines. You can then run a client program from any machine that is able to connect to the host machine.

To run the SimpleDB server, run Java on the simpledb.server.Startup class.
The argument to the class is the name of a folder that SimpleDB will use to hold the database. If you leave out the argument, it will use "studentdb". If a folder with that name does not exist, then one will be created automatically in the current directory.

If everything is working correctly, when you run the server with a new database folder the following will be printed in the server's window:

  creating new database
  new transaction: 1
  transaction 1 committed
  database server ready

If you run the server with an existing database folder, the following will be printed instead:

  recovering existing database
  database server ready

In either case, the server will then sit awaiting connections from clients. As connections arrive, the server will print additional messages in its window.

The server is implemented using RMI, on port 1099. If a registry is running when the server is started, it will use that registry; otherwise, it will create and run the registry itself.

IV. Running Client Code:

SimpleDB clients can be run in embedded mode or network mode. To run a client in embedded mode, use the EmbeddedDriver JDBC class with the connection string "jdbc:simpledb:xyz", where xyz is the name of the database. The database will be created if it does not exist, in the current directory of the client program. No server is necessary

To run a client in network mode, use the NetworkDriver class with the connection string "jdbc:simpledb://xyz", where xyz is the name or IP address of the machine running the SimpleDB server. Note that you cannot specify a database, because the client must use the database bound to the server.

SimpleDB does not require a username and password, although it is easy enough to modify the server code to do so.

The following list briefly describes the provided SimpleDB clients.

* CreateStudentDB creates and populates the student database used
  by the other clients.  It therefore must be the first client run 
  on a new database. 
* StudentMajors prints a table listing the names of students and 
  their majors.
* FindMajors asks the user for the name of a department. It then
  prints the name and graduation year of students having that major.
* SimpleIJ repeatedly prints a prompt asking you to enter a 
  single line of text containing an SQL statement.  The program then 
  executes that statement.  If the statement is a query, the output 
  table is displayed.  If the statement is an update command, then
  the number of affected records is printed.  If the statement is ill
  formed, and error message will be printed.  SimpleDB understands 
  only a limited subset of SQL, which is described below.
* ChangeMajor changes the student named Amy to be a drama major.  
  It is the only client that updates the database (although you can 
  use SQLInterpreter to run update commands).

These clients connect to the server at "localhost". If the client is
to be run from a different machine than the server, then its source code must be modified so that localhost is replaced by the domain name (or IP address) of the server machine.

V. SimpleDB Limitations

SimpleDB is a teaching tool. It deliberately implements a tiny subset of SQL and JDBC, and (for simplicity) imposes restrictions not present in the SQL standard. Here we briefly indicate these restrictions.

SimpleDB SQL

A query in SimpleDB consists only of select-from-where clauses in which the select clause contains a list of fieldnames (without the AS keyword), and the from clause contains a list of tablenames (without range variables).

The where clause is optional. The only Boolean operator is and. The only comparison operator is equality. Unlike standard SQL, there are no other comparison operators, no other Boolean operators, no arithmetic operators or built-in functions, and no parentheses. Consequently, nested queries, aggregation, and computed values are not supported.

Views can be created, but a view definition can be at most 100 characters.

Because there are no range variables and no renaming, all field names in a query must be disjoint. And because there are no group by or order by clauses, grouping and sorting are not supported. Other restrictions:

* The "*" abbreviation in the select clause is not supported.
* There are no null values.
* There are no explicit joins or outer joins in the from clause.
* The union and except keywords are not supported.
* Insert statements take explicit values only, not queries.
* Update statements can have only one assignment in the set clause.

SimpleDB JDBC

SimpleDB implements only the following JDBC methods:

Driver public Connection connect(String url, Properties prop); // The method ignores the contents of variable prop.

Connection

  public Statement createStatement();
  public void      close();

Statement

  public ResultSet executeQuery(String qry);
  public int       executeUpdate(String cmd);

ResultSet

  public boolean   next();
  public int       getInt();
  public String    getString();
  public void      close();
  public ResultSetMetaData getMetaData();

ResultSetMetaData

  public int        getColumnCount();
  public String     getColumnName(int column);
  public int        getColumnType(int column);
  public int getColumnDisplaySize(int column);

VII. The Organization of the Server Code

SimpleDB is usable without knowing anything about what the code looks like. However, the entire point of the system is to make the code easy to read and modify. The basic packages in SimpleDB are structured hierarchically, in the following order:

* file (Manages OS files as a virtual disk.)
* log (Manages the log.)
* buffer (Manages a buffer pool of pages in memory that acts as a
          cache of disk blocks.)
* tx (Implements transactions at the page level.  Does locking
      and logging.)
* record (Implements fixed-length records inside of pages.)
* metadata (Maintains metadata in the system catalog.)
* query (Implements relational algebra operations. Each operation 
         has a scan class, which can be composed to create a query tree.)
* parse (Implements the parser.)
* plan (Implements a naive planner for SQL statements.)
* jdbc (Implements the JDBC embedded and network interfaces.)
* server (The place where the startup and initialization code live. 
          The class Startup contains the main method.)

The basic server is exceptionally inefficient. The following packages enable more efficient query processing:

* index (Implements static hash and btree indexes, as well as 
         extensions to the parser and planner to take advantage
         of them.)
* materialize (Implements implementations of the relational 
               operators materialize, sort, groupby, and mergejoin.)
* multibuffer (Implements modifications to the sort and product 
               operators, in order to make optimum use of available
               buffers.)
* opt (Implements a heuristic query optimizer)

The textbook "Database Design and Implementation" describes these packages in considerably more detail. For further information, go to the Springer/Verlag web site.