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 2.10, which was
uploaded on January 16, 2013. This release provides the following
fixes to Version 2.10:
* The files simpledb.Startup and remote.SimpleDriver have been changed
to use a server-specific registry, instead of forcing the user to
run rmiregistry as a separate process.
* The files ConnectionAdapter, DriverAdapter, StatementAdapter, and
ResultSetAdapter in simpledb.remote have been changed to handle
the new Java 7 JDBC methods.
* A bug was fixed in the file SortScan.java.
* The new client file StudentMajorNoServer was added.
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 javadoc, which contains the JavaDoc documentation
of the above code.
* The folder studentClient, which contains some client-side code
for an example database.
* This document.
The author welcomes all comments, including bug reports, suggestions
for improvement, and anectodal experiences. His email address is
sciore@bc.edu
II. Installation Instructions:
1) Install the Java SDK, level 1.5 or higher.
2) If you do install Java 1.5, you need to make some minor changes
to the package simpledb.remote:
* The classes named xxxAdapter provide default implementations of
the interfaces in java.sql. Java 1.6 added several extra methods
to these interfaces. If you are using Java 1.5, just comment out
those methods. (You can tell which ones they are because you'll
get an error when you try to compile them.)
* The classes named SimpleXXX call the SQLException constructor
with a Throwable argument. This constructor is new to version
1.6. To use in 1.5, rewrite the code "throw new SQLException(e)"
to be "throw new SQLException(e.getMessage())".
3) Decide where you want the server-side software to go. Let's assume
that the code will go in the folder C:\javalib in Windows, or the
folder ~/javalib in UNIX or MacOS.
4) Add that folder to your classpath. In other words, the javalib
folder must be mentioned in your CLASSPATH environment variable.
* In UNIX, your home directory has an initialization file,
typically called .bashrc. If the file does not set CLASSPATH,
add the following line to the file:
CLASSPATH =.:~/javalib
Here, the ‘:’ character separates folder names. The command
therefore says that the folder "." (i.e., your current diretory)
and "~/javalib" are to be searched whenever Java needs to find a
class. If the file already contains a CLASSPATH setting, modify
it to include the javalib directory.
* In Windows, you must set the CLASSPATH variable via the System
control panel. From that control panel, choose the advanced tab
and click on the environment variables button. You want to have
a user variable named CLASSPATH that looks like this:
.;C:\javalib
Here, the ‘;’ character separates the two folder names.
5) Copy the simpledb folder from the distribution file to that
folder. Within the simpledb folder should be subfolders
containing all of the code for SimpleDB.
III. Running the Server:
SimpleDB has a client-server architecture. 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.
You must pass in the name of a folder that SimpleDB will use to hold the
database. For example in Windows, if you execute the command:
> start java simpledb.server.Startup studentdb
then the server will run in a new window, using studentdb as the
database folder. You can execute this command from any directory;
the server will always use the studentdb folder that exists in your
home directory. If a folder with that name does not exist, then
one will be created automatically.
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, and requires that an RMI registry
be running on port 1099. If a registry is running when the server is
started, it will use that registry; otherwise, it will run the registry
itself.
IV. Running Client Programs
The SimpleDB server accepts connections from any JDBC client. The client
program makes its connection via the following code:
Driver d = new SimpleDriver();
String host = "mymachine.com"; //any DNS name or IP address
String url = "jdbc:simpledb://" + host;
Connection conn = d.connect(url, null);
Note that SimpleDB does not require a username and password, although
it is easy enough to modify the server code to do so.
The driver class SimpleDriver is contained in the package
simpledb.remote, along with the other classes that it needs. A client
program will not run unless this package in its classpath. Note that
you could install the entire SimpleDB server code on a client machine,
but that is overkill. All you need is simpledb.remote.
The studentClient folder contains client code for a simple university
student-course database. The folder contains two subfolders, named
simpledb and derby. The simpledb subfolder contains programs that
run with the SimpleDB database server. The derby subfolder is not
relevant here. (It contains programs for the Derby database server,
which can be downloaded from db.apache.org. That code is used to
illustrate some examples from my text "Database Design and
Implementation", published by John WIley.)
The following list briefly describes the 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 requires a command-line argument denoting the name of
a department. The program then prints the name and graduation
year of all students having that major.
* SQLInterpreter 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.
Unlike the server classes, the client classes are not part of an
explicit package, and thus they need to be run from the directory that
they are stored in. For example, suppose we copy the studentClient
folder from the distribution file to our home directory. In Windows
we could execute the client programs as follows:
> cd C:\studentClient\simpledb
> java CreateStudentDB
V. Running SimpleDB as a Standalone Program
It is possible to write a program that calls the SimpleDB source code
directly, instead of calling server.Startup. The demo program
StudentMajorNoServer is an example. In this case, the entire database
source code is available to the program. Such programs are very useful
for testing changes to the source code without having to run the server
and a client.
VI. 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 plan class, used by the planner, and a scan class,
used at runtime.)
* parse (Implements the parser.)
* planner (Implements a naive planner for SQL statements.)
* remote (Implements the server using RMI.)
* 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 URL www.wiley.com/college/sciore