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<th width="60%" align="center">Chapter 2. Transactional Application</th>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="simpleprogramlisting"></a>Program Listing</h2>
</div>
</div>
<div></div>
</div>
<p>
Our example program is a fairly simple transactional
application. At this early stage of its development, the
application contains no hint that it must be network-aware
so the only command line argument that it takes is one that
allows us to specify the environment home directory.
(Eventually, we will specify things like host names and
ports from the command line).
</p>
<p>
Note that the application performs all writes under the
protection of a transaction; however, multiple database
operations are not performed per transaction. Consequently,
we simplify things a bit by using autocommit for our
database writes.
</p>
<p>
Also, this application is single-threaded. It is possible
to write a multi-threaded or multi-process application that
performs replication. That said, the concepts described in
this book are applicable to both single threaded and
multi-threaded applications so nothing
is gained by multi-threading this application other than
distracting complexity. This manual
does, however, identify where care must be taken when
performing replication with a non-single threaded
application.
</p>
<p>
Finally, remember that transaction processing is not described in
this manual. Rather, see the
<i class="citetitle">Berkeley DB Getting Started with Transaction Processing</i> guide for details on
that topic.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="repconfiginfo_cxx"></a>
<span>Class: RepConfig</span>
</h3>
</div>
</div>
<div></div>
</div>
<p>
Before we begin, we present a
class that we will use to maintain useful
information for us. Under normal circumstances,
this class would not be necessary for a simple
transactional example such as this. However, this code will
grow into a replicated example that needs to
track a lot more information for the
application, and so we lay the groundwork for
it here.
</p>
<p>
The class that we create is called
<tt class="classname">RepConfig</tt>
and its only purpose at this time is to track
the location of our environment home directory.
</p>
<pre class="programlisting">package db.simpletxn;
public class RepConfig
{
// Constant values used in the RepQuote application.
public static final String progname = "SimpleTxn";
public static final int CACHESIZE = 10 * 1024 * 1024;
// member variables containing configuration information
public String home; // String specifying the home directory for
// rep files.
public RepConfig()
{
home = "TESTDIR";
}
public java.io.File getHome()
{
return new java.io.File(home);
}
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="simpletxnusage_java"></a>Class: SimpleTxn</h3>
</div>
</div>
<div></div>
</div>
<p>
Our transactional example will
consist of a class,
<tt class="classname">SimpleTxn</tt>, that performs
all our work for us.
</p>
<p>
First, we provide the package declaration and
then a few import statements that the class
needs.
</p>
<pre class="programlisting">package db.simpletxn;
import java.io.FileNotFoundException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import com.sleepycat.db.Cursor;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseConfig;
import com.sleepycat.db.DatabaseEntry;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.DatabaseType;
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
import db.simpletxn.RepConfig;
public class SimpleTxn
{
private RepConfig repConfig;
private Environment dbenv; </pre>
<p>
Next, we provide our class constructor. This simply initializes our
class data members.
</p>
<pre class="programlisting"> public SimpleTxn()
throws DatabaseException
{
repConfig = null;
dbenv = null;
} </pre>
<p>
And then we provide our <tt class="methodname">usage()</tt> method. At
this point, this method has very little to report:
</p>
<pre class="programlisting"> public static void usage()
{
System.err.println("usage: " + repConfig.progname);
System.err.println("[-h home]");
System.err.println("\t -h home directory\n");
System.exit(1);
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="simpletxnmain_java"></a>Method: SimpleTxn.main()</h3>
</div>
</div>
<div></div>
</div>
<p>
Having implemented our
<tt class="methodname">usage()</tt>
method, we can jump directly into our
<tt class="methodname">main()</tt>
method. This method begins by instantiating a
<tt class="classname">RepConfig</tt> object, and
then collecting the command line arguments so
that it can populate the object with the
appropriate data (just the environment home
directory, at this time):
</p>
<pre class="programlisting"> public static void main(String[] argv)
throws Exception
{
RepConfig config = new RepConfig();
// Extract the command line parameters
for (int i = 0; i < argv.length; i++)
{
if (argv[i].compareTo("-h") == 0) {
// home - a string arg.
i++;
config.home = argv[i];
} else {
System.err.println("Unrecognized option: " + argv[i]);
usage();
}
} </pre>
<p>
And then perform a little sanity checking on the command line
input:
</p>
<pre class="programlisting"> // Error check command line.
if (config.home.length() == 0)
usage(); </pre>
<p>
Now we perform the class' work. To begin, we initialize the
object. The <tt class="methodname">init()</tt> method actually
opens our environment for us (shown in the next section).
</p>
<pre class="programlisting"> SimpleTxn runner = null;
try {
runner = new SimpleTxn();
runner.init(config); </pre>
<p>
And then we call our <tt class="methodname">doloop()</tt>
method. This method is where we perform all our database
activity. See <a href="simpleprogramlisting.html#doloop_java">Method: SimpleTxn.doloop()</a>
for it's details.
</p>
<pre class="programlisting"> runner.doloop(); </pre>
<p>
And then, finally terminate the application (which closes our
environment handle) and end the method.
</p>
<pre class="programlisting"> runner.terminate();
} catch (DatabaseException dbe) {
System.err.println("Caught an exception during " +
"initialization or processing: " + dbe.toString());
if (runner != null)
runner.terminate();
}
System.exit(0);
} // end main </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="simpletxn_init_java"></a>Method: SimpleTxn.init()</h3>
</div>
</div>
<div></div>
</div>
<p>
The <tt class="methodname">SimpleTxn.init()</tt>
method is used to open our environment handle.
For readers familiar with writing transactional
DB applications, there should be no surprises
here. However, we will be adding to this in later
chapters as we roll replication into this example.
</p>
<p>
The only thing worth noting in this method here is that
we relax our transactional durability guarantee for this application.
We do this because the application will eventually be replicated and
so we don't need a high durability guarantee.
</p>
<pre class="programlisting"> public int init(RepConfig config)
throws DatabaseException
{
int ret = 0;
repConfig = config;
EnvironmentConfig envConfig = new EnvironmentConfig();
envConfig.setErrorStream(System.err);
envConfig.setErrorPrefix(RepConfig.progname);
envConfig.setCacheSize(RepConfig.CACHESIZE);
envConfig.setTxnNoSync(true);
envConfig.setAllowCreate(true);
envConfig.setRunRecovery(true);
envConfig.setInitializeLocking(true);
envConfig.setInitializeLogging(true);
envConfig.setInitializeCache(true);
envConfig.setTransactional(true);
try {
dbenv = new Environment(repConfig.getHome(), envConfig);
} catch(FileNotFoundException e) {
System.err.println("FileNotFound exception: " + e.toString());
System.err.println(
"Ensure that the environment directory is pre-created.");
ret = 1;
}
return ret;
} </pre>
<p>
Finally, we present the <tt class="methodname">SimpleTxn.terminate()</tt>
method here. All this does is close the environment handle. Again,
there should be no surprises here, but we provide the
implementation for the sake of completeness anyway.
</p>
<pre class="programlisting"> public void terminate()
throws DatabaseException
{
dbenv.close();
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="doloop_java"></a>Method: SimpleTxn.doloop()</h3>
</div>
</div>
<div></div>
</div>
<p>
We now implement our application's
primary data processing method. This
method provides a command prompt at which the
user can enter a stock ticker value and a price for
that value. This information is then entered to the
database.
</p>
<p>
To display the database, simply enter
<tt class="literal">return</tt> at the prompt.
</p>
<p>
To begin, we declare a database pointer:
</p>
<pre class="programlisting"> public int doloop()
throws DatabaseException, , UnsupportedEncodingException
{
Database db = null; </pre>
<p>
Next, we begin the loop and we immediately open our
database if it has not already been opened.
</p>
<pre class="programlisting"> for (;;)
{
if (db == null) {
DatabaseConfig dbconf = new DatabaseConfig();
// Set page size small so page allocation is cheap.
dbconf.setPageSize(512);
dbconf.setType(DatabaseType.BTREE);
dbconf.setAllowCreate(true);
dbconf.setTransactional(true);
try {
db = dbenv.openDatabase(null, // Txn handle
RepConfig.progname, // db filename
null, // db name
dbconf);
} catch (FileNotFoundException fnfe) {
System.err.println("File not found exception" + fnfe.toString());
// Get here only if the environment home directory
// somehow does not exist.
}
} </pre>
<p>
Now we implement our command prompt. This is a simple and not
very robust implementation of a command prompt.
If the user enters the keywords <tt class="literal">exit</tt>
or <tt class="literal">quit</tt>, the loop is exited and the
application ends. If the user enters nothing and instead simply
presses <tt class="literal">return</tt>, the entire contents of the
database is displayed. We use our
<tt class="methodname">printStocks()</tt> method to display the
database. (That implementation is shown next in this chapter.)
</p>
<p>
Notice that very little error checking is performed on the data
entered at this prompt. If the user fails to enter at least one
space in the value string, a simple help message is printed and
the prompt is returned to the user. That is the only error
checking performed here. In a real-world application,
at a minimum the application would probably check to ensure
that the price was in fact an integer or float value.
However, in order to keep this example code as simple as
possible, we refrain from implementing a thorough user interface.
</p>
<pre class="programlisting"> BufferedReader stdin =
new BufferedReader(new InputStreamReader(System.in));
// listen for input, and add it to the database.
System.out.print("QUOTESERVER> ");
System.out.flush();
String nextline = null;
try {
nextline = stdin.readLine();
} catch (IOException ioe) {
System.err.println("Unable to get data from stdin");
break;
}
String[] words = nextline.split("\\s");
// A blank line causes the DB to be dumped to stdout.
if (words.length == 0 ||
(words.length == 1 && words[0].length() == 0)) {
try {
printStocks(db);
} catch (DatabaseException e) {
System.err.println("Got db exception reading " +
"DB: " + e.toString());
break;
}
continue;
}
if (words.length == 1 &&
(words[0].compareToIgnoreCase("quit") == 0 ||
words[0].compareToIgnoreCase("exit") == 0)) {
break;
} else if (words.length != 2) {
System.err.println("Format: TICKER VALUE");
continue;
} </pre>
<p>
Now we assign data to the <tt class="classname">DatabaseEntry</tt>
classes that we will use to write the new information to the database.
</p>
<pre class="programlisting"> DatabaseEntry key =
new DatabaseEntry(words[0].getBytes("UTF-8"));
DatabaseEntry data =
new DatabaseEntry(words[1].getBytes("UTF-8")); </pre>
<p>
Having done that, we can write the new information to the
database. Remember that because a transaction handle is not
explicitly used, but we did open the database such that it
supports transactions, then autocommit is automatically
used for this database write.
</p>
<p>
Autocommit is described in the
<i class="citetitle">Berkeley DB Getting Started with Transaction Processing</i> guide.
</p>
<p>
Also, the database is not configured for duplicate records, so
the data portion of a record is overwritten if the provided
key already exists in the database. However, in this case
DB returns <tt class="methodname">OperationStatus.KEYEXIST</tt> — which
we ignore.
</p>
<pre class="programlisting"> db.put(null, key, data); </pre>
<p>
Finally, we close our database before returning from the
method.
</p>
<pre class="programlisting"> }
if (db != null)
db.close(true);
return 0;
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="printstocks_c"></a>
<span>Method: SimpleTxn.printStocks()</span>
</h3>
</div>
</div>
<div></div>
</div>
<p>
The
<tt class="methodname">printStocks()</tt>
<span>method</span>
simply takes a database handle, opens a cursor, and uses
it to display all the information it finds in a database.
This is trivial cursor operation that should hold
no surprises for you. We simply provide it here for
the sake of completeness.
</p>
<p>
If you are unfamiliar with basic cursor operations,
please see the <i class="citetitle">Getting Started with Berkeley DB</i>
guide.
</p>
<pre class="programlisting"> public void terminate()
throws DatabaseException
{
dbenv.close();
}
/*
* void return type since error conditions are propagated
* via exceptions.
*/
private void printStocks(Database db)
throws DatabaseException
{
Cursor dbc = db.openCursor(null, null);
System.out.println("\tSymbol\tPrice");
System.out.println("\t======\t=====");
DatabaseEntry key = new DatabaseEntry();
DatabaseEntry data = new DatabaseEntry();
OperationStatus ret;
for (ret = dbc.getFirst(key, data, LockMode.DEFAULT);
ret == OperationStatus.SUCCESS;
ret = dbc.getNext(key, data, LockMode.DEFAULT)) {
String keystr = new String
(key.getData(), key.getOffset(), key.getSize());
String datastr = new String
(data.getData(), data.getOffset(), data.getSize());
System.out.println("\t"+keystr+"\t"+datastr);
}
dbc.close();
}
} // end class </pre>
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