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<div class="sect1" lang="en" xml:lang="en">
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<h2 class="title" style="clear: both"><a id="exampledoloop"></a>Example Processing Loop</h2>
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<p>
In this section we take the example
processing loop that we presented in the
previous section and we flesh it out to
provide a more complete example. We do this
by updating the
<tt class="function">doloop()</tt>
function that our original transaction
application used
<span>(see <a href="simpleprogramlisting.html#doloop_java">Method: SimpleTxn.doloop()</a>)</span>
to fully support our replicated application.
</p>
<p>
In the following example code, code that we
add to the original example is presented in
<b class="userinput"><tt>bold</tt></b>.
</p>
<p>
To begin, we must implement a way to track whether
our application is running as a master or a client.
There are many ways to do this, but in this case
what we will do is extend
<tt class="classname">com.sleepycat.db.Environment</tt>
to carry the information. We do this by creating
the <tt class="classname">RepQuoteEnvironment</tt>
class.
</p>
<pre class="programlisting">
<b class="userinput">
<tt>package db.repquote;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.Environment;
import com.sleepycat.db.EnvironmentConfig;
public class RepQuoteEnvironment extends Environment
{
private boolean isMaster;
public RepQuoteEnvironment(final java.io.File host,
EnvironmentConfig config)
throws DatabaseException, java.io.FileNotFoundException
{
super(host, config);
isMaster = false;
}
boolean getIsMaster()
{
return isMaster;
}
public void setIsMaster(boolean isMaster)
{
this.isMaster = isMaster;
}
} </tt>
</b>
</pre>
<p>
Next, we go to <tt class="filename">RepQuoteExample.java</tt> and
we include the
<tt class="classname">RepQuoteEnvironment</tt> class
as well as the
<tt class="classname">EventHandler</tt> class. We then
cause our <tt class="classname">RepQuoteExample</tt>
class to implement
<tt class="classname">EventHandler</tt>. We also
change our environment handle to be an instance
of <tt class="classname">RepQuoteEnvironment</tt>
instead of <tt class="classname">Environment</tt>.
</p>
<p>
Note that we also import the
<tt class="classname">com.sleepycat.db.ReplicationHandleDeadException</tt>
class. We will discuss what that exception is
used for a little later in this example.
</p>
<pre class="programlisting">package db.repquote;
import java.io.FileNotFoundException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.lang.Thread;
import java.lang.InterruptedException;
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.EnvironmentConfig;
<b class="userinput"><tt>import com.sleepycat.db.EventHandler;</tt></b>
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
<b class="userinput"><tt>import com.sleepycat.db.ReplicationHandleDeadException;</tt></b>
import db.repquote.RepConfig;
<b class="userinput"><tt>import db.repquote.RepQuoteEnvironment</tt></b>
public class RepQuoteExample <b class="userinput"><tt> implements EventHandler</tt></b>
{
private RepConfig repConfig;
private <b class="userinput"><tt>RepQuoteEnvironment</tt></b> dbenv; </pre>
<p>
That done, we can skip the
<span><tt class="methodname">main()</tt> method and
our class constructor, because they do not change.</span>
Instead, we skip down to our
<span><tt class="methodname">init()</tt> method
where we take care of opening our environment and setting
the event handler. </span>
</p>
<p>
To update our <tt class="methodname">init()</tt> method, we only need
to do a couple of things. First, we identify the current class as
the event handler. Then, when we open our environment, we
instantiate a <tt class="classname">RepQuoteEnvironment</tt>
class instead of an <tt class="classname">Environment</tt>
class.
</p>
<pre class="programlisting"> public int init(RepConfig config)
throws DatabaseException
{
int ret = 0;
appConfig = config;
EnvironmentConfig envConfig = new EnvironmentConfig();
envConfig.setErrorStream(System.err);
envConfig.setErrorPrefix(RepConfig.progname);
envConfig.setReplicationManagerLocalSite(appConfig.getThisHost());
for (ReplicationHostAddress host = appConfig.getFirstOtherHost();
host != null; host = appConfig.getNextOtherHost())
{
envConfig.replicationManagerAddRemoteSite(host);
}
if (appConfig.totalSites > 0)
envConfig.setReplicationNumSites(appConfig.totalSites);
envConfig.setReplicationPriority(appConfig.priority);
envConfig.setReplicationManagerAckPolicy(
ReplicationManagerAckPolicy.ALL);
envConfig.setCacheSize(RepConfig.CACHESIZE);
envConfig.setTxnNoSync(true);
<b class="userinput"><tt>envConfig.setEventHandler(this);</tt></b>
envConfig.setAllowCreate(true);
envConfig.setRunRecovery(true);
envConfig.setThreaded(true);
envConfig.setInitializeReplication(true);
envConfig.setInitializeLocking(true);
envConfig.setInitializeLogging(true);
envConfig.setInitializeCache(true);
envConfig.setTransactional(true);
envConfig.setVerboseReplication(appConfig.verbose);
try {
dbenv = new <b class="userinput"><tt>RepQuoteEnvironment</tt></b>(appConfig.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;
}
// start replication manager
dbenv.replicationManagerStart(3, appConfig.startPolicy);
return ret;
} </pre>
<p>
That done, we need to implement our
<tt class="methodname">handleEvent()</tt> method.
This method is required because we are now implementing
<tt class="classname">com.sleepycat.db.EventHandler</tt>. We use this
method to track whether we are operating as a master.
</p>
<pre class="programlisting"> public int handleEvent(EventType event)
{
int ret = 0;
if (event == EventType.REP_MASTER)
dbenv.setIsMaster(true);
else if (event == EventType.REP_CLIENT)
dbenv.setIsMaster(false);
else if (event == EventType.REP_NEW_MASTER) {
// ignored for now.
} else {
System.err.println("Unknown event callback received.\n");
ret = 1;
}
return ret;
} </pre>
<p>
That done, we need to update our
<tt class="function">doloop()</tt>
<span>method.</span>
</p>
<p>
We begin by updating our <tt class="classname">DatabaseConfig</tt>
instance to determine which options to use, depending on whether the
application is running as a master.
</p>
<pre class="programlisting"> public int doloop()
throws DatabaseException
{
Database db = null;
for (;;)
{
if (db == null) {
DatabaseConfig dbconf = new DatabaseConfig();
// Set page size small so page allocation is cheap.
dbconf.setPageSize(512);
dbconf.setType(DatabaseType.BTREE);
<b class="userinput"><tt>if (dbenv.getIsMaster()) {
dbconf.setAllowCreate(true);
}</tt></b>
dbconf.setTransactional(true); </pre>
<p>
When we open the database, we modify our error handling to
account for the case where the database does not yet exist. This can
happen if our code is running as a replica and the replication framework has not
yet had a chance to create the databases for us. Recall that replicas never
write to their own databases directly, and so they cannot
create databases on their own.
</p>
<p>
If we detect that the database does not yet exist, we simply
close the database handle, sleep for a short period of time
and then continue processing. This gives the replication framework a chance to
create the database so that our replica can continue
operations.
</p>
<pre class="programlisting"> try {
db = dbenv.openDatabase
(null, RepConfig.progname, null, dbconf);
} catch (java.io.FileNotFoundException e) {
<b class="userinput"><tt>System.err.println("no stock database available yet.");
if (db != null) {
db.close(true);
db = null;
}
try {
Thread.sleep(RepConfig.SLEEPTIME);
} catch (InterruptedException ie) {}
continue;</tt></b>
}
} </pre>
<p>
Next we modify our prompt, so that if the local process is running
as a replica, we can tell from the shell that the prompt is for a
read-only process.
</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");
<b class="userinput"><tt>if (!dbenv.getIsMaster())
System.out.print("(read-only)");
System.out.print("> ");</tt></b>
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"); </pre>
<p>
When we collect data from the prompt, there is a case that says
if no data is entered then show the entire stocks database.
This display is performed by our
<tt class="function">print_stocks()</tt>
<span>method</span>
(which has not
required a modification since we first introduced it in
<a href="simpleprogramlisting.html#printstocks_c">
<span>Method: SimpleTxn.printStocks()</span>
</a>).
</p>
<p>
When we call
<span><tt class="function">printStocks()</tt>, </span>
we check for a dead replication handle. Dead
replication handles happen whenever a replication election
results in a previously committed transaction becoming
invalid. This is an error scenario caused by a new master having a
slightly older version of the data than the original
master and so all replicas must modify their database(s) to
reflect that of the new master. In this situation, some
number of previously committed transactions may have to be
unrolled. From the replica's perspective, the database
handles should all be closed and then opened again.
</p>
<pre class="programlisting"> // 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);
<b class="userinput"><tt>} catch (DeadlockException de) {
continue;
// Dead replication handles are caused by an election
// resulting in a previously committing read becoming
// invalid. Close the db handle and reopen.
} catch (ReplicationHandleDeadException rhde) {
db.close(true); // close no sync.
db = null;
continue;</tt></b>
} catch (DatabaseException e) {
System.err.println("Got db exception reading " +
"replication 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>
That done, we need to add a little error checking to our
command prompt to make sure the user is not attempting to
modify the database at a replica. Remember, replicas must never
modify their local databases on their own. This guards against
that happening due to user input at the prompt.
</p>
<pre class="programlisting"> <b class="userinput"><tt>if (!dbenv.getIsMaster()) {
System.err.println("Can't update client.");
continue;
}</tt></b>
DatabaseEntry key = new DatabaseEntry(words[0].getBytes());
DatabaseEntry data = new DatabaseEntry(words[1].getBytes());
db.put(null, key, data);
}
if (db != null)
db.close(true);
return 0;
} </pre>
<p>
With that completed, we are all done updating our application
for replication.
The only remaining
<span>method, <tt class="function">printStocks()</tt>,</span>
is unmodified from when we
originally introduced it. For details on that function, see
<a href="simpleprogramlisting.html#printstocks_c">
<span>Method: SimpleTxn.printStocks()</span>
</a>.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="runningit"></a>Running It</h3>
</div>
</div>
<div></div>
</div>
<p>
To run our replicated application, we need to make
sure each participating environment has its own unique
home directory. We can do this by running
each site on a separate networked machine, but that
is not strictly necessary; multiple instances of this
code can run on the same machine provided the
environment home restriction is observed.
</p>
<p>
To run a process, make sure the environment home
exists and then start the process using the
<tt class="literal">-h</tt> option to specify that
directory. You must also use the <tt class="literal">-m</tt>
option to identify the local host and port that this
process will use to listen for replication messages, and
the <tt class="literal">-o</tt> option to identify the other
processes in the replication group. Finally, use the
<tt class="literal">-p</tt> option to specify a priority.
The process that you designate to have the highest priority will become
the master.
</p>
<pre class="programlisting">> mkdir env1
> java db.repquote.RepQuoteExample -h env1 -n 2 -m localhost:8080 \
-o localhost:8081 -p 10
No stock database yet available.
No stock database yet available. </pre>
<p>
Now, start another process. This time, change the environment
home to something else, use the <tt class="literal">-m</tt> to at
least change the port number the process is listening on, and
use the <tt class="literal">-o</tt> option to identify the host and
port of the other replication process:
</p>
<pre class="programlisting">> mkdir env2
> java db.repquote.RepQuoteExample -h env2 -n 2 -m localhost:8081 \
-o localhost:8080 -p 20</pre>
<p>
After a short pause, the second process should display the master
prompt:
</p>
<pre class="programlisting">
QUOTESERVER > </pre>
<p>
And the first process should
display the read-only prompt:
</p>
<pre class="programlisting">
QUOTESERVER (read-only)> </pre>
<p>
Now go to the master process and give it a couple of stocks and stock
prices:
</p>
<pre class="programlisting">QUOTESERVER> FAKECO 9.87
QUOTESERVER> NOINC .23
QUOTESERVER> </pre>
<p>
Then, go to the replica and hit <b class="userinput"><tt>return</tt></b> at the prompt to
see the new values:
</p>
<pre class="programlisting">QUOTESERVER (read-only)>
Symbol Price
====== =====
FAKECO 9.87
NOINC .23
QUOTESERVER (read-only)> </pre>
<p>
Doing the same at the master results in the same thing:
</p>
<pre class="programlisting">QUOTESERVER>
Symbol Price
====== =====
FAKECO 9.87
NOINC .23
QUOTESERVER> </pre>
<p>
You can change a stock by simply entering the stock value and
new price at the master's prompt:
</p>
<pre class="programlisting">QUOTESERVER> FAKECO 10.01
QUOTESERVER> </pre>
<p>
Then, go to either the master or the replica to see the updated
database:
</p>
<pre class="programlisting">QUOTESERVER>
Symbol Price
====== =====
FAKECO 10.01
NOINC .23
QUOTESERVER> </pre>
<p>
And on the replica:
</p>
<pre class="programlisting">QUOTESERVER (read-only)>
Symbol Price
====== =====
FAKECO 10.01
NOINC .23
QUOTESERVER (read-only)> </pre>
<p>
Finally, to quit the applications, simply type
<tt class="literal">quit</tt> at both prompts:
</p>
<pre class="programlisting">QUOTESERVER (read-only)> quit
> </pre>
<p>
And on the master as well:
</p>
<pre class="programlisting">QUOTESERVER> quit
> </pre>
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