gsg_db_rep/JAVA/exampledoloop.html

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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
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          <div>
            <h2 class="title" style="clear: both"><a id="exampledoloop"></a>Example Processing Loop</h2>
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      </div>
      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="exampledoloop.html#runningit">Running It</a>
            </span>
          </dt>
        </dl>
      </div>
      <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
                          <code class="function">doloop()</code>
                          function that our original transaction
                          application used


                          <span>(see <a class="xref" href="simpleprogramlisting.html#doloop_java" title="Method: SimpleTxn.doloop()">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
                          <strong class="userinput"><code>bold</code></strong>.
                  </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
                        <code class="classname">com.sleepycat.db.Environment</code>
                        to carry the information. We do this by creating
                        the <code class="classname">RepQuoteEnvironment</code>
                        class.
                   </p>
      <pre class="programlisting">
        <strong class="userinput">
          <code>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;
    }
} </code>
        </strong>
      </pre>
      <p>
                          Next, we go to <code class="filename">RepQuoteExampleGSG.java</code> and
                          we include the
                          <code class="classname">RepQuoteEnvironment</code> class
                          as well as the
                          <code class="classname">EventHandler</code> class. We then
                          cause our <code class="classname">RepQuoteExampleGSG</code>
                          class to implement
                          <code class="classname">EventHandler</code>. We also
                          change our environment handle to be an instance
                          of <code class="classname">RepQuoteEnvironment</code>
                          instead of <code class="classname">Environment</code>.
                  </p>
      <p>
                          Note that we also import the
                          <code class="classname">com.sleepycat.db.ReplicationHandleDeadException</code>
                          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;
<strong class="userinput"><code>import com.sleepycat.db.EventHandler;</code></strong>
import com.sleepycat.db.LockMode;
import com.sleepycat.db.OperationStatus;
<strong class="userinput"><code>import com.sleepycat.db.ReplicationHandleDeadException;</code></strong>

import db.repquote.RepConfig;
<strong class="userinput"><code>import db.repquote.RepQuoteEnvironment</code></strong>

public class RepQuoteExampleGSG <strong class="userinput"><code> implements EventHandler</code></strong>
{
    private RepConfig repConfig;
    private <strong class="userinput"><code>RepQuoteEnvironment</code></strong> dbenv; </pre>
      <p>
        That done, we can skip the


        <span><code class="methodname">main()</code> method and
                our class constructor, because they do not change.</span>

        Instead, we skip down to our


        <span><code class="methodname">init()</code> method
                where we take care of opening our environment and setting
                the event handler.  </span>
</p>
      <p>
        To update our <code class="methodname">init()</code> 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 <code class="classname">RepQuoteEnvironment</code>
        class instead of an <code class="classname">Environment</code>
        class.
</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.setReplicationManagerLocalSite(repConfig.getThisHost());
        for (ReplicationHostAddress host = repConfig.getFirstOtherHost();
            host != null; host = repConfig.getNextOtherHost())
        {
            envConfig.replicationManagerAddRemoteSite(host);
        }

        if (appConfig.totalSites &gt; 0)
            envConfig.setReplicationNumSites(repConfig.totalSites);
        envConfig.setReplicationPriority(repConfig.priority);

        envConfig.setReplicationManagerAckPolicy(
            ReplicationManagerAckPolicy.ALL);
        envConfig.setCacheSize(RepConfig.CACHESIZE);
        envConfig.setTxnNoSync(true);

        <strong class="userinput"><code>envConfig.setEventHandler(this);</code></strong>

        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 <strong class="userinput"><code>RepQuoteEnvironment</code></strong>(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;
        }

        // start Replication Manager
        dbenv.replicationManagerStart(3, repConfig.startPolicy);
        return ret;
    } </pre>
      <p>
    That done, we need to implement the methods
    required for responding to replication events.
        These methods are required because we are now implementing
        <code class="classname">com.sleepycat.db.EventHandler</code>.  While
        we are required to provide an implementation for all of these
        methods, for our simple application we are really only interested
        in these because they allow us to track whether a replication
        instance is a master.
</p>
      <pre class="programlisting">    public void handleRepClientEvent()
    {
        dbenv.setIsMaster(false);
    }

    public void handleRepMasterEvent()
    {
        dbenv.setIsMaster(true);
    }

    public void handleRepNewMasterEvent(int envId)
    {
        // Ignored for now
    }

    public void handleWriteFailedEvent(int errorCode)
    {
        // Ignored for now
    }

    public void handleRepStartupDoneEvent()
    {
        // Ignored for now
    }

    public void handleRepPermFailedEvent()
    {
        // Ignored for now
    }

    public void handleRepElectedEvent()
    {
        // Safely ignored for Replication Manager applications.
    }

    public void handlePanicEvent()
    {
        System.err.println("Panic encountered!");
        System.err.println("Shutting down.");
        System.err.println("You should restart, running recovery.");
        try {
            terminate();
        } catch (DatabaseException dbe) {
            System.err.println("Caught an exception during " +
                "termination in handlePanicEvent: " + dbe.toString());
        }
        System.exit(-1);
    } </pre>
      <p>
        That done, we need to update our
        <code class="function">doloop()</code>


        <span>method.</span>
</p>
      <p>
            We begin by updating our <code class="classname">DatabaseConfig</code>
            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();
                dbconf.setType(DatabaseType.BTREE);
                <strong class="userinput"><code>if (dbenv.getIsMaster()) {
                    dbconf.setAllowCreate(true);
                }</code></strong>
                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 Manager 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 Manager 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) {
                    <strong class="userinput"><code>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;</code></strong>
                }
            } </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");
            <strong class="userinput"><code>if (!dbenv.getIsMaster())
                System.out.print("(read-only)");
            System.out.print("&gt; ");</code></strong>
            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
            <code class="function">print_stocks()</code>

            <span>method</span>
            (which has not
            required a modification since we first introduced it in
            <a class="xref" href="simpleprogramlisting.html#printstocks_c" title="Method: SimpleTxn.printStocks()">


                            <span>Method: SimpleTxn.printStocks()</span>
                    </a>).
    </p>
      <p>
            When we call

               <span><code class="function">printStocks()</code>, </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 &amp;&amp; words[0].length() == 0)) {
                try {
                    printStocks(db);
                <strong class="userinput"><code>} 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;</code></strong>
                } catch (DatabaseException e) {
                    System.err.println("Got db exception reading " +
                        "replication DB: " + e.toString());
                    break;
                }
                continue;
            }

            if (words.length == 1 &amp;&amp;
                (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">            <strong class="userinput"><code>if (!dbenv.getIsMaster()) {
                System.err.println("Can't update client.");
                continue;
            }</code></strong>

            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, <code class="function">printStocks()</code>,</span>

        is unmodified from when we
        originally introduced it. For details on that function, see
        <a class="xref" href="simpleprogramlisting.html#printstocks_c" title="Method: SimpleTxn.printStocks()">


                            <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>
        <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
                <code class="literal">-h</code> option to specify that
                directory. You must also use the <code class="literal">-l</code>
                option to identify the local host and port that this
                process will use to listen for replication messages, the
                <code class="literal">-n</code> option to specify the number of sites
                in the replication group, and
                the <code class="literal">-r</code> option to identify the other
                processes in the replication group. Finally, use the
                <code class="literal">-p</code> option to specify a priority.
                The process that you designate to have the highest priority will become
                the master.
        </p>
        <pre class="programlisting">&gt; mkdir env1
&gt; java db.repquote.RepQuoteExampleGSG -h env1 -n 2 -l localhost:8080 \
-r 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 <code class="literal">-l</code> flag to at
        least change the port number the process is listening on, and
        use the <code class="literal">-r</code> option to identify the host and
        port of the other replication process:
</p>
        <pre class="programlisting">&gt; mkdir env2
&gt; java db.repquote.RepQuoteExampleGSG -h env2 -n 2 -l localhost:8081 \
-r localhost:8080 -p 20</pre>
        <p>
        After a short pause, the second process should display the master
        prompt:
</p>
        <pre class="programlisting">
QUOTESERVER &gt; </pre>
        <p>
        And the first process should
        display the read-only prompt:
</p>
        <pre class="programlisting">
QUOTESERVER (read-only)&gt; </pre>
        <p>
        Now go to the master process and give it a couple of stocks and stock
        prices:
</p>
        <pre class="programlisting">QUOTESERVER&gt; FAKECO 9.87
QUOTESERVER&gt; NOINC .23
QUOTESERVER&gt; </pre>
        <p>
    Then, go to the replica and hit <strong class="userinput"><code>return</code></strong> at the prompt to
    see the new values:
</p>
        <pre class="programlisting">QUOTESERVER (read-only)&gt;
        Symbol  Price
        ======  =====
        FAKECO  9.87
        NOINC    .23
QUOTESERVER (read-only)&gt; </pre>
        <p>
        Doing the same at the master results in the same thing:
</p>
        <pre class="programlisting">QUOTESERVER&gt;
        Symbol  Price
        ======  =====
        FAKECO  9.87
        NOINC    .23
QUOTESERVER&gt; </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&gt; FAKECO 10.01
QUOTESERVER&gt; </pre>
        <p>
        Then, go to either the master or the replica to see the updated
        database. On the master:
</p>
        <pre class="programlisting">QUOTESERVER&gt;
        Symbol  Price
        ======  =====
        FAKECO  10.01
        NOINC    .23
QUOTESERVER&gt; </pre>
        <p>
        And on the replica:
</p>
        <pre class="programlisting">QUOTESERVER (read-only)&gt;
        Symbol  Price
        ======  =====
        FAKECO  10.01
        NOINC    .23
QUOTESERVER (read-only)&gt; </pre>
        <p>
        Finally, to quit the applications, simply type
        <code class="literal">quit</code> at both prompts. On the replica:
</p>
        <pre class="programlisting">QUOTESERVER (read-only)&gt; quit
&gt; </pre>
        <p>
        And on the master as well:
</p>
        <pre class="programlisting">QUOTESERVER&gt; quit
&gt; </pre>
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