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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: RepConfigInfo</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">RepConfigInfo</tt>
and its only purpose at this time is to track
the location of our environment home directory.
</p>
<pre class="programlisting">#include <db_cxx.h>
#include <iostream>
class RepConfigInfo {
public:
RepConfigInfo();
virtual ~RepConfigInfo();
public:
char* home;
};
RepConfigInfo::RepConfigInfo()
{
home = "TESTDIR";
}
RepConfigInfo::~RepConfigInfo()
{
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="repmgr_cxx"></a>Class: RepMgr</h3>
</div>
</div>
<div></div>
</div>
<p>
Our transactional example will
instantiate a class,
<tt class="classname">RepMgr</tt>, that performs
all our work for us. Before we implement our
<tt class="function">main()</tt> function, we show
the <tt class="classname">RepMgr</tt> class
declaration.
</p>
<p>
First, we provide some declarations and
definitions that are needed later in
our example:
</p>
<pre class="programlisting">using std::cout;
using std::cin;
using std::cerr;
using std::endl;
using std::flush;
#define CACHESIZE (10 * 1024 * 1024)
#define DATABASE "quote.db"
const char *progname = "SimpleTxn"; </pre>
<p>
And then we define our <tt class="classname">RepMgr</tt> class:
</p>
<pre class="programlisting">class RepMgr
{
public:
// Constructor.
RepMgr();
// Initialization method. Creates and opens our environment handle.
int init(RepConfigInfo* config);
// The doloop is where all the work is performed.
int doloop();
// terminate() provides our shutdown code.
int terminate();
private:
// disable copy constructor.
RepMgr(const RepMgr &);
void operator = (const RepMgr &);
// internal data members.
RepConfigInfo *app_config;
DbEnv dbenv;
// private methods.
// print_stocks() is used to display the contents of our database.
static int print_stocks(Db *dbp);
}; </pre>
<p>
Note that we show the implementation of the various
<tt class="classname">RepMgr</tt> methods later in this section.
</p>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="usage_cxx"></a>Function: usage()</h3>
</div>
</div>
<div></div>
</div>
<p>
Our <tt class="function">usage()</tt> is at this
stage of development trivial because we only
have one command line argument to manage.
Still, we show it here for the sake of
completeness.
</p>
<pre class="programlisting">static void usage()
{
cerr << "usage: " << progname << endl
<< "-h home" << endl;
exit(EXIT_FAILURE);
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="main_cxx"></a>Function: main()</h3>
</div>
</div>
<div></div>
</div>
<p>
Now we provide our <tt class="function">main()</tt>
function. This is a trivial function whose only
job is to collect command line information,
then instantiate a <tt class="classname">RepMgr</tt>
object, run it, then terminate it.
</p>
<p>
We begin by declaring some useful variables. Of
these, note that we instantiate our
<tt class="classname">RepConfigInfo</tt>
object here. Recall that this is used to store
information useful to our code. This class becomes more
interesting later in this book.
</p>
<pre class="programlisting">int main(int argc, char **argv)
{
RepConfigInfo config;
char ch;
int ret; </pre>
<p>
Then we collect our command line information. Again, this is at
this point fairly trivial:
</p>
<pre class="programlisting"> // Extract the command line parameters
while ((ch = getopt(argc, argv, "h:")) != EOF) {
switch (ch) {
case 'h':
config.home = optarg;
break;
case '?':
default:
usage();
}
}
// Error check command line.
if (config.home == NULL)
usage(); </pre>
<p>
Now we instantiate and initialize our <tt class="classname">RepMgr</tt>
class, which is what is responsible for doing all our real work.
The <tt class="methodname">RepMgr::init()</tt> method creates and
opens our environment handle.
</p>
<pre class="programlisting"> RepMgr runner;
try {
if((ret = runner.init(&config)) != 0)
goto err; </pre>
<p>
Then we call the <tt class="methodname">RepMgr::doloop()</tt>
method, which is where the actual transactional work is
performed for this application.
</p>
<pre class="programlisting"> if((ret = runner.doloop()) != 0)
goto err; </pre>
<p>
Finally, catch exceptions and terminate the program:
</p>
<pre class="programlisting"> } catch (DbException dbe) {
cerr << "Caught an exception during initialization or"
<< " processing: " << dbe.what() << endl;
}
err:
runner.terminate();
return 0;
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="repmgr_init_cxx"></a>Method: RepMgr::init()</h3>
</div>
</div>
<div></div>
</div>
<p>
The <tt class="methodname">RepMgr::init()</tt>
method is used to create and 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>
First, we show the class constructor
implementation, which is only used to initialize a
few variables:
</p>
<pre class="programlisting">RepMgr::RepMgr() : app_config(0), dbenv(0)
{
} </pre>
<p>
We now provide the <tt class="methodname">init()</tt> method
implementation. The only thing of interest here is that we specify
<tt class="literal">DB_TXN_NOSYNC</tt> to our environment. This causes
our transactional commits to become non-durable, which is something
that we are doing only because of the nature of our example.
</p>
<pre class="programlisting">int RepMgr::init(RepConfigInfo *config)
{
int ret = 0;
app_config = config;
dbenv.set_errfile(stderr);
dbenv.set_errpfx(progname);
/*
* We can now open our environment.
*/
dbenv.set_cachesize(0, CACHESIZE, 0);
dbenv.set_flags(DB_TXN_NOSYNC, 1);
try {
dbenv.open(app_config->home,
DB_CREATE |
DB_RECOVER |
DB_INIT_LOCK |
DB_INIT_LOG |
DB_INIT_MPOOL |
DB_INIT_TXN,
0);
} catch(DbException dbe) {
cerr << "Caught an exception during DB environment open." << endl
<< "Ensure that the home directory is created prior to starting"
<< " the application." << endl;
ret = ENOENT;
goto err;
}
err:
return ret;
} </pre>
<p>
Finally, we present the <tt class="methodname">RepMgr::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">int RepMgr::terminate()
{
try {
dbenv.close(0);
} catch (DbException dbe) {
cerr << "error closing environment: " << dbe.what() << endl;
}
return 0;
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="doloop_cxx"></a>Method: RepMgr::doloop()</h3>
</div>
</div>
<div></div>
</div>
<p>
Having written our <tt class="function">main()</tt>
function and support utility methods, 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,
several <tt class="classname">Dbt</tt> variables, and
the usual assortment of variables used for buffers
and return codes. We also initialize all of this.
</p>
<pre class="programlisting">#define BUFSIZE 1024
int RepMgr::doloop()
{
Db *dbp;
Dbt key, data;
char buf[BUFSIZE], *rbuf;
int ret;
dbp = NULL;
memset(&key, 0, sizeof(key));
memset(&data, 0, sizeof(data));
ret = 0; </pre>
<p>
Next, we begin the loop and we immediately open our
database if it has not already been opened. Notice that
we specify autocommit when we open the database. In
this case, autocommit is important because we will only
ever write to our database using it. There is no need
for explicit transaction handles and commit/abort code
in this application, because we are not combining
multiple database operations together under a single
transaction.
</p>
<p>
Autocommit is described in greater detail in the
<i class="citetitle">Berkeley DB Getting Started with Transaction Processing</i> guide.
</p>
<pre class="programlisting"> for (;;) {
if (dbp == NULL) {
dbp = new Db(&dbenv, 0);
// Set page size small so page allocation is cheap.
if ((ret = dbp->set_pagesize(512)) != 0)
goto err;
try {
dbp->open(NULL, DATABASE, NULL, DB_BTREE,
DB_CREATE | DB_AUTO_COMMIT, 0);
} catch(DbException dbe) {
dbenv.err(ret, "DB->open");
throw dbe;
}
} </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="function">print_stocks()</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"> cout << "QUOTESERVER" ;
cout << "> " << flush;
if (fgets(buf, sizeof(buf), stdin) == NULL)
break;
if (strtok(&buf[0], " \t\n") == NULL) {
switch ((ret = print_stocks(dbp))) {
case 0:
continue;
default:
dbp->err(ret, "Error traversing data");
goto err;
}
}
rbuf = strtok(NULL, " \t\n");
if (rbuf == NULL || rbuf[0] == '\0') {
if (strncmp(buf, "exit", 4) == 0 ||
strncmp(buf, "quit", 4) == 0)
break;
dbenv.errx("Format: TICKER VALUE");
continue;
} </pre>
<p>
Now we assign data to the <tt class="classname">Dbt</tt>s that
we will use to write the new information to the database.
</p>
<pre class="programlisting"> key.set_data(buf);
key.set_size((u_int32_t)strlen(buf));
data.set_data(rbuf);
data.set_size((u_int32_t)strlen(rbuf)); </pre>
<p>
Having done that, we can write the new information to the
database. Remember that this application uses autocommit,
so no explicit transaction management is required. 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="literal">DB_KEYEXIST</tt> — which
we ignore.
</p>
<pre class="programlisting"> if ((ret = dbp->put(NULL, &key, &data, 0)) != 0)
{
dbp->err(ret, "DB->put");
if (ret != DB_KEYEXIST)
goto err;
}
} </pre>
<p>
Finally, we close our database before returning from the
method.
</p>
<pre class="programlisting">err: if (dbp != NULL) {
(void)dbp->close(DB_NOSYNC);
cout << "database closed" << endl;
}
return (ret);
} </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: RepMgr::print_stocks()</span>
</h3>
</div>
</div>
<div></div>
</div>
<p>
The <tt class="function">print_stocks()</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">int RepMgr::print_stocks(Db *dbp)
{
Dbc *dbc;
Dbt key, data;
#define MAXKEYSIZE 10
#define MAXDATASIZE 20
char keybuf[MAXKEYSIZE + 1], databuf[MAXDATASIZE + 1];
int ret, t_ret;
u_int32_t keysize, datasize;
if ((ret = dbp->cursor(NULL, &dbc, 0)) != 0) {
dbp->err(ret, "can't open cursor");
return (ret);
}
memset(&key, 0, sizeof(key));
memset(&data, 0, sizeof(data));
cout << "\tSymbol\tPrice" << endl
<< "\t======\t=====" << endl;
for (ret = dbc->get(&key, &data, DB_FIRST);
ret == 0;
ret = dbc->get(&key, &data, DB_NEXT)) {
keysize = key.get_size() > MAXKEYSIZE ? MAXKEYSIZE : key.get_size();
memcpy(keybuf, key.get_data(), keysize);
keybuf[keysize] = '\0';
datasize = data.get_size() >=
MAXDATASIZE ? MAXDATASIZE : data.get_size();
memcpy(databuf, data.get_data(), datasize);
databuf[datasize] = '\0';
cout << "\t" << keybuf << "\t" << databuf << endl;
}
cout << endl << flush;
if ((t_ret = dbc->close()) != 0 && ret == 0) {
cout << "closed cursor" << endl;
ret = t_ret;
}
switch (ret) {
case 0:
case DB_NOTFOUND:
return (0);
default:
return (ret);
}
} </pre>
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