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Release 5.2.28 on 6/10/2011
2011-09-13 17:44:24 +00:00
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<th colspan="3" align="center">Program Listing</th>
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<td width="20%" align="left"><a accesskey="p" href="txnapp.html">Prev</a> </td>
<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 class="toc">
<dl>
<dt>
<span class="sect2">
<a href="simpleprogramlisting.html#main_c">Function: main()</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="simpleprogramlisting.html#create_env_c">Function: create_env()</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="simpleprogramlisting.html#env_init_c">Function: env_init()</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="simpleprogramlisting.html#doloop_c">Function: doloop()</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="simpleprogramlisting.html#printstocks_c">
<span>Function: print_stocks()</span>
</a>
</span>
</dt>
</dl>
</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
<em class="citetitle">Berkeley DB Getting Started with Transaction Processing</em> 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="main_c"></a>Function: main()</h3>
</div>
</div>
</div>
<p>
Our program begins with the usual assortment of
include statements.
</p>
<a id="c_simpletxn1"></a>
<pre class="programlisting">/*
* File: ex_rep_gsg_simple.c
*/
#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
#ifndef _WIN32
#include &lt;unistd.h&gt;
#endif
#include &lt;db.h&gt;
#ifdef _WIN32
extern int getopt(int, char * const *, const char *);
#endif </pre>
<p>
We then define a few values. One is the size of our cache,
which we keep deliberately small for this example, and the
other is the name of our database. We also provide a global
variable that is the name of our program; this is used for
error reporting later on.
</p>
<a id="c_simpletxn2"></a>
<pre class="programlisting">#define CACHESIZE (10 * 1024 * 1024)
#define DATABASE "quote.db"
const char *progname = "ex_rep_gsg_simple"; </pre>
<p>
Then we perform a couple of forward declarations. The first
of these, <code class="function">create_env()</code> and
<code class="function">env_init()</code> are used to open
and initialize our environment.
</p>
<p>
Next we declare
<code class="function">doloop()</code>, which is the function that we use to
add data to the database and then display its contents. This is
essentially a big <code class="literal">do</code> loop, hence the
function's name.
</p>
<p>
Finally, we have <code class="function">print_stocks</code>, which is
used to display a database record once it has been retrieved from the
database.
</p>
<a id="c_simpletxn3"></a>
<pre class="programlisting">int create_env(const char *, DB_ENV **);
int env_init(DB_ENV *, const char *);
int doloop (DB_ENV *);
int print_stocks(DB *); </pre>
<p>
Next we need our <code class="function">usage()</code> function,
which is fairly trivial at this point:
</p>
<a id="c_simpletxn4"></a>
<pre class="programlisting">/* Usage function */
static void
usage()
{
fprintf(stderr, "usage: %s ", progname);
fprintf(stderr, "-h home\n");
exit(EXIT_FAILURE);
} </pre>
<p>
That completed, we can jump into our application's
<code class="function">main()</code> function. If you are familiar with
DB transactional applications, you will not find any
surprises here. We begin by declaring and initializing the
usual set of variables:
</p>
<a id="c_simpletxn5"></a>
<pre class="programlisting">int
main(int argc, char *argv[])
{
extern char *optarg;
DB_ENV *dbenv;
const char *home;
char ch;
int ret;
dbenv = NULL;
ret = 0;
home = NULL; </pre>
<p>
Now we create and configure our environment handle.
We do this with our <code class="function">create_env()</code> function, which we will
show a little later in this example.
</p>
<a id="c_simpletxn6"></a>
<pre class="programlisting"> if ((ret = create_env(progname, &amp;dbenv)) != 0)
goto err; </pre>
<p>
Then we parse the command line arguments:
</p>
<a id="c_simpletxn7"></a>
<pre class="programlisting"> while ((ch = getopt(argc, argv, "h:")) != EOF)
switch (ch) {
case 'h':
home = optarg;
break;
case '?':
default:
usage();
}
/* Error check command line. */
if (home == NULL)
usage(); </pre>
<p>
Now we can open our environment. We do this with our
<code class="function">env_init()</code> function which we will describe
a little later in this chapter.
</p>
<a id="c_simpletxn8"></a>
<pre class="programlisting"> if ((ret = env_init(dbenv, home)) != 0)
goto err; </pre>
<p>
Now that we have opened the environment, we can call our
<code class="function">doloop()</code> function. This function performs the basic
database interaction. Notice that we have not yet opened any databases. In
a traditional transactional application we would probably open the
databases before calling our our main data processing function.
However, the eventual replicated application will want to handle
database open and close in the main processing loop, so in a nod to what this
application will eventually become we do a slightly unusual thing
here.
</p>
<a id="c_simpletxn9"></a>
<pre class="programlisting"> if ((ret = doloop(dbenv)) != 0) {
dbenv-&gt;err(dbenv, ret, "Application failed");
goto err;
} </pre>
<p>
Finally, we provide our application shutdown code. Note, again,
that in a traditional transactional application all databases would
also be closed here. But, again, due to the way this application
will eventually behave, we cause the database close to occur in the
<code class="function">doloop()</code> function.
</p>
<a id="c_simpletxn10"></a>
<pre class="programlisting">err: if (dbenv != NULL)
(void)dbenv-&gt;close(dbenv, 0);
return (ret);
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="create_env_c"></a>Function: create_env()</h3>
</div>
</div>
</div>
<p>
Having written our <code class="function">main()</code>
function, we now implement the first of our utility
functions that we use to manage our environments.
This function exists only to make our code easier
to manage, and all it does is create an environment
handle for us.
</p>
<a id="c_create_env1"></a>
<pre class="programlisting">int
create_env(const char *progname, DB_ENV **dbenvp)
{
DB_ENV *dbenv;
int ret;
if ((ret = db_env_create(&amp;dbenv, 0)) != 0) {
fprintf(stderr, "can't create env handle: %s\n",
db_strerror(ret));
return (ret);
}
dbenv-&gt;set_errfile(dbenv, stderr);
dbenv-&gt;set_errpfx(dbenv, progname);
*dbenvp = dbenv;
return (0);
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="env_init_c"></a>Function: env_init()</h3>
</div>
</div>
</div>
<p>
Having written the function that initializes an
environment handle, we now implement the function
that opens the handle. Again, there should be no
surprises here for anyone familiar with DB
applications. The open flags that we use are those
normally used for a transactional application.
</p>
<a id="c_env_init"></a>
<pre class="programlisting">int
env_init(DB_ENV *dbenv, const char *home)
{
u_int32_t flags;
int ret;
(void)dbenv-&gt;set_cachesize(dbenv, 0, CACHESIZE, 0);
(void)dbenv-&gt;set_flags(dbenv, DB_TXN_NOSYNC, 1);
flags = DB_CREATE |
DB_INIT_LOCK |
DB_INIT_LOG |
DB_INIT_MPOOL |
DB_INIT_TXN |
DB_RECOVER;
if ((ret = dbenv-&gt;open(dbenv, home, flags, 0)) != 0)
dbenv-&gt;err(dbenv, ret, "can't open environment");
return (ret);
} </pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="doloop_c"></a>Function: doloop()</h3>
</div>
</div>
</div>
<p>
Having written our <code class="function">main()</code>
function and utility functions, we now implement
our application's
primary data processing function. This
function 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
<code class="literal">return</code> at the prompt.
</p>
<p>
To begin, we declare a database pointer,
several <code class="classname">DBT</code> variables, and
the usual assortment of variables used for buffers
and return codes. We also initialize all of this.
</p>
<a id="c_doloop1"></a>
<pre class="programlisting">#define BUFSIZE 1024
int
doloop(DB_ENV *dbenv)
{
DB *dbp;
DBT key, data;
char buf[BUFSIZE], *rbuf;
int ret;
u_int32_t db_flags;
dbp = NULL;
memset(&amp;key, 0, sizeof(key));
memset(&amp;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
<em class="citetitle">Berkeley DB Getting Started with Transaction Processing</em> guide.
</p>
<a id="c_doloop2"></a>
<pre class="programlisting"> for (;;) {
if (dbp == NULL) {
if ((ret = db_create(&amp;dbp, dbenv, 0)) != 0)
return (ret);
db_flags = DB_AUTO_COMMIT | DB_CREATE;
if ((ret = dbp-&gt;open(dbp, NULL, DATABASE,
NULL, DB_BTREE, db_flags, 0)) != 0) {
dbenv-&gt;err(dbenv, ret, "DB-&gt;open");
goto err;
}
} </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 <code class="literal">exit</code>
or <code class="literal">quit</code>, the loop is exited and the
application ends. If the user enters nothing and instead simply
presses <code class="literal">return</code>, the entire contents of the
database is displayed. We use our
<code class="function">print_stocks()</code> function 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>
<a id="c_doloop3"></a>
<pre class="programlisting"> printf("QUOTESERVER &gt; ");
fflush(stdout);
if (fgets(buf, sizeof(buf), stdin) == NULL)
break;
if (strtok(&amp;buf[0], " \t\n") == NULL) {
switch ((ret = print_stocks(dbp))) {
case 0:
continue;
default:
dbp-&gt;err(dbp, 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-&gt;errx(dbenv, "Format: TICKER VALUE");
continue;
} </pre>
<p>
Now we assign data to the <code class="classname">DBT</code>s that
we will use to write the new information to the database.
</p>
<a id="c_doloop4"></a>
<pre class="programlisting"> key.data = buf;
key.size = (u_int32_t)strlen(buf);
data.data = rbuf;
data.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 <code class="literal">DB_KEYEXIST</code> — which
we ignore.
</p>
<a id="c_doloop5"></a>
<pre class="programlisting"> if ((ret = dbp-&gt;put(dbp, NULL, &amp;key, &amp;data, 0)) != 0)
{
dbp-&gt;err(dbp, ret, "DB-&gt;put");
if (ret != DB_KEYEXIST)
goto err;
}
} </pre>
<p>
Finally, we close our database before returning from the
function.
</p>
<a id="c_doloop6"></a>
<pre class="programlisting">err: if (dbp != NULL)
(void)dbp-&gt;close(dbp, DB_NOSYNC);
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>Function: print_stocks()</span>
</h3>
</div>
</div>
</div>
<p>
The <code class="function">print_stocks()</code>
<span>function</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 <em class="citetitle">Getting Started with Berkeley DB</em>
guide.
</p>
<a id="c_printstocks1"></a>
<pre class="programlisting">/* Displays all stock quote information in the database. */
int
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-&gt;cursor(dbp, NULL, &amp;dbc, 0)) != 0) {
dbp-&gt;err(dbp, ret, "can't open cursor");
return (ret);
}
memset(&amp;key, 0, sizeof(key));
memset(&amp;data, 0, sizeof(data));
printf("\tSymbol\tPrice\n");
printf("\t======\t=====\n");
for (ret = dbc-&gt;get(dbc, &amp;key, &amp;data, DB_FIRST);
ret == 0;
ret = dbc-&gt;get(dbc, &amp;key, &amp;data, DB_NEXT)) {
keysize = key.size &gt; MAXKEYSIZE ? MAXKEYSIZE : key.size;
memcpy(keybuf, key.data, keysize);
keybuf[keysize] = '\0';
datasize = data.size &gt;= MAXDATASIZE ? MAXDATASIZE : data.size;
memcpy(databuf, data.data, datasize);
databuf[datasize] = '\0';
printf("\t%s\t%s\n", keybuf, databuf);
}
printf("\n");
fflush(stdout);
if ((t_ret = dbc-&gt;close(dbc)) != 0 &amp;&amp; ret == 0)
ret = t_ret;
switch (ret) {
case 0:
case DB_NOTFOUND:
return (0);
default:
return (ret);
}
} </pre>
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