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<p>Library Version 18.1.40</p>
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<th colspan="3" align="center">Creating a database
environment</th>
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<th width="60%" align="center">Chapter 9. The Berkeley DB Environment </th>
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<h2 class="title" style="clear: both"><a id="env_create"></a>Creating a database
environment</h2>
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<p>
The Berkeley DB environment is created and described by the
<a href="../api_reference/C/envcreate.html" class="olink">db_env_create()</a> and <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> interfaces. In situations where
customization is desired, such as storing log files on a
separate disk drive or selection of a particular cache size,
applications must describe the customization by either
creating an environment configuration file in the environment
home directory or by arguments passed to other <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> handle
methods.
</p>
<p>
Once an environment has been created, database files
specified using relative pathnames will be named relative to
the home directory. Using pathnames relative to the home
directory allows the entire environment to be easily moved,
simplifying restoration and recovery of a database in a
different directory or on a different system.
</p>
<p>
Applications first obtain an environment handle using the
<a href="../api_reference/C/envcreate.html" class="olink">db_env_create()</a> method, then call the <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> method which
creates or joins the database environment. There are a number
of options you can set to customize <a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> for your
environment. These options fall into four broad
categories:
</p>
<div class="variablelist">
<dl>
<dt>
<span class="term">Subsystem Initialization:</span>
</dt>
<dd>
These flags indicate which Berkeley DB
subsystems will be initialized for the environment,
and what operations will happen automatically when
databases are accessed within the environment. The
flags include <a href="../api_reference/C/envopen.html#envopen_DB_INIT_CDB" class="olink">DB_INIT_CDB</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a>,
<a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOG" class="olink">DB_INIT_LOG</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_MPOOL" class="olink">DB_INIT_MPOOL</a>, and <a href="../api_reference/C/envopen.html#envopen_DB_INIT_TXN" class="olink">DB_INIT_TXN</a>. The
<a href="../api_reference/C/envopen.html#envopen_DB_INIT_CDB" class="olink">DB_INIT_CDB</a> flag does initialization for Berkeley DB
Concurrent Data Store applications. (See <a class="xref" href="cam.html#cam_intro" title="Concurrent Data Store introduction">Concurrent Data Store
introduction</a>
for more information.) The rest of the flags
initialize a single subsystem; that is, when
<a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a> is specified, applications reading and
writing databases opened in this environment will be
using locking to ensure that they do not overwrite
each other's changes.
</dd>
<dt>
<span class="term">Recovery options:</span>
</dt>
<dd>
These flags, which include <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> and
<a href="../api_reference/C/envopen.html#envopen_DB_RECOVER_FATAL" class="olink">DB_RECOVER_FATAL</a>, indicate what recovery is to be
performed on the environment before it is opened for
normal use.
</dd>
<dt>
<span class="term">Naming options:</span>
</dt>
<dd>
These flags, which include <a href="../api_reference/C/envopen.html#envopen_DB_USE_ENVIRON" class="olink">DB_USE_ENVIRON</a> and
<a href="../api_reference/C/envopen.html#envopen_DB_USE_ENVIRON_ROOT" class="olink">DB_USE_ENVIRON_ROOT</a>, modify how file naming happens
in the environment.
</dd>
<dt>
<span class="term">Miscellaneous:</span>
</dt>
<dd>
Finally, there are a number of miscellaneous
flags, for example, <a href="../api_reference/C/dbopen.html#open_DB_CREATE" class="olink">DB_CREATE</a> which causes
underlying files to be created as necessary. See the
<a href="../api_reference/C/envopen.html" class="olink">DB_ENV->open()</a> manual pages for further
information.
</dd>
</dl>
</div>
<p>
Most applications either specify only the <a href="../api_reference/C/envopen.html#envopen_DB_INIT_MPOOL" class="olink">DB_INIT_MPOOL</a>
flag or they specify all four subsystem initialization flags
(<a href="../api_reference/C/envopen.html#envopen_DB_INIT_MPOOL" class="olink">DB_INIT_MPOOL</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOCK" class="olink">DB_INIT_LOCK</a>, <a href="../api_reference/C/envopen.html#envopen_DB_INIT_LOG" class="olink">DB_INIT_LOG</a>, and
<a href="../api_reference/C/envopen.html#envopen_DB_INIT_TXN" class="olink">DB_INIT_TXN</a>). The former configuration is for applications
that simply want to use the basic Access Method interfaces
with a shared underlying buffer pool, but don't care about
recoverability after application or system failure. The latter
is for applications that need recoverability. There are
situations in which other combinations of the initialization
flags make sense, but they are rare.
</p>
<p>
The <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER" class="olink">DB_RECOVER</a> flag is specified by applications that want
to perform any necessary database recovery when they start
running. That is, if there was a system or application failure
the last time they ran, they want the databases to be made
consistent before they start running again. It is not an error
to specify this flag when no recovery needs to be done.
</p>
<p>
The <a href="../api_reference/C/envopen.html#envopen_DB_RECOVER_FATAL" class="olink">DB_RECOVER_FATAL</a> flag is more special-purpose. It
performs catastrophic database recovery, and normally requires
that some initial arrangements be made; that is, archived log
files be brought back into the filesystem. Applications should
not normally specify this flag. Instead, under these rare
conditions, the <a href="../api_reference/C/db_recover.html" class="olink">db_recover</a> utility should be used.
</p>
<p>
The following is a simple example of a function that opens a
database environment for a transactional program.
</p>
<a id="prog_env27"></a>
<pre class="programlisting">DB_ENV *
db_setup(char *home, char *data_dir, FILE *errfp, char *progname)
{
DB_ENV *dbenv;
int ret;
/*
* Create an environment and initialize it for additional error
* reporting.
*/
if ((ret = db_env_create(&dbenv, 0)) != 0) {
fprintf(errfp, "%s: %s\n", progname, db_strerror(ret));
return (NULL);
}
dbenv->set_errfile(dbenv, errfp);
dbenv->set_errpfx(dbenv, progname);
/*
* Specify the shared memory buffer pool cachesize: 5MB.
* Databases are in a subdirectory of the environment home.
*/
if ((ret = dbenv->set_cachesize(dbenv, 0,
5 * 1024 * 1024, 0)) != 0) {
dbenv->err(dbenv, ret, "set_cachesize");
goto err;
}
if ((ret = dbenv->add_data_dir(dbenv, data_dir)) != 0) {
dbenv->err(dbenv, ret, "add_data_dir: %s", data_dir);
goto err;
}
/* Open the environment with full transactional support. */
if ((ret = dbenv->open(dbenv, home, DB_CREATE | DB_INIT_LOG |
DB_INIT_LOCK | DB_INIT_MPOOL |
DB_INIT_TXN, 0)) != 0) {
dbenv->err(dbenv, ret, "environment open: %s", home);
goto err;
}
return (dbenv);
err: (void)dbenv->close(dbenv, 0);
return (NULL);
}</pre>
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