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tim@threads.polyesthetic.msg 89dad52004 Import changeset
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<!--$Id: env_open.so,v 10.61 2000/12/01 15:50:31 bostic Exp $-->
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<h1>DbEnv.open</h1>
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<h3><pre>
import com.sleepycat.db.*;
import java.io.FileNotFoundException;
<p>
public void open(String db_home, int flags, int mode)
throws DbException, FileNotFoundException;
</pre></h3>
<h1>Description</h1>
<p>The DbEnv.open method is the interface for opening the Berkeley DB
environment. It provides a structure for creating a consistent
environment for processes using one or more of the features of Berkeley DB.
<p>The <b>db_home</b> argument to DbEnv.open (and file name
resolution in general) is described in
<a href="../ref/env/naming.html">Berkeley DB File Naming</a>.
<p>The <b>flags</b> argument specifies the subsystems that are initialized
and how the application's environment affects Berkeley DB file naming, among
other things.
<p>The <b>flags</b> value must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one or more
of the following values.
<p>As there are a large number of flags that can be specified, they have been
grouped together by functionality. The first group of flags indicate
which of the Berkeley DB subsystems should be initialized:
<p><dl compact>
<p><dt><a name="Db.DB_JOINENV">Db.DB_JOINENV</a><dd>Join an existing environment. This option allows applications to
join an existing environment without knowing which Berkeley DB subsystems
the environment supports.
<p><dt><a name="Db.DB_INIT_CDB">Db.DB_INIT_CDB</a><dd>Initialize locking for the <a href="../ref/cam/intro.html">Berkeley DB Concurrent Data Store</a>
product. In this mode, Berkeley DB provides multiple reader/single writer
access. The only other subsystem that should be specified with the
Db.DB_INIT_CDB flag is Db.DB_INIT_MPOOL.
<p>Access method calls are largely unchanged when using this flag, although
any cursors through which update operations (e.g., <a href="../api_java/dbc_put.html">Dbc.put</a>,
<a href="../api_java/dbc_del.html">Dbc.del</a>) will be made must have the <a href="../api_java/db_cursor.html#DB_WRITECURSOR">Db.DB_WRITECURSOR</a> value
set in the flags parameter to the cursor call that creates the cursor.
See <a href="../api_java/db_cursor.html">Db.cursor</a> for more information.
<p><dt><a name="Db.DB_INIT_LOCK">Db.DB_INIT_LOCK</a><dd>Initialize the locking subsystem. This subsystem should be used when
multiple processes or threads are going to be reading and writing a Berkeley DB
database, so that they do not interfere with each other. If all threads
are accessing the database(s) read-only, then locking is unnecessary.
When the DB_INIT_LOCK flag is specified, it is usually necessary to run
the deadlock detector, as well. See <a href="../utility/db_deadlock.html">db_deadlock</a> and
<a href="../api_java/lock_detect.html">DbEnv.lock_detect</a> for more information.
<p><dt><a name="Db.DB_INIT_LOG">Db.DB_INIT_LOG</a><dd>Initialize the logging subsystem. This subsystem is used when recovery
from application or system failure is necessary.
<p>The log is stored in one or more files in the environment directory.
Each file is named using the format <i>log.NNNNNNNNNN</i>, where
<i>NNNNNNNNNN</i> is the sequence number of the file within the log.
For further information, see
<a href="../ref/log/limits.html">Log File Limits</a>.
<p>If the log region is being created and log files are already present, the
log files are reviewed and subsequent log writes are appended
to the end of the log, rather than overwriting current log entries.
<p><dt><a name="Db.DB_INIT_MPOOL">Db.DB_INIT_MPOOL</a><dd>Initialize the shared memory buffer pool subsystem. This subsystem is
used whenever the application is using any Berkeley DB access method.
<p><dt><a name="Db.DB_INIT_TXN">Db.DB_INIT_TXN</a><dd>Initialize the transaction subsystem. This subsystem is used when
recovery and atomicity of multiple operations and recovery are important.
The DB_INIT_TXN flag implies the DB_INIT_LOG flag.
</dl>
<p>The second group of flags govern what recovery, if any, is performed when
the environment is initialized:
<p><dl compact>
<p><dt><a name="Db.DB_RECOVER">Db.DB_RECOVER</a><dd>Run normal recovery on this environment before opening it for normal use.
If this flag is set, the DB_CREATE flag must also be set since the regions
will be removed and recreated.
<p><dt><a name="Db.DB_RECOVER_FATAL">Db.DB_RECOVER_FATAL</a><dd>Run catastrophic recovery on this environment before opening it for normal
use. If this flag is set, the DB_CREATE flag must also be set since the
regions will be removed and recreated.
</dl>
<p>A standard part of the recovery process is to remove the existing Berkeley DB
environment and create a new one in which to perform recovery. If the
thread of control performing recovery does not specify the correct region
initialization information (e.g., the correct memory pool cache size),
the result can be an application running in an environment with incorrect
cache and other subsystem sizes. For this reason, the thread of control
performing recovery should either specify correct configuration
information before calling the DbEnv.open method, or it should remove
the environment after recovery is completed, leaving creation of the
correctly sized environment to a subsequent call to DbEnv.open.
<p>All Berkeley DB recovery processing must be single-threaded, that is, only a
single thread of control may perform recovery or access a Berkeley DB
environment while recovery is being performed. As it is not an error to
specify Db.DB_RECOVER for an environment for which no recovery is
required, it is reasonable programming practice for the thread of control
responsible for performing recovery and creating the environment to always
specify the Db.DB_RECOVER flag during startup.
<p>The DbEnv.open function returns successfully if Db.DB_RECOVER
or Db.DB_RECOVER_FATAL is specified and no log files exist, so it is
necessary to ensure all necessary log files are present before running
recovery. For further information, consult <a href="../utility/db_archive.html">db_archive</a> and
<a href="../utility/db_recover.html">db_recover</a>.
<p>The third group of flags govern file naming extensions in the environment:
<p><dl compact>
<!--$Id: m4.env_flags,v 10.9 2000/06/29 22:54:10 bostic Exp $-->
<p><dt><a name="Db.DB_USE_ENVIRON">Db.DB_USE_ENVIRON</a><dd>The Berkeley DB process' environment may be permitted to specify information to
be used when naming files; see <a href="../ref/env/naming.html">Berkeley DB
File Naming</a>. As permitting users to specify which files are used can
create security problems, environment information will be used in file
naming for all users only if the DB_USE_ENVIRON flag is set.
<p><dt><a name="Db.DB_USE_ENVIRON_ROOT">Db.DB_USE_ENVIRON_ROOT</a><dd>The Berkeley DB process' environment may be permitted to specify information to
be used when naming files; see <a href="../ref/env/naming.html">Berkeley DB
File Naming</a>. As permitting users to specify which files are used can
create security problems, if the DB_USE_ENVIRON_ROOT flag is set,
environment information will be used for file naming only for users with
appropriate permissions (e.g., on UNIX systems, users with a user-ID of 0).
</dl>
<p>Finally, there are a few additional, unrelated flags:
<p><dl compact>
<p><dt><a name="Db.DB_CREATE">Db.DB_CREATE</a><dd>Cause Berkeley DB subsystems to create any underlying files, as necessary.
<p><dt><a name="Db.DB_LOCKDOWN">Db.DB_LOCKDOWN</a><dd>Lock shared Berkeley DB environment files and memory mapped databases into memory.
<p><dt><a name="Db.DB_PRIVATE">Db.DB_PRIVATE</a><dd>Specify that the environment will only be accessed by a single process
(although that process may be multi-threaded). This flag has two effects
on the Berkeley DB environment. First, all underlying data structures are
allocated from per-process memory instead of from shared memory that is
potentially accessible to more than a single process. Second, mutexes
are only configured to work between threads.
<p>This flag should not be specified if more than a single process is
accessing the environment, as it is likely to cause database corruption
and unpredictable behavior, e.g., if both a server application and the
Berkeley DB utility <a href="../utility/db_stat.html">db_stat</a> will access the environment, the
Db.DB_PRIVATE flag should not be specified.
<p><dt><a name="Db.DB_SYSTEM_MEM">Db.DB_SYSTEM_MEM</a><dd>Allocate memory from system shared memory instead of from memory backed
by the filesystem. See <a href="../ref/env/region.html">Shared Memory
Regions</a> for more information.
<p><dt><a name="Db.DB_THREAD">Db.DB_THREAD</a><dd>Cause the <a href="../api_java/dbenv_class.html">DbEnv</a> handle returned by DbEnv.open to be
<i>free-threaded</i>, that is, useable by multiple threads within a
single address space.
<p>Threading is always assumed in the Java API, so no special flags are
required and Berkeley DB functions will always behave as if the Db.DB_THREAD
flag was specified.
</dl>
<p>On UNIX systems, or in IEEE/ANSI Std 1003.1 (POSIX) environments, all files created by Berkeley DB
are created with mode <b>mode</b> (as described in <b>chmod</b>(2)) and
modified by the process' umask value at the time of creation (see
<b>umask</b>(2)). The group ownership of created files is based on
the system and directory defaults, and is not further specified by Berkeley DB.
If <b>mode</b> is 0, files are created readable and writeable by both
owner and group. On Windows systems, the mode argument is ignored.
<p>The DbEnv.open method throws an exception that encapsulates a non-zero error value on
failure.
<h1>Environment Variables</h1>
<p><dl compact>
<p><dt>DB_HOME<dd>The environment variable <b>DB_HOME</b> may be used as the path of
the database home as described in
<a href="../ref/env/naming.html">Berkeley DB File Naming</a>.
</dl>
<h1>Errors</h1>
<p>The DbEnv.open method may fail and throw an exception encapsulating a non-zero error for the following conditions:
<p><dl compact>
<p><dt>EAGAIN<dd>The shared memory region was locked and (repeatedly) unavailable.
</dl>
<p><dl compact>
<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
<p>
The Db.DB_THREAD flag was specified and spinlocks are not
implemented for this architecture.
<p>The DB_HOME or TMPDIR environment variables were set but empty.
<p>An incorrectly formatted <b>NAME VALUE</b> entry or line was found.
</dl>
<p><dl compact>
<p><dt>ENOSPC<dd>HP-UX only: a previously created Berkeley DB environment for this process still
exists.
</dl>
<p>If the file or directory does not exist, the DbEnv.open method will
fail and
throw a FileNotFoundException exception.
<p>The DbEnv.open method may fail and throw an exception for errors specified for other Berkeley DB and C library or system methods.
If a catastrophic error has occurred, the DbEnv.open method may fail and throw
a <a href="../api_java/runrec_class.html">DbRunRecoveryException</a>, in which case all subsequent Berkeley DB calls
will fail in the same way.
<h3>Class</h3>
<a href="../api_java/dbenv_class.html">DbEnv</a>
<h1>See Also</h1>
<a href="../api_java/env_close.html">DbEnv.close</a>,
<a href="../api_java/env_open.html">DbEnv.open</a>,
<a href="../api_java/env_remove.html">DbEnv.remove</a>,
<a href="../api_java/env_strerror.html">DbEnv.strerror</a>,
<a href="../api_java/env_version.html">DbEnv.get_version_string</a>
<a href="../api_java/env_set_cachesize.html">DbEnv.set_cachesize</a>,
<a href="../api_java/env_set_errcall.html">DbEnv.set_errcall</a>,
<a href="../api_java/env_set_error_stream.html">DbEnv.set_error_stream</a>,
<a href="../api_java/env_set_errpfx.html">DbEnv.set_errpfx</a>,
<a href="../api_java/env_set_flags.html">DbEnv.set_flags</a>,
<a href="../api_java/env_set_mutexlocks.html">DbEnv.set_mutexlocks</a>,
and
<a href="../api_java/env_set_verbose.html">DbEnv.set_verbose</a>.
</tt>
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