<!--$Id: env_open.html,v 1.1 2003/02/15 04:55:51 zarzycki Exp $--> <!--Copyright 1997-2002 by Sleepycat Software, Inc.--> <!--All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> <title>Berkeley DB: DbEnv.open</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,b+tree,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,java,C,C++"> </head> <body bgcolor=white> <a name="2"><!--meow--></a> <table width="100%"><tr valign=top> <td> <h1>DbEnv.open</h1> </td> <td align=right> <a href="../api_java/c_index.html"><img src="../images/api.gif" alt="API"></a><a href="../reftoc.html"><img src="../images/ref.gif" alt="Ref"></a> </td></tr></table> <hr size=1 noshade> <tt> <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 filename 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>Because there are a large number of flags that can be specified, they have been grouped together by functionality. The first group of flags indicates 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><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, locking is unnecessary. When the Db.DB_INIT_LOCK flag is specified, it is usually necessary to run a 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 should be used when recovery from application or system failure is necessary. If the log region is being created and log files are already present, the log files are reviewed; 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 should be used whenever an 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 should be used when recovery and atomicity of multiple operations are important. The Db.DB_INIT_TXN flag implies the Db.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.DB_CREATE flag must also be set because the regions will be removed and re-created. <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.DB_CREATE flag must also be set because the regions will be removed and re-created. </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 (for example, 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 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. Because 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_CREATE and Db.DB_RECOVER flags 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 that 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> <a name="3"><!--meow--></a> <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>. Because 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.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>. Because permitting users to specify which files are used can create security problems, if the Db.DB_USE_ENVIRON_ROOT flag is set, environment information will be used for file naming only for users with appropriate permissions (for example, users with a user-ID of 0 on UNIX systems). </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 multithreaded). 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 because it is likely to cause database corruption and unpredictable behavior. For example, if both a server application and the Berkeley DB utility <a href="../utility/db_stat.html">db_stat</a> are expected to 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/env_class.html">DbEnv</a> handle returned by DbEnv.open to be <i>free-threaded</i>; that is, usable 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)). If <b>mode</b> is 0, Berkeley DB will use a default mode of readable and writable by both owner and group. On Windows systems, the mode argument is ignored. The group ownership of created files is based on the system and directory defaults, and is not further specified by Berkeley DB. <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 fast mutexes are not available 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. <h1>Class</h1> <a href="../api_java/env_class.html">DbEnv</a> <h1>See Also</h1> <a href="../api_java/env_list.html">Database Environments and Related Methods</a> </tt> <table width="100%"><tr><td><br></td><td align=right> <a href="../api_java/c_index.html"><img src="../images/api.gif" alt="API"></a><a href="../reftoc.html"><img src="../images/ref.gif" alt="Ref"></a> </td></tr></table> <p><font size=1><a href="http://www.sleepycat.com">Copyright Sleepycat Software</a></font> </body> </html>