m4_comment([$Id: create.so,v 10.30 2005/09/23 16:22:42 bostic Exp $])

m4_ref_title(Environment,
    Creating a database environment,
    database @environment, env/intro, env/open)

m4_p([dnl
The m4_db environment is created and described by the m4_ref(dbenv_create)
and m4_ref(dbenv_open) 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 m4_ref(DbEnv) handle methods.])

m4_p([dnl
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.])

m4_p([dnl
Applications first obtain an environment handle using the
m4_refT(dbenv_create), then call the m4_refT(dbenv_open) which creates
or joins the database environment.  There are a number of options you
can set to customize m4_ref(dbenv_open) for your environment.  These
options fall into four broad categories:])

m4_tagbegin

m4_tag([Subsystem Initialization:], [dnl
These flags indicate which m4_db subsystems will be initialized for the
environment, and what operations will happen automatically when
databases are accessed within the environment.  The flags include
m4_ref(DB_INIT_CDB), m4_ref(DB_INIT_LOCK), m4_ref(DB_INIT_LOG),
m4_ref(DB_INIT_MPOOL), and m4_ref(DB_INIT_TXN).  The m4_ref(DB_INIT_CDB)
flag does initialization for m4_cam applications.  (See
m4_link(M4RELDIR/ref/cam/intro, Building m4_cam applications) for more
information.)  The rest of the flags initialize a single subsystem; that
is, when m4_ref(DB_INIT_LOCK) 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.])

m4_tag([Recovery options:], [dnl
These flags, which include m4_ref(DB_RECOVER) and
m4_ref(DB_RECOVER_FATAL), indicate what recovery is to be performed on
the environment before it is opened for normal use.])

m4_tag([Naming options:], [dnl
These flags, which include m4_ref(DB_USE_ENVIRON) and
m4_ref(DB_USE_ENVIRON_ROOT), modify how file naming happens in the
environment.])

m4_tag([Miscellaneous:], [dnl
Finally, there are a number of miscellaneous flags, for example,
m4_ref(DB_CREATE) which causes underlying files to be created as
necessary.  See the m4_ref(dbenv_open) manual pages for further
information.])

m4_tagend

m4_p([dnl
Most applications either specify only the m4_ref(DB_INIT_MPOOL) flag or
they specify all four subsystem initialization flags
(m4_ref(DB_INIT_MPOOL), m4_ref(DB_INIT_LOCK), m4_ref(DB_INIT_LOG), and
m4_ref(DB_INIT_TXN)).  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.])

m4_p([dnl
The m4_ref(DB_RECOVER) 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.])

m4_p([dnl
The m4_ref(DB_RECOVER_FATAL) 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 m4_ref(db_recover) utility
should be used.])

m4_p([dnl
The following is a simple example of a function that opens a database
environment for a transactional program.])

include(ref/env/create.cs)

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