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SWI-Prolog BerkeleyDB interface |
The SWI-Prolog interface for DB allows for fast storage of arbitrary Prolog terms in the database.
The native Prolog database is not very well suited for either very large data-sets or dynamically changing large data-sets that need to be communicated between Prolog instances or need to be safely guarded against system failure. These cases ask for an external database that can be attached quickly and provides protection against system failure.
The Berkeley DB package is an open source library realising the bottom-layers of a database. It is a modular system, which in it's simplest deals with resource management on a mapped file and in its most complex form deals with network transparency, transaction management, locking, recovery, life-backup, etc.
The DB library maps keys to values. Optionally multiple values can be associated with a key. Both key and value are arbitrary-length binary objects.
This package stores arbitrary Prolog terms into the database, serializing them using PL_record_external(). This provides an interface similar to the recorded-database (recorda/3), which supports terms with internal sharing, cycles and attributes. In addition, it can store restricted data types such as atoms, strings and integers using standard representations which allows for sharing the database with other languages.
This manual is by no means complete. The Berkeley DB documentation should be consulted directly to resolve details on security, resource usage, formats, configuration options etc. This interface passed default values for most DB API calls. Supported options hint to the corresponding DB API calls, which should be consulted for details.
This package realises a binding to Berkeley DB, originally by Sleepycat Software, now managed by Oracle. The DB library implements modular support for the bottom layers of a database. In can be configured for single-threaded access to a file, multi-threaded access with transactions, remote access as well as database replication.
Berkeley DB is an embedded database. This implies the library provides access to a file containing one or more database tables. The Berkeley DB database tables are always binary, mapping a key to a value. The SWI-Prolog interface to Berkeley DB allows for fast storage of arbitrary Prolog terms including cycles and constraints in the database.
Accessing a database consists of four steps:
Errors reported by the underlying database are mapped to an
exception of the form error(bdb(Code,Message,Object), _)
,
where Code is an atom for well known errors and an integer
for less known ones. Message is the return from the db_strerror()
function and Object is the most related Prolog object,
typically a database or database environment handle. If Code
is an atom, it is the lowercase version of the associated C macro after
string the DB_
prefix. Currently the following atom-typed
codes are defined: lock_deadlock
, runrecovery
,
notfound
, keyempty
, keyexist
, lock_notgranted
and
secondary_bad
.
environment(+Environment)
option. If bdb_init/1
is called, it must be called before the first call to bdb_open/4
that uses the default environment. If bdb_init/1
is not called, the default environment can only handle plain files and
does not support multiple threads, locking, crash recovery, etc.
Initializing a BDB environment always requires the home(+Dir)
option. If the environment contains no databases, the argument
create(true)
must be supplied as well.
The currently supported options are listed below. The name of the
boolean options are derived from the DB flags by dropping the =DB_=
prefix and using lowercase, e.g. DB_INIT_LOCK
becomes init_lock
.
For details, please refer to the DB manual.
true
, create any underlying file as required. By
default, no new files are created. This option should be set for
prograns that create new databases.DB_INIT_LOCK
). Implied if transactions are
used.DB_INIT_LOG
). Logging
enables recovery of databases in case of system failure. Normally it is
used in combination with transactions.mp_size(+Size)
or
mp_mmapsize(+Size)
is specified.init_log(true)
.DB_INIT_MPOOL
). The
mp_size
option sets the memory-pool used for caching, while
the mp_mmapsize
controls the maximum size of a DB file
mapped entirely into memory.berkeley_db_svc
. Optionally additional options may be
specified:
DB_ENV->set_thread_count()
.read
,
providing read-only access or update
, providing read/write
access.
Options is a list of options. Supported options are below.
The boolean options are passed as flags to DB->open()
.
The option name is derived from the flag name by stripping the
DB_
prefix and converting to lower case. Consult the
Berkeley DB documentation for details.
create(true)
, fail if the database already
exists.DB | is unified with a blob of type db .
Database handles are subject to atom garbage collection. |
permission_error(access, bdb_environment, Env)
if an
environment is not thread-enabled and accessed from multiple threads.bdb_put(DB, f(X), 42)
, we get the following query results:
bdb_get(DB, f(Y), V)
binds Value to 42
,
while Y is left unbound.bdb_get(DB, f(a), V)
fails.bdb_enum(DB, f(a), V)
succeeds, but does not perform
any indexing, i.e., it enumerates all key-value pairs and performs the
unification.error(package(db, deadlock), _)
This exception indicates a deadlock was raised by one of the DB predicates. Deadlocks may arise if multiple processes or threads access the same keys in a different order. The DB infra-structure causes one of the processes involved in the deadlock to abort its transaction. This process may choose to restart the transaction.
For example, a DB application may define {Goal}
to
realise transactions and restart these automatically is a deadlock is
raised:
{Goal} :- catch(bdb_transaction(Goal), E, true), ( var(E) -> true ; E = error(package(db, deadlock), _) -> {Goal} ; throw(E) ).
Environment | defines the environment to which the transaction applies. If omitted, the default environment is used. See bdb_init/1 and bdb_init/2. |
DB_VERSION_MAJOR*10000 + DB_VERSION_MINOR*100 + DB_VERSION_PATCH
The SWI-Prolog interface code is licensed under the same conditions as SWI-Prolog itself.
The following is the license that applies to this copy of the Berkeley DB software. For a license to use the Berkeley DB software under conditions other than those described here, or to purchase support for this software, please contact Oracle at berkeleydb-info_us@oracle.com.
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