SWI-Prolog BerkeleyDB interface
Jan Wielemaker
VU university Amsterdam
The Netherlands
E-mail: J.Wielemaker@vu.nl
Abstract
This package realised external storage of Prolog terms based on the Berkeley DB library from Sleepycat Software. The DB library implements modular support for the bottom layers of a database. The database itself maps unconstrained keys onto values.

The SWI-Prolog interface for DB allows for fast storage of arbitrary Prolog terms in the database.

# Table of Contents

## 1 Introduction

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.

### 1.1 About this manual

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.

## 2 library(bdb): Berkeley DB interface

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:

1. Initialise the default DB environment using bdb_init/1 or create an explicit DB environment using bdb_init/2. This step is optional, providing simple non-transactional file access when omitted.
2. Open a database using bdb_open/4, returning a handle to the database.
3. Accessing the data using bdb_put/3, bdb_get/3, etc.
4. Closing a database using bdb_close/1. When omitted, all open databases are closed on program halt (see at_halt/1).

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`.

[det]bdb_init(+Options)
[det]bdb_init(-Environment, +Options)
  Initialise a DB environment. The predicate bdb_init/1 initialises the default environment, while bdb_init/2 creates an explicit environment that can be passed to bdb_open/4 using the `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.

  create(+Bool)
    If `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.
  failchk(+Bool)
  home(+Home)
    Specify the DB home directory, the directory holding the database files. The directory must exist prior to calling these predicates.
  init_lock(+Bool)
    Enable locking (`DB_INIT_LOCK`). Implied if transactions are used.
  init_log(+Bool)
    Enable logging the DB modifications (`DB_INIT_LOG`). Logging enables recovery of databases in case of system failure. Normally it is used in combination with transactions.
  init_mpool(+Bool)
    Initialize memory pool. Impicit if `mp_size(+Size)` or `mp_mmapsize(+Size)` is specified.
  init_rep(+Bool)
    Init database replication. The rest of the replication logic is not yet supported.
  init_txn(+Bool)
    Init transactions. Implies `init_log(true)`.
  lockdown(+Bool)
  mp_size(+Integer)
  mp_mmapsize(+Integer)
    Control memory pool handling (`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.
  private(+Bool)
  recover(+Bool)
    Perform recovery before opening the database.
  recover_fatal(+Bool)
    Perform fatal recovery before opening the database.
  register(+Bool)
  server(+Host, [+ServerOptions])
    Initialise the DB package for accessing a remote database. Host specifies the name of the machine running `berkeley_db_svc`. Optionally additional options may be specified:
    server_timeout(+Seconds)
      Specify the timeout time the server uses to determine that the client has gone. This implies the server will terminate the connection to this client if this client does not issue any requests for the indicated time.
    client_timeout(+Seconds)
      Specify the time the client waits for the server to handle a request.
  system_mem(+Bool)
  transactions(+Bool)
    Enable transactions, providing atomicy of changes and security. Implies logging and locking. See bdb_transaction/1.
  thread(+Bool)
    Make the environment accessible from multiple threads.
  thread_count(+Integer)
    Declare an approximate number of threads in the database environment. See `DB_ENV->set_thread_count()`.
  use_environ(+Bool)
  use_environ_root(+Bool)
  config(+ListOfConfig)
    Specify a list of configuration options, each option is of the form Name(Value). Currently unused.
[det]bdb_close_environment(+Environment)
  Close a database environment that was explicitly created using bdb_init/2.
[nondet]bdb_current_environment(-Environment)
  True when Environment is a currently known environment.
[nondet]bdb_environment_property(?Environment, ?Property)
  True when Property is a property of Environment. Defined properties are all boolean options defined with bdb_init/2 and the following options:
  home(-Path)
    Path is the absolute path name for the directory used as database environment.
  open(-Boolean)
    True if the environment is open.
[det]bdb_open(+File, +Mode, -DB, +Options)
  Open File holding a database. Mode is one of `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.
  auto_commit(+Boolean)
    Open the database in a transaction. Ensures no database is created in case of failure.
  create(+Boolean)
    Create a new database of the database does not exist.
  dup(+Boolean)
    Do/do not allow for duplicate values on the same key. Default is not to allow for duplicates.
  excl(+Boolean)
    Combined with `create(true)`, fail if the database already exists.
  multiversion(+Boolean)
    Open the database with support for multiversion concurrency control. The flag is passed, but no further support is provided yet.
  nommap(+Boolean)
    Do not map this database into process memory.
  rdonly(+Boolean)
    Open the database for reading only.
  read_uncommitted(+Boolean)
    Read operations on the database may request the return of modified but not yet committed data. This flag must be specified on all DB handles used to perform dirty reads or database updates, otherwise requests for dirty reads may not be honored and the read may block.
  thread(+Boolean)
    Enable access to the database handle from multiple threads. This is default if the corresponding flag is specified for the environment.
  truncate(+Boolean)
    When specified, truncate the underlying file, i.e., start with an empty database.
  database(+Name)
    If File contains multiple databases, address the named database in the file. A DB file can only consist of multiple databases if the bdb_open/4 call that created it specified this argument. Each database in the file has its own characteristics.
  environment(+Environment)
    Specify a database environment created using bdb_init/2.
  key(+Type)
  value(+Type)
    Specify the type of the key or value. Allowed values are:
    term
      Key/Value is a Prolog term (default). This type allows for representing arbitrary Prolog data in both keys and value. The representation is space-efficient, but Prolog specific. See PL_record_external() in the SWI-Prolog Reference Manual for details on the representation. The other representations are more neutral. This implies they are more stable and sharing the DB with other languages is feasible.
    atom
      Key/Value is an atom. The text is represented as a UTF-8 string and its length.
    c_blob
      Key/Value is a blob (sequence of bytes). On output, a Prolog string is used. The input is either a Prolog string or an atom holding only characters in the range [0..255].
    c_string
      Key/Value is an atom. The text is represented as a C 0-terminated UTF-8 string.
    c_long
      Key/Value is an integer. The value is represented as a native C long in machine byte-order.

  DB | is unified with a blob of type `db`. Database handles are subject to atom garbage collection.

  Errors
    `permission_error(access, bdb_environment, Env)` if an environment is not thread-enabled and accessed from multiple threads.
[det]bdb_close(+DB)
  Close BerkeleyDB database indicated by DB. DB becomes invalid after this operation. An attempt to access a closed database is detected reliably and results in a permission_error exception.
[det]bdb_put(+DB, +Key, +Value)
  Add a new key-value pair to the database. If the database does not allow for duplicates the possible previous associated with Key is replaced by Value.
[nondet]bdb_del(+DB, ?Key, ?Value)
  Delete the first matching key-value pair from the database. If the database allows for duplicates, this predicate is non-deterministic, otherwise it is semidet. The enumeration performed by this predicate is the same as for bdb_get/3. See also bdb_delall/3.
[det]bdb_delall(+DB, +Key, ?Value)
  Delete all matching key-value pairs from the database. With unbound Value the key and all values are removed efficiently.
[nondet]bdb_get(+DB, ?Key, -Value)
  Query the database. If the database allows for duplicates this predicate is non-deterministic, otherwise it is semidet. Note that if Key is a term this matches stored keys that are variants of Key, not unification. See =@=/2. Thus, after `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.
bdb_enum(+DB, -Key, -Value)
  Enumerate the whole database, unifying the key-value pairs to Key and Value. Though this predicate can be used with an instantiated Key to enumerate only the keys unifying with Key, no indexing is used by bdb_enum/3.
[semidet]bdb_getall(+DB, +Key, -Values)
  Get all values associated with Key. Fails if the key does not exist (as bagof/3).
[nondet]bdb_current(?DB)
  True when DB is a handle to a currently open database.
[det]bdb_closeall
  Close all currently open databases and environments. This is called automatically after loading this library on process terminatation using at_halt/1.
[semidet]bdb_transaction(:Goal)
[semidet]bdb_transaction(+Environment, :Goal)
  Start a transaction, execute Goal and terminate the transaction. Only if Goal succeeds, the transaction is commited. If Goal fails or raises an exception, the transaction is aborted and bdb_transaction/1 either fails or rethrows the exception. Of special interest is the exception

      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.

[det]bdb_version(-Version:integer)
  True when Version identifies the database version. Version is an integer defined as:

      DB_VERSION_MAJOR*10000 +
      DB_VERSION_MINOR*100   +
      DB_VERSION_PATCH

## 3 Copyright and license

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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