/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2006-2015, University of Amsterdam
                              VU University Amsterdam
    All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
    are met:

    1. Redistributions of source code must retain the above copyright
       notice, this list of conditions and the following disclaimer.

    2. Redistributions in binary form must reproduce the above copyright
       notice, this list of conditions and the following disclaimer in
       the documentation and/or other materials provided with the
       distribution.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
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*/

:- module(rdf_persistency,
          [ rdf_attach_db/2,            % +Directory, +Options
            rdf_detach_db/0,            % +Detach current Graph
            rdf_current_db/1,           % -Directory
            rdf_persistency/2,          % +Graph, +Bool
            rdf_flush_journals/1,       % +Options
            rdf_persistency_property/1, % ?Property
            rdf_journal_file/2,         % ?Graph, ?JournalFile
            rdf_snapshot_file/2,        % ?Graph, ?SnapshotFile
            rdf_db_to_file/2            % ?Graph, ?FileBase
          ]).
:- use_module(library(semweb/rdf_db)).
:- use_module(library(filesex)).
:- use_module(library(lists)).
:- use_module(library(uri)).
:- use_module(library(debug)).
:- use_module(library(option)).
:- use_module(library(error)).
:- use_module(library(thread)).
:- use_module(library(apply)).


/** <module> RDF persistency plugin

This  module  provides  persistency   for    rdf_db.pl   based   on  the
rdf_monitor/2 predicate to  track  changes   to  the  repository.  Where
previous  versions  used  autosafe  of  the  whole  database  using  the
quick-load format of rdf_db, this version is  based on a quick-load file
per source (4th argument of rdf/4), and journalling for edit operations.

The result is safe, avoids frequent small   changes to large files which
makes synchronisation and backup expensive and avoids long disruption of
the server doing the autosafe. Only loading large files disrupts service
for some time.

The persistent backup of the database is  realised in a directory, using
a lock file to avoid corruption due to concurrent access. Each source is
represented by two files, the latest snapshot   and a journal. The state
is restored by loading  the  snapshot   and  replaying  the journal. The
predicate rdf_flush_journals/1 can be used to create fresh snapshots and
delete the journals.

@tbd If there is a complete `.new'   snapshot  and no journal, we should
     move the .new to the plain snapshot name as a means of recovery.

@tbd Backup of each graph using one or two files is very costly if there
     are many graphs.  Although the currently used subdirectories avoid
     hitting OS limits early, this is still not ideal. Probably we
     should collect (small, older?) files and combine them into a single
     quick load file.  We could call this (similar to GIT) a `pack'.

@see    rdf_edit.pl
*/

:- volatile
    rdf_directory/1,
    rdf_lock/2,
    rdf_option/1,
    source_journal_fd/2,
    file_base_db/2.
:- dynamic
    rdf_directory/1,                % Absolute path
    rdf_lock/2,                     % Dir, Lock
    rdf_option/1,                   % Defined options
    source_journal_fd/2,            % DB, JournalFD
    file_base_db/2.                 % FileBase, DB

:- meta_predicate
    no_agc(0).

:- predicate_options(rdf_attach_db/2, 2,
                     [ access(oneof([read_write,read_only])),
                       concurrency(positive_integer),
                       max_open_journals(positive_integer),
                       silent(oneof([true,false,brief])),
                       log_nested_transactions(boolean)
                     ]).

%!  rdf_attach_db(+Directory, +Options) is det.
%
%   Start persistent operations using Directory   as  place to store
%   files.   There are several cases:
%
%           * Empty DB, existing directory
%           Load the DB from the existing directory
%
%           * Full DB, empty directory
%           Create snapshots for all sources in directory
%
%   Options:
%
%           * access(+AccessMode)
%           One of =auto= (default), =read_write= or
%           =read_only=. Read-only access implies that the RDF
%           store is not locked. It is read at startup and all
%           modifications to the data are temporary. The default
%           =auto= mode is =read_write= if the directory is
%           writeable and the lock can be acquired.  Otherwise
%           it reverts to =read_only=.
%
%           * concurrency(+Jobs)
%           Number of threads to use for loading the initial
%           database.  If not provided it is the number of CPUs
%           as optained from the flag =cpu_count=.
%
%           * max_open_journals(+Count)
%           Maximum number of journals kept open.  If not provided,
%           the default is 10.  See limit_fd_pool/0.
%
%           * directory_levels(+Count)
%           Number of levels of intermediate directories for storing
%           the graph files.  Default is 2.
%
%           * silent(+BoolOrBrief)
%           If =true= (default =false=), do not print informational
%           messages.  Finally, if =brief= it will show minimal
%           feedback.
%
%           * log_nested_transactions(+Boolean)
%           If =true=, nested _log_ transactions are added to the
%           journal information.  By default (=false=), no log-term
%           is added for nested transactions.\\
%
%   @error existence_error(source_sink, Directory)
%   @error permission_error(write, directory, Directory)

rdf_attach_db(DirSpec, Options) :-
    option(access(read_only), Options),
    !,
    absolute_file_name(DirSpec,
                       Directory,
                       [ access(read),
                         file_type(directory)
                       ]),
    rdf_attach_db_ro(Directory, Options).
rdf_attach_db(DirSpec, Options) :-
    option(access(read_write), Options),
    !,
    rdf_attach_db_rw(DirSpec, Options).
rdf_attach_db(DirSpec, Options) :-
    absolute_file_name(DirSpec,
                       Directory,
                       [ access(exist),
                         file_type(directory),
                         file_errors(fail)
                       ]),
    !,
    (   access_file(Directory, write)
    ->  catch(rdf_attach_db_rw(Directory, Options), E, true),
        (   var(E)
        ->  true
        ;   E = error(permission_error(lock, rdf_db, _), _)
        ->  print_message(warning, E),
            print_message(warning, rdf(read_only)),
            rdf_attach_db(DirSpec, [access(read_only)|Options])
        ;   throw(E)
        )
    ;   print_message(warning,
                      error(permission_error(write, directory, Directory))),
        print_message(warning, rdf(read_only)),
        rdf_attach_db_ro(Directory, Options)
    ).
rdf_attach_db(DirSpec, Options) :-
    catch(rdf_attach_db_rw(DirSpec, Options), E, true),
    (   var(E)
    ->  true
    ;   print_message(warning, E),
        print_message(warning, rdf(read_only)),
        rdf_attach_db(DirSpec, [access(read_only)|Options])
    ).


rdf_attach_db_rw(DirSpec, Options) :-
    absolute_file_name(DirSpec,
                       Directory,
                       [ access(write),
                         file_type(directory),
                         file_errors(fail)
                       ]),
    !,
    (   rdf_directory(Directory)
    ->  true                        % update settings?
    ;   rdf_detach_db,
        mkdir(Directory),
        lock_db(Directory),
        assert(rdf_directory(Directory)),
        assert_options(Options),
        stop_monitor,               % make sure not to register load
        no_agc(load_db),
        at_halt(rdf_detach_db),
        start_monitor
    ).
rdf_attach_db_rw(DirSpec, Options) :-
    absolute_file_name(DirSpec,
                       Directory,
                       [ solutions(all)
                       ]),
    (   exists_directory(Directory)
    ->  access_file(Directory, write)
    ;   catch(make_directory(Directory), _, fail)
    ),
    !,
    rdf_attach_db(Directory, Options).
rdf_attach_db_rw(DirSpec, _) :-         % Generate an existence or
    absolute_file_name(DirSpec,     % permission error
                       Directory,
                       [ access(exist),
                         file_type(directory)
                       ]),
    permission_error(write, directory, Directory).

%!  rdf_attach_db_ro(+Directory, +Options)
%
%   Open an RDF database in read-only mode.

rdf_attach_db_ro(Directory, Options) :-
    rdf_detach_db,
    assert(rdf_directory(Directory)),
    assert_options(Options),
    stop_monitor,           % make sure not to register load
    no_agc(load_db).


assert_options([]).
assert_options([H|T]) :-
    (   option_type(H, Check)
    ->  Check,
        assert(rdf_option(H))
    ;   true                        % ignore options we do not understand
    ),
    assert_options(T).

option_type(concurrency(X),             must_be(positive_integer, X)).
option_type(max_open_journals(X),       must_be(positive_integer, X)).
option_type(directory_levels(X),        must_be(positive_integer, X)).
option_type(silent(X),                  must_be(oneof([true,false,brief]), X)).
option_type(log_nested_transactions(X), must_be(boolean, X)).
option_type(access(X),                  must_be(oneof([read_write,
                                                       read_only]), X)).


%!  rdf_persistency_property(?Property) is nondet.
%
%   True when Property is a property of the current persistent database.
%   Exposes  the  properties  that  can   be    passed   as  options  to
%   rdf_attach_db/2.                                       Specifically,
%   rdf_persistency_property(access(read_only)) is true iff the database
%   is mounted in read-only mode. In addition, the following property is
%   supported:
%
%     - directory(Dir)
%     The directory in which the database resides.

rdf_persistency_property(Property) :-
    var(Property),
    !,
    rdf_persistency_property_(Property).
rdf_persistency_property(Property) :-
    rdf_persistency_property_(Property),
    !.

rdf_persistency_property_(Property) :-
    rdf_option(Property).
rdf_persistency_property_(directory(Dir)) :-
    rdf_directory(Dir).

%!  no_agc(:Goal)
%
%   Run Goal with atom garbage collection   disabled. Loading an RDF
%   database creates large amounts  of  atoms   we  *know*  are  not
%   garbage.

no_agc(Goal) :-
    current_prolog_flag(agc_margin, Old),
    setup_call_cleanup(
        set_prolog_flag(agc_margin, 0),
        Goal,
        set_prolog_flag(agc_margin, Old)).


%!  rdf_detach_db is det.
%
%   Detach from the  current  database.   Succeeds  silently  if  no
%   database is attached. Normally called at  the end of the program
%   through at_halt/1.

rdf_detach_db :-
    debug(halt, 'Detaching RDF database', []),
    stop_monitor,
    close_journals,
    (   retract(rdf_directory(Dir))
    ->  debug(halt, 'DB Directory: ~w', [Dir]),
        save_prefixes(Dir),
        retractall(rdf_option(_)),
        retractall(source_journal_fd(_,_)),
        unlock_db(Dir)
    ;   true
    ).


%!  rdf_current_db(?Dir)
%
%   True if Dir is the current RDF persistent database.

rdf_current_db(Directory) :-
    rdf_directory(Dir),
    !,
    Dir = Directory.


%!  rdf_flush_journals(+Options)
%
%   Flush dirty journals.  Options:
%
%           * min_size(+KB)
%           Only flush if journal is over KB in size.
%           * graph(+Graph)
%           Only flush the journal of Graph
%
%   @tbd Provide a default for min_size?

rdf_flush_journals(Options) :-
    option(graph(Graph), Options, _),
    forall(rdf_graph(Graph),
           rdf_flush_journal(Graph, Options)).

rdf_flush_journal(Graph, Options) :-
    db_files(Graph, _SnapshotFile, JournalFile),
    db_file(JournalFile, File),
    (   \+ exists_file(File)
    ->  true
    ;   memberchk(min_size(KB), Options),
        size_file(JournalFile, Size),
        Size / 1024 < KB
    ->  true
    ;   create_db(Graph)
    ).

                 /*******************************
                 *             LOAD             *
                 *******************************/

%!  load_db is det.
%
%   Reload database from the directory specified by rdf_directory/1.
%   First we find all names graphs using find_dbs/1 and then we load
%   them.

load_db :-
    rdf_directory(Dir),
    concurrency(Jobs),
    cpu_stat_key(Jobs, StatKey),
    get_time(Wall0),
    statistics(StatKey, T0),
    load_prefixes(Dir),
    verbosity(Silent),
    find_dbs(Dir, Graphs, SnapShots, Journals),
    length(Graphs, GraphCount),
    maplist(rdf_unload_graph, Graphs),
    rdf_statistics(triples(Triples0)),
    load_sources(snapshots, SnapShots, Silent, Jobs),
    load_sources(journals, Journals, Silent, Jobs),
    rdf_statistics(triples(Triples1)),
    statistics(StatKey, T1),
    get_time(Wall1),
    T is T1 - T0,
    Wall is Wall1 - Wall0,
    Triples = Triples1 - Triples0,
    message_level(Silent, Level),
    print_message(Level, rdf(restore(attached(GraphCount, Triples, T/Wall)))).

load_sources(_, [], _, _) :- !.
load_sources(Type, Sources, Silent, Jobs) :-
    length(Sources, Count),
    RunJobs is min(Count, Jobs),
    print_message(informational, rdf(restoring(Type, Count, RunJobs))),
    make_goals(Sources, Silent, 1, Count, Goals),
    concurrent(RunJobs, Goals, []).


%!  make_goals(+DBs, +Silent, +Index, +Total, -Goals)

make_goals([], _, _, _, []).
make_goals([DB|T0], Silent, I,  Total,
           [load_source(DB, Silent, I, Total)|T]) :-
    I2 is I + 1,
    make_goals(T0, Silent, I2, Total, T).

verbosity(Silent) :-
    rdf_option(silent(Silent)),
    !.
verbosity(Silent) :-
    current_prolog_flag(verbose, silent),
    !,
    Silent = true.
verbosity(brief).


%!  concurrency(-Jobs)
%
%   Number of jobs to run concurrently.

concurrency(Jobs) :-
    rdf_option(concurrency(Jobs)),
    !.
concurrency(Jobs) :-
    current_prolog_flag(cpu_count, Jobs),
    Jobs > 0,
    !.
concurrency(1).

cpu_stat_key(1, cputime) :- !.
cpu_stat_key(_, process_cputime).


%!  find_dbs(+Dir, -Graphs, -SnapBySize, -JournalBySize) is det.
%
%   Scan the persistent database and return a list of snapshots and
%   journals, both sorted by file-size.  Each term is of the form
%
%     ==
%     db(Size, Ext, DB, DBFile, Depth)
%     ==

find_dbs(Dir, Graphs, SnapBySize, JournalBySize) :-
    directory_files(Dir, Files),
    phrase(scan_db_files(Files, Dir, '.', 0), Scanned),
    maplist(db_graph, Scanned, UnsortedGraphs),
    sort(UnsortedGraphs, Graphs),
    (   consider_reindex_db(Dir, Graphs, Scanned)
    ->  find_dbs(Dir, Graphs, SnapBySize, JournalBySize)
    ;   partition(db_is_snapshot, Scanned, Snapshots, Journals),
        sort(Snapshots, SnapBySize),
        sort(Journals, JournalBySize)
    ).

consider_reindex_db(Dir, Graphs, Scanned) :-
    length(Graphs, Count),
    Count > 0,
    DepthNeeded is floor(log(Count)/log(256)),
    (   maplist(depth_db(DepthNow), Scanned)
    ->  (   DepthNeeded > DepthNow
        ->  true
        ;   retractall(rdf_option(directory_levels(_))),
            assertz(rdf_option(directory_levels(DepthNow))),
            fail
        )
    ;   true
    ),
    reindex_db(Dir, DepthNeeded).

db_is_snapshot(Term) :-
    arg(2, Term, trp).

db_graph(Term, DB) :-
    arg(3, Term, DB).

db_file_name(Term, File) :-
    arg(4, Term, File).

depth_db(Depth, DB) :-
    arg(5, DB, Depth).

%!  scan_db_files(+Files, +Dir, +Prefix, +Depth)// is det.
%
%   Produces a list of db(DB,  Size,   File)  for all recognised RDF
%   database files.  File is relative to the database directory Dir.

scan_db_files([], _, _, _) -->
    [].
scan_db_files([Nofollow|T], Dir, Prefix, Depth) -->
    { nofollow(Nofollow) },
    !,
    scan_db_files(T, Dir, Prefix, Depth).
scan_db_files([File|T], Dir, Prefix, Depth) -->
    { file_name_extension(Base, Ext, File),
      db_extension(Ext),
      !,
      rdf_db_to_file(DB, Base),
      directory_file_path(Prefix, File, DBFile),
      directory_file_path(Dir, DBFile, AbsFile),
      size_file(AbsFile, Size)
    },
    [ db(Size, Ext, DB, AbsFile, Depth) ],
    scan_db_files(T, Dir, Prefix, Depth).
scan_db_files([D|T], Dir, Prefix, Depth) -->
    { directory_file_path(Prefix, D, SubD),
      directory_file_path(Dir, SubD, AbsD),
      exists_directory(AbsD),
      \+ read_link(AbsD, _, _),    % Do not follow links
      !,
      directory_files(AbsD, SubFiles),
      SubDepth is Depth + 1
    },
    scan_db_files(SubFiles, Dir, SubD, SubDepth),
    scan_db_files(T, Dir, Prefix, Depth).
scan_db_files([_|T], Dir, Prefix, Depth) -->
    scan_db_files(T, Dir, Prefix, Depth).

nofollow(.).
nofollow(..).

db_extension(trp).
db_extension(jrn).

:- public load_source/4.                % called through make_goals/5

load_source(DB, Silent, Nth, Total) :-
    db_file_name(DB, File),
    db_graph(DB, Graph),
    message_level(Silent, Level),
    graph_triple_count(Graph, Count0),
    statistics(cputime, T0),
    (   db_is_snapshot(DB)
    ->  print_message(Level, rdf(restore(Silent, snapshot(Graph, File)))),
        rdf_load_db(File)
    ;   print_message(Level, rdf(restore(Silent, journal(Graph, File)))),
        load_journal(File, Graph)
    ),
    statistics(cputime, T1),
    T is T1 - T0,
    graph_triple_count(Graph, Count1),
    Count is Count1 - Count0,
    print_message(Level, rdf(restore(Silent,
                                     done(Graph, T, Count, Nth, Total)))).


graph_triple_count(Graph, Count) :-
    rdf_statistics(triples_by_graph(Graph, Count)),
    !.
graph_triple_count(_, 0).


%!  attach_graph(+Graph, +Options) is det.
%
%   Load triples and reload  journal   from  the  indicated snapshot
%   file.

attach_graph(Graph, Options) :-
    (   option(silent(true), Options)
    ->  Level = silent
    ;   Level = informational
    ),
    db_files(Graph, SnapshotFile, JournalFile),
    rdf_retractall(_,_,_,Graph),
    statistics(cputime, T0),
    print_message(Level, rdf(restore(Silent, Graph))),
    db_file(SnapshotFile, AbsSnapShot),
    (   exists_file(AbsSnapShot)
    ->  print_message(Level, rdf(restore(Silent, snapshot(SnapshotFile)))),
        rdf_load_db(AbsSnapShot)
    ;   true
    ),
    (   exists_db(JournalFile)
    ->  print_message(Level, rdf(restore(Silent, journal(JournalFile)))),
        load_journal(JournalFile, Graph)
    ;   true
    ),
    statistics(cputime, T1),
    T is T1 - T0,
    (   rdf_statistics(triples_by_graph(Graph, Count))
    ->  true
    ;   Count = 0
    ),
    print_message(Level, rdf(restore(Silent,
                                     done(Graph, T, Count)))).

message_level(true, silent) :- !.
message_level(_, informational).


                 /*******************************
                 *         LOAD JOURNAL         *
                 *******************************/

%!  load_journal(+File:atom, +DB:atom) is det.
%
%   Process transactions from the RDF journal File, adding the given
%   named graph.

load_journal(File, DB) :-
    rdf_create_graph(DB),
    setup_call_cleanup(
        open(File, read, In, [encoding(utf8)]),
        ( read(In, T0),
          process_journal(T0, In, DB)
        ),
        close(In)).

process_journal(end_of_file, _, _) :- !.
process_journal(Term, In, DB) :-
    (   process_journal_term(Term, DB)
    ->  true
    ;   throw(error(type_error(journal_term, Term), _))
    ),
    read(In, T2),
    process_journal(T2, In, DB).

process_journal_term(assert(S,P,O), DB) :-
    rdf_assert(S,P,O,DB).
process_journal_term(assert(S,P,O,Line), DB) :-
    rdf_assert(S,P,O,DB:Line).
process_journal_term(retract(S,P,O), DB) :-
    rdf_retractall(S,P,O,DB).
process_journal_term(retract(S,P,O,Line), DB) :-
    rdf_retractall(S,P,O,DB:Line).
process_journal_term(update(S,P,O,Action), DB) :-
    (   rdf_update(S,P,O,DB, Action)
    ->  true
    ;   print_message(warning, rdf(update_failed(S,P,O,Action)))
    ).
process_journal_term(start(_), _).      % journal open/close
process_journal_term(end(_), _).
process_journal_term(begin(_), _).      % logged transaction (compatibility)
process_journal_term(end, _).
process_journal_term(begin(_,_,_,_), _). % logged transaction (current)
process_journal_term(end(_,_,_), _).


                 /*******************************
                 *         CREATE JOURNAL       *
                 *******************************/

:- dynamic
    blocked_db/2,                   % DB, Reason
    transaction_message/3,          % Nesting, Time, Message
    transaction_db/3.               % Nesting, DB, Id

%!  rdf_persistency(+DB, Bool)
%
%   Specify whether a database is persistent.  Switching to =false=
%   kills the persistent state.  Switching to =true= creates it.

rdf_persistency(DB, Bool) :-
    must_be(atom, DB),
    must_be(boolean, Bool),
    fail.
rdf_persistency(DB, false) :-
    !,
    (   blocked_db(DB, persistency)
    ->  true
    ;   assert(blocked_db(DB, persistency)),
        delete_db(DB)
    ).
rdf_persistency(DB, true) :-
    (   retract(blocked_db(DB, persistency))
    ->  create_db(DB)
    ;   true
    ).

%!  rdf_db:property_of_graph(?Property, +Graph) is nondet.
%
%   Extend rdf_graph_property/2 with new properties.

:- multifile
    rdf_db:property_of_graph/2.

rdf_db:property_of_graph(persistent(State), Graph) :-
    (   blocked_db(Graph, persistency)
    ->  State = false
    ;   State = true
    ).


%!  start_monitor is det.
%!  stop_monitor is det.
%
%   Start/stop monitoring the RDF database   for  changes and update
%   the journal.

start_monitor :-
    rdf_monitor(monitor,
                [ -assert(load)
                ]).
stop_monitor :-
    rdf_monitor(monitor,
                [ -all
                ]).

%!  monitor(+Term) is semidet.
%
%   Handle an rdf_monitor/2 callback to  deal with persistency. Note
%   that the monitor calls that come   from rdf_db.pl that deal with
%   database changes are serialized.  They   do  come from different
%   threads though.

monitor(Msg) :-
    debug(monitor, 'Monitor: ~p~n', [Msg]),
    fail.
monitor(assert(S,P,O,DB:Line)) :-
    !,
    \+ blocked_db(DB, _),
    journal_fd(DB, Fd),
    open_transaction(DB, Fd),
    format(Fd, '~q.~n', [assert(S,P,O,Line)]),
    sync_journal(DB, Fd).
monitor(assert(S,P,O,DB)) :-
    \+ blocked_db(DB, _),
    journal_fd(DB, Fd),
    open_transaction(DB, Fd),
    format(Fd, '~q.~n', [assert(S,P,O)]),
    sync_journal(DB, Fd).
monitor(retract(S,P,O,DB:Line)) :-
    !,
    \+ blocked_db(DB, _),
    journal_fd(DB, Fd),
    open_transaction(DB, Fd),
    format(Fd, '~q.~n', [retract(S,P,O,Line)]),
    sync_journal(DB, Fd).
monitor(retract(S,P,O,DB)) :-
    \+ blocked_db(DB, _),
    journal_fd(DB, Fd),
    open_transaction(DB, Fd),
    format(Fd, '~q.~n', [retract(S,P,O)]),
    sync_journal(DB, Fd).
monitor(update(S,P,O,DB:Line,Action)) :-
    !,
    \+ blocked_db(DB, _),
    (   Action = graph(NewDB)
    ->  monitor(assert(S,P,O,NewDB)),
        monitor(retract(S,P,O,DB:Line))
    ;   journal_fd(DB, Fd),
        format(Fd, '~q.~n', [update(S,P,O,Action)]),
        sync_journal(DB, Fd)
    ).
monitor(update(S,P,O,DB,Action)) :-
    \+ blocked_db(DB, _),
    (   Action = graph(NewDB)
    ->  monitor(assert(S,P,O,NewDB)),
        monitor(retract(S,P,O,DB))
    ;   journal_fd(DB, Fd),
        open_transaction(DB, Fd),
        format(Fd, '~q.~n', [update(S,P,O,Action)]),
        sync_journal(DB, Fd)
    ).
monitor(load(BE, _DumpFileURI)) :-
    (   BE = end(Graphs)
    ->  sync_loaded_graphs(Graphs)
    ;   true
    ).
monitor(create_graph(Graph)) :-
    \+ blocked_db(Graph, _),
    journal_fd(Graph, Fd),
    open_transaction(Graph, Fd),
    sync_journal(Graph, Fd).
monitor(reset) :-
    forall(rdf_graph(Graph), delete_db(Graph)).
                                        % TBD: Remove empty directories?

monitor(transaction(BE, Id)) :-
    monitor_transaction(Id, BE).

monitor_transaction(load_journal(DB), begin(_)) :-
    !,
    assert(blocked_db(DB, journal)).
monitor_transaction(load_journal(DB), end(_)) :-
    !,
    retractall(blocked_db(DB, journal)).

monitor_transaction(parse(URI), begin(_)) :-
    !,
    (   blocked_db(URI, persistency)
    ->  true
    ;   assert(blocked_db(URI, parse))
    ).
monitor_transaction(parse(URI), end(_)) :-
    !,
    (   retract(blocked_db(URI, parse))
    ->  create_db(URI)
    ;   true
    ).
monitor_transaction(unload(DB), begin(_)) :-
    !,
    (   blocked_db(DB, persistency)
    ->  true
    ;   assert(blocked_db(DB, unload))
    ).
monitor_transaction(unload(DB), end(_)) :-
    !,
    (   retract(blocked_db(DB, unload))
    ->  delete_db(DB)
    ;   true
    ).
monitor_transaction(log(Msg), begin(N)) :-
    !,
    check_nested(N),
    get_time(Time),
    asserta(transaction_message(N, Time, Msg)).
monitor_transaction(log(_), end(N)) :-
    check_nested(N),
    retract(transaction_message(N, _, _)),
    !,
    findall(DB:Id, retract(transaction_db(N, DB, Id)), DBs),
    end_transactions(DBs, N).
monitor_transaction(log(Msg, DB), begin(N)) :-
    !,
    check_nested(N),
    get_time(Time),
    asserta(transaction_message(N, Time, Msg)),
    journal_fd(DB, Fd),
    open_transaction(DB, Fd).
monitor_transaction(log(Msg, _DB), end(N)) :-
    monitor_transaction(log(Msg), end(N)).


%!  check_nested(+Level) is semidet.
%
%   True if we must log this transaction.   This  is always the case
%   for toplevel transactions. Nested transactions   are only logged
%   if log_nested_transactions(true) is defined.

check_nested(0) :- !.
check_nested(_) :-
    rdf_option(log_nested_transactions(true)).


%!  open_transaction(+DB, +Fd) is det.
%
%   Add a begin(Id, Level, Time,  Message)   term  if  a transaction
%   involves DB. Id is an incremental   integer, where each database
%   has its own counter. Level is the nesting level, Time a floating
%   point timestamp and Message te message   provided as argument to
%   the log message.

open_transaction(DB, Fd) :-
    transaction_message(N, Time, Msg),
    !,
    (   transaction_db(N, DB, _)
    ->  true
    ;   next_transaction_id(DB, Id),
        assert(transaction_db(N, DB, Id)),
        RoundedTime is round(Time*100)/100,
        format(Fd, '~q.~n', [begin(Id, N, RoundedTime, Msg)])
    ).
open_transaction(_,_).


%!  next_transaction_id(+DB, -Id) is det.
%
%   Id is the number to user for  the next logged transaction on DB.
%   Transactions in each  named  graph   are  numbered  in sequence.
%   Searching the Id of the last transaction is performed by the 2nd
%   clause starting 1Kb from the end   and doubling this offset each
%   failure.

:- dynamic
    current_transaction_id/2.

next_transaction_id(DB, Id) :-
    retract(current_transaction_id(DB, Last)),
    !,
    Id is Last + 1,
    assert(current_transaction_id(DB, Id)).
next_transaction_id(DB, Id) :-
    db_files(DB, _, Journal),
    exists_file(Journal),
    !,
    size_file(Journal, Size),
    open_db(Journal, read, In, []),
    call_cleanup(iterative_expand(In, Size, Last), close(In)),
    Id is Last + 1,
    assert(current_transaction_id(DB, Id)).
next_transaction_id(DB, 1) :-
    assert(current_transaction_id(DB, 1)).

iterative_expand(_, 0, 0) :- !.
iterative_expand(In, Size, Last) :-     % Scan growing sections from the end
    Max is floor(log(Size)/log(2)),
    between(10, Max, Step),
    Offset is -(1<<Step),
    seek(In, Offset, eof, _),
    skip(In, 10),                   % records are line-based
    read(In, T0),
    last_transaction_id(T0, In, 0, Last),
    Last > 0,
    !.
iterative_expand(In, _, Last) :-        % Scan the whole file
    seek(In, 0, bof, _),
    read(In, T0),
    last_transaction_id(T0, In, 0, Last).

last_transaction_id(end_of_file, _, Last, Last) :- !.
last_transaction_id(end(Id, _, _), In, _, Last) :-
    read(In, T1),
    last_transaction_id(T1, In, Id, Last).
last_transaction_id(_, In, Id, Last) :-
    read(In, T1),
    last_transaction_id(T1, In, Id, Last).


%!  end_transactions(+DBs:list(atom:id)) is det.
%
%   End a transaction that affected the  given list of databases. We
%   write the list of other affected databases as an argument to the
%   end-term to facilitate fast finding of the related transactions.
%
%   In each database, the transaction is   ended with a term end(Id,
%   Nesting, Others), where  Id  and   Nesting  are  the transaction
%   identifier and nesting (see open_transaction/2)  and Others is a
%   list of DB:Id,  indicating  other   databases  affected  by  the
%   transaction.

end_transactions(DBs, N) :-
    end_transactions(DBs, DBs, N).

end_transactions([], _, _).
end_transactions([DB:Id|T], DBs, N) :-
    journal_fd(DB, Fd),
    once(select(DB:Id, DBs, Others)),
    format(Fd, 'end(~q, ~q, ~q).~n', [Id, N, Others]),
    sync_journal(DB, Fd),
    end_transactions(T, DBs, N).


%!  sync_loaded_graphs(+Graphs)
%
%   Called after a binary triple has been loaded that added triples
%   to the given graphs.

sync_loaded_graphs(Graphs) :-
    maplist(create_db, Graphs).


                 /*******************************
                 *         JOURNAL FILES        *
                 *******************************/

%!  journal_fd(+DB, -Stream) is det.
%
%   Get an open stream to a journal. If the journal is not open, old
%   journals are closed to satisfy   the =max_open_journals= option.
%   Then the journal is opened in   =append= mode. Journal files are
%   always encoded as UTF-8 for  portability   as  well as to ensure
%   full coverage of Unicode.

journal_fd(DB, Fd) :-
    source_journal_fd(DB, Fd),
    !.
journal_fd(DB, Fd) :-
    with_mutex(rdf_journal_file,
               journal_fd_(DB, Out)),
    Fd = Out.

journal_fd_(DB, Fd) :-
    source_journal_fd(DB, Fd),
    !.
journal_fd_(DB, Fd) :-
    limit_fd_pool,
    db_files(DB, _Snapshot, Journal),
    open_db(Journal, append, Fd,
            [ close_on_abort(false)
            ]),
    time_stamp(Now),
    format(Fd, '~q.~n', [start([time(Now)])]),
    assert(source_journal_fd(DB, Fd)).              % new one at the end

%!  limit_fd_pool is det.
%
%   Limit the number of  open   journals  to max_open_journals (10).
%   Note that calls  from  rdf_monitor/2   are  issued  in different
%   threads, but as they are part of write operations they are fully
%   synchronised.

limit_fd_pool :-
    predicate_property(source_journal_fd(_, _), number_of_clauses(N)),
    !,
    (   rdf_option(max_open_journals(Max))
    ->  true
    ;   Max = 10
    ),
    Close is N - Max,
    forall(between(1, Close, _),
           close_oldest_journal).
limit_fd_pool.

close_oldest_journal :-
    source_journal_fd(DB, _Fd),
    !,
    debug(rdf_persistency, 'Closing old journal for ~q', [DB]),
    close_journal(DB).
close_oldest_journal.


%!  sync_journal(+DB, +Fd)
%
%   Sync journal represented by database and   stream.  If the DB is
%   involved in a transaction there is   no point flushing until the
%   end of the transaction.

sync_journal(DB, _) :-
    transaction_db(_, DB, _),
    !.
sync_journal(_, Fd) :-
    flush_output(Fd).

%!  close_journal(+DB) is det.
%
%   Close the journal associated with DB if it is open.

close_journal(DB) :-
    with_mutex(rdf_journal_file,
               close_journal_(DB)).

close_journal_(DB) :-
    (   retract(source_journal_fd(DB, Fd))
    ->  time_stamp(Now),
        format(Fd, '~q.~n', [end([time(Now)])]),
        close(Fd, [force(true)])
    ;   true
    ).

%!  close_journals
%
%   Close all open journals.

close_journals :-
    forall(source_journal_fd(DB, _),
           catch(close_journal(DB), E,
                 print_message(error, E))).

%!  create_db(+Graph)
%
%   Create a saved version of Graph in corresponding file, close and
%   delete journals.

create_db(Graph) :-
    \+ rdf(_,_,_,Graph),
    !,
    debug(rdf_persistency, 'Deleting empty Graph ~w', [Graph]),
    delete_db(Graph).
create_db(Graph) :-
    debug(rdf_persistency, 'Saving Graph ~w', [Graph]),
    close_journal(Graph),
    db_abs_files(Graph, Snapshot, Journal),
    atom_concat(Snapshot, '.new', NewSnapshot),
    (   catch(( create_directory_levels(Snapshot),
                rdf_save_db(NewSnapshot, Graph)
              ), Error,
              ( print_message(warning, Error),
                fail
              ))
    ->  (   exists_file(Journal)
        ->  delete_file(Journal)
        ;   true
        ),
        rename_file(NewSnapshot, Snapshot),
        debug(rdf_persistency, 'Saved Graph ~w', [Graph])
    ;   catch(delete_file(NewSnapshot), _, true)
    ).


%!  delete_db(+DB)
%
%   Remove snapshot and journal file for DB.

delete_db(DB) :-
    with_mutex(rdf_journal_file,
               delete_db_(DB)).

delete_db_(DB) :-
    close_journal_(DB),
    db_abs_files(DB, Snapshot, Journal),
    !,
    (   exists_file(Journal)
    ->  delete_file(Journal)
    ;   true
    ),
    (   exists_file(Snapshot)
    ->  delete_file(Snapshot)
    ;   true
    ).
delete_db_(_).

                 /*******************************
                 *             LOCKING          *
                 *******************************/

%!  lock_db(+Dir)
%
%   Lock the database directory Dir.

lock_db(Dir) :-
    lockfile(Dir, File),
    catch(open(File, update, Out, [lock(write), wait(false)]),
          error(permission_error(Access, _, _), _),
          locked_error(Access, Dir)),
    (   current_prolog_flag(pid, PID)
    ->  true
    ;   PID = 0                     % TBD: Fix in Prolog
    ),
    time_stamp(Now),
    gethostname(Host),
    format(Out, '/* RDF Database is in use */~n~n', []),
    format(Out, '~q.~n', [ locked([ time(Now),
                                    pid(PID),
                                    host(Host)
                                  ])
                         ]),
    flush_output(Out),
    set_end_of_stream(Out),
    assert(rdf_lock(Dir, lock(Out, File))),
    at_halt(unlock_db(Dir)).

locked_error(lock, Dir) :-
    lockfile(Dir, File),
    (   catch(read_file_to_terms(File, Terms, []), _, fail),
        Terms = [locked(Args)]
    ->  Context = rdf_locked(Args)
    ;   Context = context(_, 'Database is in use')
    ),
    throw(error(permission_error(lock, rdf_db, Dir), Context)).
locked_error(open, Dir) :-
    throw(error(permission_error(lock, rdf_db, Dir),
                context(_, 'Lock file cannot be opened'))).

%!  unlock_db(+Dir) is det.
%!  unlock_db(+Stream, +File) is det.

unlock_db(Dir) :-
    retract(rdf_lock(Dir, lock(Out, File))),
    !,
    unlock_db(Out, File).
unlock_db(_).

unlock_db(Out, File) :-
    close(Out),
    delete_file(File).

                 /*******************************
                 *           FILENAMES          *
                 *******************************/

lockfile(Dir, LockFile) :-
    atomic_list_concat([Dir, /, lock], LockFile).

directory_levels(Levels) :-
    rdf_option(directory_levels(Levels)),
    !.
directory_levels(2).

db_file(Base, File) :-
    rdf_directory(Dir),
    directory_levels(Levels),
    db_file(Dir, Base, Levels, File).

db_file(Dir, Base, Levels, File) :-
    dir_levels(Base, Levels, Segments, [Base]),
    atomic_list_concat([Dir|Segments], /, File).

open_db(Base, Mode, Stream, Options) :-
    db_file(Base, File),
    create_directory_levels(File),
    open(File, Mode, Stream, [encoding(utf8)|Options]).

create_directory_levels(_File) :-
    rdf_option(directory_levels(0)),
    !.
create_directory_levels(File) :-
    file_directory_name(File, Dir),
    make_directory_path(Dir).

exists_db(Base) :-
    db_file(Base, File),
    exists_file(File).

%!  dir_levels(+File, +Levels, ?Segments, ?Tail) is det.
%
%   Create a list of intermediate directory names for File.  Each
%   directory consists of two hexadecimal digits.

dir_levels(_, 0, Segments, Segments) :- !.
dir_levels(File, Levels, Segments, Tail) :-
    rdf_atom_md5(File, 1, Hash),
    create_dir_levels(Levels, 0, Hash, Segments, Tail).

create_dir_levels(0, _, _, Segments, Segments) :- !.
create_dir_levels(N, S, Hash, [S1|Segments0], Tail) :-
    sub_atom(Hash, S, 2, _, S1),
    S2 is S+2,
    N2 is N-1,
    create_dir_levels(N2, S2, Hash, Segments0, Tail).


%!  db_files(+DB, -Snapshot, -Journal).
%!  db_files(-DB, +Snapshot, -Journal).
%!  db_files(-DB, -Snapshot, +Journal).
%
%   True if named graph DB is represented  by the files Snapshot and
%   Journal. The filenames are local   to the directory representing
%   the store.

db_files(DB, Snapshot, Journal) :-
    nonvar(DB),
    !,
    rdf_db_to_file(DB, Base),
    atom_concat(Base, '.trp', Snapshot),
    atom_concat(Base, '.jrn', Journal).
db_files(DB, Snapshot, Journal) :-
    nonvar(Snapshot),
    !,
    atom_concat(Base, '.trp', Snapshot),
    atom_concat(Base, '.jrn', Journal),
    rdf_db_to_file(DB, Base).
db_files(DB, Snapshot, Journal) :-
    nonvar(Journal),
    !,
    atom_concat(Base, '.jrn', Journal),
    atom_concat(Base, '.trp', Snapshot),
    rdf_db_to_file(DB, Base).

db_abs_files(DB, Snapshot, Journal) :-
    db_files(DB, Snapshot0, Journal0),
    db_file(Snapshot0, Snapshot),
    db_file(Journal0, Journal).


%!  rdf_journal_file(+Graph, -File) is semidet.
%!  rdf_journal_file(-Graph, -File) is nondet.
%
%   True if File the name of the existing journal file for Graph.

rdf_journal_file(Graph, Journal) :-
    (   var(Graph)
    ->  rdf_graph(Graph)
    ;   true
    ),
    db_abs_files(Graph, _Snapshot, Journal),
    exists_file(Journal).


%!  rdf_snapshot_file(+Graph, -File) is semidet.
%!  rdf_snapshot_file(-Graph, -File) is nondet.
%
%   True if File the name of the existing snapshot file for Graph.

rdf_snapshot_file(Graph, Snapshot) :-
    (   var(Graph)
    ->  rdf_graph(Graph)    % also pick the empty graphs
    ;   true
    ),
    db_abs_files(Graph, Snapshot, _Journal),
    exists_file(Snapshot).


%!  rdf_db_to_file(+DB, -File) is det.
%!  rdf_db_to_file(-DB, +File) is det.
%
%   Translate between database encoding (often an   file or URL) and
%   the name we store in the  directory.   We  keep  a cache for two
%   reasons. Speed, but much more important   is that the mapping of
%   raw --> encoded provided by  www_form_encode/2 is not guaranteed
%   to be unique by the W3C standards.

rdf_db_to_file(DB, File) :-
    file_base_db(File, DB),
    !.
rdf_db_to_file(DB, File) :-
    url_to_filename(DB, File),
    assert(file_base_db(File, DB)).

%!  url_to_filename(+URL, -FileName) is det.
%!  url_to_filename(-URL, +FileName) is det.
%
%   Turn  a  valid  URL  into  a  filename.  Earlier  versions  used
%   www_form_encode/2, but this can produce  characters that are not
%   valid  in  filenames.  We  will  use    the   same  encoding  as
%   www_form_encode/2,  but  using  our  own    rules   for  allowed
%   characters. The only requirement is that   we avoid any filename
%   special character in use.  The   current  encoding  use US-ASCII
%   alnum characters, _ and %

url_to_filename(URL, FileName) :-
    atomic(URL),
    !,
    atom_codes(URL, Codes),
    phrase(url_encode(EncCodes), Codes),
    atom_codes(FileName, EncCodes).
url_to_filename(URL, FileName) :-
    uri_encoded(path, URL, FileName).

url_encode([0'+|T]) -->
    " ",
    !,
    url_encode(T).
url_encode([C|T]) -->
    alphanum(C),
    !,
    url_encode(T).
url_encode([C|T]) -->
    no_enc_extra(C),
    !,
    url_encode(T).
url_encode(Enc) -->
    (   "\r\n"
    ;   "\n"
    ),
    !,
    { string_codes("%0D%0A", Codes),
      append(Codes, T, Enc)
    },
    url_encode(T).
url_encode([]) -->
    eos,
    !.
url_encode([0'%,D1,D2|T]) -->
    [C],
    { Dv1 is (C>>4 /\ 0xf),
      Dv2 is (C /\ 0xf),
      code_type(D1, xdigit(Dv1)),
      code_type(D2, xdigit(Dv2))
    },
    url_encode(T).

eos([], []).

alphanum(C) -->
    [C],
    { C < 128,                      % US-ASCII
      code_type(C, alnum)
    }.

no_enc_extra(0'_) --> "_".


                 /*******************************
                 *             REINDEX          *
                 *******************************/

%!  reindex_db(+Dir, +Levels)
%
%   Reindex the database by creating intermediate directories.

reindex_db(Dir, Levels) :-
    directory_files(Dir, Files),
    reindex_files(Files, Dir, '.', 0, Levels),
    remove_empty_directories(Files, Dir).

reindex_files([], _, _, _, _).
reindex_files([Nofollow|Files], Dir, Prefix, CLevel, Levels) :-
    nofollow(Nofollow),
    !,
    reindex_files(Files, Dir, Prefix, CLevel, Levels).
reindex_files([File|Files], Dir, Prefix, CLevel, Levels) :-
    CLevel \== Levels,
    file_name_extension(_Base, Ext, File),
    db_extension(Ext),
    !,
    directory_file_path(Prefix, File, DBFile),
    directory_file_path(Dir, DBFile, OldPath),
    db_file(Dir, File, Levels, NewPath),
    debug(rdf_persistency, 'Rename ~q --> ~q', [OldPath, NewPath]),
    file_directory_name(NewPath, NewDir),
    make_directory_path(NewDir),
    rename_file(OldPath, NewPath),
    reindex_files(Files, Dir, Prefix, CLevel, Levels).
reindex_files([D|Files], Dir, Prefix, CLevel, Levels) :-
    directory_file_path(Prefix, D, SubD),
    directory_file_path(Dir, SubD, AbsD),
    exists_directory(AbsD),
    \+ read_link(AbsD, _, _),      % Do not follow links
    !,
    directory_files(AbsD, SubFiles),
    CLevel2 is CLevel + 1,
    reindex_files(SubFiles, Dir, SubD, CLevel2, Levels),
    reindex_files(Files, Dir, Prefix, CLevel, Levels).
reindex_files([_|Files], Dir, Prefix, CLevel, Levels) :-
    reindex_files(Files, Dir, Prefix, CLevel, Levels).


remove_empty_directories([], _).
remove_empty_directories([File|Files], Dir) :-
    \+ nofollow(File),
    directory_file_path(Dir, File, Path),
    exists_directory(Path),
    \+ read_link(Path, _, _),
    !,
    directory_files(Path, Content),
    exclude(nofollow, Content, RealContent),
    (   RealContent == []
    ->  debug(rdf_persistency, 'Remove empty dir ~q', [Path]),
        delete_directory(Path)
    ;   remove_empty_directories(RealContent, Path)
    ),
    remove_empty_directories(Files, Dir).
remove_empty_directories([_|Files], Dir) :-
    remove_empty_directories(Files, Dir).


                 /*******************************
                 *            PREFIXES          *
                 *******************************/

save_prefixes(Dir) :-
    atomic_list_concat([Dir, /, 'prefixes.db'], PrefixFile),
    setup_call_cleanup(open(PrefixFile, write, Out, [encoding(utf8)]),
                       write_prefixes(Out),
                       close(Out)).

write_prefixes(Out) :-
    format(Out, '% Snapshot of defined RDF prefixes~n~n', []),
    forall(rdf_current_ns(Alias, URI),
           format(Out, 'prefix(~q, ~q).~n', [Alias, URI])).

%!  load_prefixes(+RDFDBDir) is det.
%
%   If the file RDFDBDir/prefixes.db exists,  load the prefixes. The
%   prefixes are registered using rdf_register_ns/3. Possible errors
%   because the prefix  definitions  have   changed  are  printed as
%   warnings, retaining the  old  definition.   Note  that  changing
%   prefixes generally requires reloading all RDF from the source.

load_prefixes(Dir) :-
    atomic_list_concat([Dir, /, 'prefixes.db'], PrefixFile),
    (   exists_file(PrefixFile)
    ->  setup_call_cleanup(open(PrefixFile, read, In, [encoding(utf8)]),
                           read_prefixes(In),
                           close(In))
    ;   true
    ).

read_prefixes(Stream) :-
    read_term(Stream, T0, []),
    read_prefixes(T0, Stream).

read_prefixes(end_of_file, _) :- !.
read_prefixes(prefix(Alias, URI), Stream) :-
    !,
    must_be(atom, Alias),
    must_be(atom, URI),
    catch(rdf_register_ns(Alias, URI, []), E,
          print_message(warning, E)),
    read_term(Stream, T, []),
    read_prefixes(T, Stream).
read_prefixes(Term, _) :-
    domain_error(prefix_term, Term).


                 /*******************************
                 *              UTIL            *
                 *******************************/

%!  mkdir(+Directory)
%
%   Create a directory if it does not already exist.

mkdir(Directory) :-
    exists_directory(Directory),
    !.
mkdir(Directory) :-
    make_directory(Directory).

%!  time_stamp(-Integer)
%
%   Return time-stamp rounded to integer.

time_stamp(Int) :-
    get_time(Now),
    Int is round(Now).


                 /*******************************
                 *            MESSAGES          *
                 *******************************/

:- multifile
    prolog:message/3,
    prolog:message_context/3.

prolog:message(rdf(Term)) -->
    message(Term).

message(restoring(Type, Count, Jobs)) -->
    [ 'Restoring ~D ~w using ~D concurrent workers'-[Count, Type, Jobs] ].
message(restore(attached(Graphs, Triples, Time/Wall))) -->
    { catch(Percent is round(100*Time/Wall), _, Percent = 0) },
    [ 'Loaded ~D graphs (~D triples) in ~2f sec. (~d% CPU = ~2f sec.)'-
      [Graphs, Triples, Wall, Percent, Time] ].
% attach_graph/2
message(restore(true, Action)) -->
    !,
    silent_message(Action).
message(restore(brief, Action)) -->
    !,
    brief_message(Action).
message(restore(_, Graph)) -->
    [ 'Restoring ~p ... '-[Graph], flush ].
message(restore(_, snapshot(_))) -->
    [ at_same_line, '(snapshot) '-[], flush ].
message(restore(_, journal(_))) -->
    [ at_same_line, '(journal) '-[], flush ].
message(restore(_, done(_, Time, Count))) -->
    [ at_same_line, '~D triples in ~2f sec.'-[Count, Time] ].
% load_source/4
message(restore(_, snapshot(G, _))) -->
    [ 'Restoring ~p\t(snapshot)'-[G], flush ].
message(restore(_, journal(G, _))) -->
    [ 'Restoring ~p\t(journal)'-[G], flush ].
message(restore(_, done(_, Time, Count))) -->
    [ at_same_line, '~D triples in ~2f sec.'-[Count, Time] ].
% journal handling
message(update_failed(S,P,O,Action)) -->
    [ 'Failed to update <~p ~p ~p> with ~p'-[S,P,O,Action] ].
% directory reindexing
message(reindex(Count, Depth)) -->
    [ 'Restructuring database with ~d levels (~D graphs)'-[Depth, Count] ].
message(reindex(Depth)) -->
    [ 'Fixing database directory structure (~d levels)'-[Depth] ].
message(read_only) -->
    [ 'Cannot write persistent store; continuing in read-only mode.', nl,
      'All changes to the RDF store will be lost if this process terminates.'
    ].

silent_message(_Action) --> [].

brief_message(done(Graph, _Time, _Count, Nth, Total)) -->
    { file_base_name(Graph, Base) },
    [ at_same_line,
      '\r~p~`.t ~D of ~D graphs~72|'-[Base, Nth, Total],
      flush
    ].
brief_message(_) --> [].


prolog:message_context(rdf_locked(Args)) -->
    { memberchk(time(Time), Args),
      memberchk(pid(Pid), Args),
      format_time(string(S), '%+', Time)
    },
    [ nl,
      'locked at ~s by process id ~w'-[S,Pid]
    ].
