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    1/*  Part of SWI-Prolog
    2
    3    Author:        Jeffrey Rosenwald, Jan Wielemaker
    4    E-mail:        jeffrose@acm.org
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2009-2019, Jeffrey Rosenwald
    7                   CWI, Amsterdam
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
   13
   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
   16
   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
   21
   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   26    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   27    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   28    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   29    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   30    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   31    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   32    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   33    POSSIBILITY OF SUCH DAMAGE.
   34*/
   35
   36:- module(paxos,
   37          [ paxos_get/1,                        % ?Term
   38            paxos_get/2,                        % +Key, -Value
   39            paxos_get/3,                        % +Key, -Value, +Options
   40            paxos_set/1,                        % ?Term
   41            paxos_set/2,                        % +Key, +Value
   42            paxos_set/3,                        % +Key, +Value, +Options
   43            paxos_on_change/2,                  % ?Term, +Goal
   44            paxos_on_change/3,                  % ?Key, ?Value, +Goal
   45
   46            paxos_initialize/1,			% +Options
   47
   48            paxos_admin_key/2,                  % ?Name, ?Key
   49            paxos_property/1,                   % ?Property
   50            paxos_quorum_ask/4,                 % ?Templ, +Msg, -Result, +Options
   51                                                % Hook support
   52            paxos_replicate_key/3               % +Nodes, ?Key, +Options
   53          ]).   54:- autoload(library(apply),[partition/4,maplist/3]).   55:- autoload(library(broadcast),
   56	    [ listen/3,
   57	      broadcast_request/1,
   58	      broadcast/1,
   59	      unlisten/1,
   60	      listen/2,
   61	      unlisten/2
   62	    ]).   63:- autoload(library(debug),[debug/3]).   64:- autoload(library(error),
   65	    [permission_error/3,resource_error/1,must_be/2]).   66:- autoload(library(lists),[select/3,nth1/3,max_list/2,member/2]).   67:- autoload(library(option),[option/2,option/3]).   68:- autoload(library(solution_sequences),[call_nth/2]).   69:- use_module(library(settings),[setting/4,setting/2]).

A Replicated Data Store

This module provides a replicated data store that is coordinated using a variation on Lamport's Paxos concensus protocol. The original method is described in his paper entitled, "The Part-time Parliament", which was published in 1998. The algorithm is tolerant of non-Byzantine failure. That is late or lost delivery or reply, but not senseless delivery or reply. The present algorithm takes advantage of the convenience offered by multicast to the quorum's membership, who can remain anonymous and who can come and go as they please without effecting Liveness or Safety properties.

Paxos' quorum is a set of one or more attentive members, whose processes respond to queries within some known time limit (< 20ms), which includes roundtrip delivery delay. This property is easy to satisfy given that every coordinator is necessarily a member of the quorum as well, and a quorum of one is permitted. An inattentive member (e.g. one whose actions are late or lost) is deemed to be "not-present" for the purposes of the present transaction and consistency cannot be assured for that member. As long as there is at least one attentive member of the quorum, then persistence of the database is assured.

Each member maintains a ledger of terms along with information about when they were originally recorded. The member's ledger is deterministic. That is to say that there can only be one entry per functor/arity combination. No member will accept a new term proposal that has a line number that is equal-to or lower-than the one that is already recorded in the ledger.

Paxos is a three-phase protocol:

1: A coordinator first prepares the quorum for a new proposal by broadcasting a proposed term. The quorum responds by returning the last known line number for that functor/arity combination that is recorded in their respective ledgers.
2: The coordinator selects the highest line number it receives, increments it by one, and then asks the quorum to finally accept the new term with the new line number. The quorum checks their respective ledgers once again and if there is still no other ledger entry for that functor/arity combination that is equal-to or higher than the specified line, then each member records the term in the ledger at the specified line. The member indicates consent by returning the specified line number back to the coordinator. If consent is withheld by a member, then the member returns a nack instead. The coordinator requires unanimous consent. If it isn't achieved then the proposal fails and the coordinator must start over from the beginning.
3: Finally, the coordinator concludes the successful negotiation by broadcasting the agreement to the quorum in the form of a paxos_changed(Key,Value) event. This is the only event that should be of interest to user programs.

For practical reasons, we rely on the partially synchronous behavior (e.g. limited upper time bound for replies) of broadcast_request/1 over TIPC to ensure Progress. Perhaps more importantly, we rely on the fact that the TIPC broadcast listener state machine guarantees the atomicity of broadcast_request/1 at the process level, thus obviating the need for external mutual exclusion mechanisms.

Note that this algorithm does not guarantee the rightness of the value proposed. It only guarantees that if successful, the value proposed is identical for all attentive members of the quorum.

author
- Jeffrey Rosenwald (JeffRose@acm.org)
See also
- tipc_broadcast.pl, udp_broadcast.pl */
license
- BSD-2
  141:- meta_predicate
  142    paxos_on_change(?, 0),
  143    paxos_on_change(?, ?, 0).  144
  145:- multifile
  146    paxos_message_hook/3,               % +PaxOS, +TimeOut, -Message
  147    paxos_ledger_hook/5.                % +Op, ?Key, ?Gen, ?Value, ?Status
  148
  149:- setting(max_sets, nonneg, 20,
  150           "Max Retries to get to an agreement").  151:- setting(max_gets, nonneg, 5,
  152           "Max Retries to get a value from the forum").  153:- setting(response_timeout, float, 0.020,
  154           "Max time to wait for a response").  155:- setting(replication_rate, number, 1000,
  156           "Number of keys replicated per second").  157:- setting(death_half_life, number, 10,
  158           "Half-time for failure score").  159:- setting(death_score, number, 100,
  160           "Consider a node dead if cummulative failure \c
  161            score exceeds this number").
 paxos_initialize(+Options) is det
Initialize this Prolog process as a paxos node. The initialization requires an initialized and configured TIPC, UDP or other broadcast protocol. Calling this initialization may be omitted, in which case the equivant of paxos_initialize([]) is executed lazily as part of the first paxos operation. Defined options:
node(?NodeID)
When instantiated, this node rejoins the network with the given node id. A fixed node idea should be used if the node is configured for persistency and causes the new node to receive updates for keys that have been created or modified since the node left the network. If NodeID is a variable it is unified with the discovered NodeID.

NodeID must be a small non-negative integer as these identifiers are used in bitmaps.

  183:- dynamic  paxos_initialized/0.  184:- volatile paxos_initialized/0.  185
  186paxos_initialize(_Options) :-
  187    paxos_initialized,
  188    !.
  189paxos_initialize(Options) :-
  190    with_mutex(paxos, paxos_initialize_sync(Options)).
  191
  192paxos_initialize_sync(_Options) :-
  193    paxos_initialized,
  194    !.
  195paxos_initialize_sync(Options) :-
  196    at_halt(paxos_leave),
  197    listen(paxos, paxos(X), paxos_message(X)),
  198    paxos_assign_node(Options),
  199    start_replicator,
  200    asserta(paxos_initialized).
  201
  202paxos_initialize :-
  203    paxos_initialize([]).
  204
  205
  206		 /*******************************
  207		 *            ADMIN		*
  208		 *******************************/
 paxos_get_admin(+Name, -Value) is semidet
 paxos_set_admin(+Name, +Value) is semidet
Set administrative keys. We use a wrapper such that we can hide the key identity.
  216paxos_admin_key(quorum, '$paxos_quorum').
  217paxos_admin_key(dead,   '$paxos_dead_nodes').
  218
  219paxos_get_admin(Name, Value) :-
  220    paxos_admin_key(Name, Key),
  221    paxos_get(Key, Value).
  222
  223paxos_set_admin(Name, Value) :-
  224    paxos_admin_key(Name, Key),
  225    paxos_set(Key, Value).
  226
  227paxos_set_admin_bg(Name, Value) :-
  228    thread_create(ignore(paxos_set_admin(Name, Value)), _,
  229                  [ detached(true)
  230                  ]).
  231
  232
  233		 /*******************************
  234		 *           NODE DATA		*
  235		 *******************************/
 node(?NodeId)
 quorum(?Bitmap)
 dead(?Bitmap)
 failed(?Bitmap)
 failed(?NodeId, ?LastTried, ?Score)
Track our identity as well as as the status of our peers in the network. NodeId is a small integer. Multiple NodeIds are combined in a Bitmap.
  255:- dynamic
  256    node/1,                             % NodeID
  257    quorum/1,                           % Bitmap
  258    failed/1,                           % Bitmap
  259    failed/3,                           % NodeID, LastTried, Score
  260    leaving/0,                          % Node is leaving
  261    dead/1,                             % Bitmap
  262    salt/1.                             % Unique key
  263:- volatile
  264    node/1,
  265    quorum/1,
  266    failed/1,
  267    failed/3,
  268    leaving/0,
  269    dead/1,
  270    salt/1.
 paxos_assign_node(+Options) is det
Assign a node for this paxos instance. If node is given as an option, this is the node id that is used. Otherwise the network is analysed and the system selects a new node.
  278paxos_assign_node(Options) :-
  279    (   option(node(Node), Options)
  280    ->  node(Node)
  281    ;   node(_)
  282    ),                                          % already done
  283    !.
  284paxos_assign_node(Options) :-
  285    between(1, 20, Retry),
  286    option(node(Node), Options, Node),
  287    (   node(_)
  288    ->  permission_error(set, paxos_node, Node)
  289    ;   true
  290    ),
  291    retractall(dead(_)),
  292    retractall(quorum(_)),
  293    retractall(failed(_)),
  294    retractall(failed(_,_,_)),
  295    retractall(leaving),
  296    Salt is random(1<<63),
  297    asserta(salt(Salt)),
  298    paxos_message(node(N,Q,D):From, 0.25, NodeQuery),
  299    findall(t(N,Q,D,From),
  300            broadcast_request(NodeQuery),
  301            Network),
  302    select(t(self,0,Salt,Me), Network, AllNodeStatus),
  303    partition(starting, AllNodeStatus, Starting, Running),
  304    nth_starting(Starting, Salt, Offset),
  305    retractall(salt(_)),
  306    debug(paxos(node), 'Me@~p; starting: ~p; running: ~p',
  307          [Me, Starting, Running]),
  308    arg_union(2, Running, Quorum),
  309    arg_union(3, Running, Dead),
  310    (   var(Node)
  311    ->  (   call_nth(( between(0, 1000, Node),
  312                       \+ memberchk(t(Node,_,_,_), Running),
  313                       Dead /\ (1<<Node) =:= 0),
  314                     Offset)
  315        ->  debug(paxos(node), 'Assigning myself node ~d', [Node])
  316        ;   resource_error(paxos_nodes)
  317        )
  318    ;   memberchk(t(Node,_,_,_), Running)
  319    ->  permission_error(set, paxos_node, Node)
  320    ;   Rejoin = true
  321    ),
  322    asserta(node(Node)),
  323    (   claim_node(Node, Me)
  324    ->  !,
  325        asserta(dead(Dead)),
  326        set_quorum(Node, Quorum),
  327        (   Rejoin == true
  328        ->  paxos_rejoin
  329        ;   true
  330        )
  331    ;   debug(paxos(node), 'Node ~p already claimed; retrying (~p)',
  332              [Node, Retry]),
  333        retractall(node(Node)),
  334        fail
  335    ).
  336
  337starting(t(self,_Quorum,_Salt,_Address)).
  338
  339nth_starting(Starting, Salt, N) :-
  340    maplist(arg(3), Starting, Salts),
  341    sort([Salt|Salts], Sorted),
  342    nth1(N, Sorted, Salt),
  343    !.
  344
  345claim_node(Node, Me) :-
  346    paxos_message(claim_node(Node, Ok):From, 0.25, NodeQuery),
  347    forall((   broadcast_request(NodeQuery),
  348               From \== Me,
  349               debug(paxos(node), 'Claim ~p ~p: ~p', [Node, From, Ok])
  350           ),
  351           Ok == true).
  352
  353set_quorum(Node, Quorum0) :-
  354    Quorum is Quorum0 \/ (1<<Node),
  355    debug(paxos(node), 'Adding ~d to quorum (now 0x~16r)', [Node, Quorum]),
  356    asserta(quorum(Quorum)),
  357    paxos_set_admin(quorum, Quorum).
 paxos_rejoin
Re-join the network. Tasks:
  367paxos_rejoin :-
  368    node(Node),
  369    repeat,
  370        (   paxos_get_admin(dead, Dead0)
  371        ->  Dead is Dead0 /\ \(1<<Node),
  372            (   Dead == Dead0
  373            ->  true
  374            ;   paxos_set_admin(dead, Dead)
  375            )
  376        ;   true
  377        ),
  378    !.
 paxos_leave is det
 paxos_leave(+Node) is det
Leave the network. The predicate paxos_leave/0 is called from at_halt/1 to ensure the node is deleted as the process dies. The paxos_leave/1 version is called to discard other nodes if they repeatedly did not respond to queries.
  388paxos_leave :-
  389    node(Node),
  390    !,
  391    asserta(leaving),
  392    paxos_leave(Node),
  393    Set is 1<<Node,
  394    paxos_message(forget(Set), -, Forget),
  395    broadcast(Forget),
  396    unlisten(paxos),
  397    retractall(leaving).
  398paxos_leave.
  399
  400paxos_leave(Node) :-
  401    !,
  402    paxos_update_set(quorum, del(Node)),
  403    paxos_update_set(dead,   add(Node)).
  404paxos_leave(_).
  405
  406paxos_update_set(Set, How) :-
  407    repeat,
  408      Term =.. [Set,Value],
  409      call(Term),
  410      (   How = add(Node)
  411      ->  NewValue is Value \/  (1<<Node)
  412      ;   How = del(Node)
  413      ->  NewValue is Value /\ \(1<<Node)
  414      ),
  415      (   Value == NewValue
  416      ->  true
  417      ;   paxos_set_admin(Set, NewValue)
  418      ->  true
  419      ;   leaving
  420      ),
  421    !.
  422
  423		 /*******************************
  424		 *          NODE STATUS		*
  425		 *******************************/
 update_failed(+Action, +Quorum, +Alive) is det
We just sent the Quorum a message and got a reply from the set Alive.
Arguments:
is- one of set, get or replicate and indicates the intended action.
  435update_failed(Action, Quorum, Alive) :-
  436    Failed is Quorum /\ \Alive,
  437    alive(Alive),
  438    consider_dead(Failed),
  439    (   failed(Failed)
  440    ->  true
  441    ;   (   clause(failed(_Old), true, Ref)
  442        ->  asserta(failed(Failed)),
  443            erase(Ref),
  444            debug(paxos(node), 'Updated failed quorum to 0x~16r', [Failed])
  445        ;   asserta(failed(Failed))
  446        ),
  447        (   Action == set
  448        ->  start_replicator
  449        ;   true
  450        )
  451    ).
  452
  453consider_dead(0) :-
  454    !.
  455consider_dead(Failed) :-
  456    Node is lsb(Failed),
  457    consider_dead1(Node),
  458    Rest is Failed /\ \(1<<Node),
  459    consider_dead(Rest).
  460
  461consider_dead1(Node) :-
  462    clause(failed(Node, Last, Score), true, Ref),
  463    !,
  464    setting(death_half_life, HalfLife),
  465    setting(death_score, DeathScore),
  466    get_time(Now),
  467    Passed is Now-Last,
  468    NewScore is Score*(2**(-Passed/HalfLife)) + 10,
  469    asserta(failed(Node, Now, NewScore)),
  470    erase(Ref),
  471    (   NewScore < DeathScore
  472    ->  debug(paxos(node), 'Consider node ~d dead', [Node]),
  473        paxos_leave(Node)
  474    ;   true
  475    ).
  476consider_dead1(Node) :-
  477    get_time(Now),
  478    asserta(failed(Node, Now, 10)).
  479
  480alive(Bitmap) :-
  481    (   clause(failed(Node, _Last, _Score), true, Ref),
  482        Bitmap /\ (1<<Node) =\= 0,
  483        erase(Ref),
  484        fail
  485    ;   true
  486    ).
 life_quorum(-Quorum, -LifeQuorum) is det
Find the Quorum and the living nodes from the Quorum. This is the set for which we wait. If the LifeQuorum is not a majority we address the whole Quorum.
To be done
- At some point in time we must remove a node from the quorum.
  497life_quorum(Quorum, LifeQuorum) :-
  498    quorum(Quorum),
  499    (   failed(Failed),
  500        Failed \== 0,
  501        LifeQuorum is Quorum /\ \Failed,
  502        majority(LifeQuorum, Quorum)
  503    ->  true
  504    ;   LifeQuorum = Quorum
  505    ).
  506
  507
  508		 /*******************************
  509		 *        NETWORK STATUS	*
  510		 *******************************/
  511
  512:- paxos_admin_key(quorum, Key),
  513   listen(paxos_changed(Key, Quorum),
  514          update_quorum(Quorum)).  515:- paxos_admin_key(dead, Key),
  516   listen(paxos_changed(Key, Death),
  517          update_dead(Death)).  518
  519update_quorum(Proposed) :-
  520    debug(paxos(node), 'Received quorum proposal 0x~16r', [Proposed]),
  521    quorum(Proposed),
  522    !.
  523update_quorum(Proposed) :-
  524    leaving,
  525    !,
  526    update(quorum(Proposed)).
  527update_quorum(Proposed) :-
  528    node(Node),
  529    Proposed /\ (1<<Node) =\= 0,
  530    !,
  531    update(quorum(Proposed)).
  532update_quorum(Proposed) :-
  533    node(Node),
  534    NewQuorum is Proposed \/ (1<<Node),
  535    update(quorum(NewQuorum)),
  536    debug(paxos(node), 'I''m not in the quorum! Proposing 0x~16r', [NewQuorum]),
  537    paxos_set_admin_bg(quorum, NewQuorum).
  538
  539update_dead(Proposed) :-
  540    debug(paxos(node), 'Received dead proposal 0x~16r', [Proposed]),
  541    dead(Proposed),
  542    !.
  543update_dead(Proposed) :-
  544    leaving,
  545    !,
  546    update(dead(Proposed)).
  547update_dead(Proposed) :-
  548    node(Node),
  549    Proposed /\ (1<<Node) =:= 0,
  550    !,
  551    update(dead(Proposed)).
  552update_dead(Proposed) :-
  553    node(Node),
  554    NewDead is Proposed /\ \(1<<Node),
  555    update(dead(NewDead)),
  556    paxos_set_admin_bg(dead, NewDead).
  557
  558update(Clause) :-
  559    functor(Clause, Name, Arity),
  560    functor(Generic, Name, Arity),
  561    (   clause(Generic, true, Ref)
  562    ->  asserta(Clause),
  563        erase(Ref)
  564    ;   asserta(Clause)
  565    ).
 paxos_property(?Property)
True if Property is a current property for the paxos network. Defined properties are:
  576paxos_property(node(NodeID)) :-
  577    node(NodeID).
  578paxos_property(quorum(Quorum)) :-
  579    quorum(Quorum).
  580paxos_property(failed(Nodes)) :-
  581    failed(Nodes).
  582
  583
  584		 /*******************************
  585		 *         INBOUND EVENTS	*
  586		 *******************************/
 paxos_message(?Message)
Handle inbound actions from our peers. Defines values for Message are:
prepare(+Key, -Node, -Gen, +Value)
A request message to set Key to Value. Returns the current generation at which we have a value or 0 for Gen and the our node id for Node.
accept(+Key, -Node, +Gen, -GenA, +Value)
A request message to set Key to Value if Gen is newer than the generation we have for Key. In that case GenA is Gen. Otherwise we reject using GenA = nack.
changed(+Key, +Gen, +Value, +Acceptors)
The leader got enough accepts for setting Key to Value at Gen. Acceptors is the set of nodes that accepted this value.
learn(+Key, -Node, +Gen, -GenA, +Value)
Request message peforming phase one for replication to learner nodes.
learned(+Key, +Gen, +Value, +Acceptors)
Phase two of the replication. Confirm the newly learned knowledge.
retrieve(+Key, -Node, -Gen, -Value)
A request message to retrieve our value for Key. Also provides our node id and the generation.
forget(+Nodes)
Forget the existence of Nodes.
node(-Node, -Quorum, -Dead)
Get my view about the network. Node is the (integer) node id of this node, Quorum is the idea of the quorum and Dead is the idea about non-responsive nodes.
To be done
- : originally the changed was handled by a get and when not successful with a new set, named paxos_audit. I don't really see why we need this.
  623paxos_message(prepare(Key,Node,Gen,Value)) :-
  624    node(Node),
  625    (   ledger(Key, Gen, _)
  626    ->  true
  627    ;   Gen = 0,
  628        ledger_create(Key, Gen, Value)
  629    ),
  630    debug(paxos, 'Prepared ~p-~p@~d', [Key,Value,Gen]).
  631paxos_message(accept(Key,Node,Gen,GenA,Value)) :-
  632    node(Node),
  633    (   ledger_update(Key, Gen, Value)
  634    ->  debug(paxos, 'Accepted ~p-~p@~d', [Key,Value,Gen]),
  635        GenA = Gen
  636    ;   debug(paxos, 'Rejected ~p-~p@~d', [Key,Value,Gen]),
  637        GenA = nack
  638    ).
  639paxos_message(changed(Key,Gen,Value,Acceptors)) :-
  640    debug(paxos, 'Changed ~p-~p@~d for ~p', [Key, Value, Gen, Acceptors]),
  641    ledger_update_holders(Key,Gen,Acceptors),
  642    broadcast(paxos_changed(Key,Value)).
  643paxos_message(learn(Key,Node,Gen,GenA,Value)) :-
  644    node(Node),
  645    debug(paxos, 'Learn ~p-~p@~p?', [Key, Value, Gen]),
  646    (   ledger_learn(Key,Gen,Value)
  647    ->  debug(paxos, 'Learned ~p-~p@~d', [Key,Value,Gen]),
  648        GenA = Gen
  649    ;   debug(paxos, 'Rejected ~p@~d', [Key, Gen]),
  650        GenA = nack
  651    ).
  652paxos_message(learned(Key,Gen,_Value,Acceptors)) :-
  653    ledger_update_holders(Key,Gen,Acceptors).
  654paxos_message(retrieve(Key,Node,K,Value)) :-
  655    node(Node),
  656    debug(paxos, 'Retrieving ~p', [Key]),
  657    ledger(Key,K,Value),
  658    debug(paxos, 'Retrieved ~p-~p@~d', [Key,Value,K]),
  659    !.
  660paxos_message(forget(Nodes)) :-
  661    ledger_forget(Nodes).
  662paxos_message(node(Node,Quorum,Dead)) :-
  663    (   node(Node),
  664        quorum(Quorum),
  665        dead(Dead)
  666    ->  true
  667    ;   salt(Salt),
  668        Node = self,
  669        Quorum = 0,
  670        Dead = Salt
  671    ).
  672paxos_message(claim_node(Node, Ok)) :-
  673    (   node(Node)
  674    ->  Ok = false
  675    ;   Ok = true
  676    ).
  677paxos_message(ask(Node, Message)) :-
  678    node(Node),
  679    broadcast_request(Message).
  680
  681
  682		 /*******************************
  683		 *     KEY-VALUE OPERATIONS	*
  684		 *******************************/
 paxos_set(+Term) is semidet
Equivalent to paxos_key(Term,Key), pasox_set(Key,Term). I.e., Term is a ground compound term and its key is the name/arity pair. This version provides compatibility with older versions of this library.
 paxos_set(+Key, +Value) is semidet
 paxos_set(+Key, +Value, +Options) is semidet
negotiates to have Key-Value recorded in the ledger for each of the quorum's members. This predicate succeeds if the quorum unanimously accepts the proposed term. If no such entry exists in the Paxon's ledger, then one is silently created. paxos_set/1 will retry the transaction several times (default: 20) before failing. Failure is rare and is usually the result of a collision of two or more writers writing to the same term at precisely the same time. On failure, it may be useful to wait some random period of time, and then retry the transaction. By specifying a retry count of zero, paxos_set/2 will succeed iff the first ballot succeeds.

On success, paxos_set/1 will also broadcast the term paxos_changed(Key,Value), to the quorum.

Options processed:

retry(Retries)
is a non-negative integer specifying the number of retries that will be performed before a set is abandoned. Defaults to the setting max_sets (20).
timeout(+Seconds)
Max time to wait for the forum to reply. Defaults to the setting response_timeout (0.020, 20ms).
Arguments:
Term- is a compound that may have unbound variables.
To be done
- If the Value is already current, should we simply do nothing?
  722paxos_set(Term) :-
  723    paxos_key(Term, Key),
  724    paxos_set(Key, Term, []).
  725
  726paxos_set(Key, Value) :-
  727    paxos_set(Key, Value, []).
  728
  729paxos_set(Key, Value, Options) :-
  730    must_be(ground, Key-Value),
  731    paxos_initialize,
  732    option(retry(Retries), Options, Retries),
  733    option(timeout(TMO), Options, TMO),
  734    apply_default(Retries, max_sets),
  735    apply_default(TMO, response_timeout),
  736    paxos_message(prepare(Key,Np,Rp,Value), TMO, Prepare),
  737    between(0, Retries, _),
  738      life_quorum(Quorum, Alive),
  739      Alive \== 0,
  740      debug(paxos, 'Set: ~p -> ~p', [Key, Value]),
  741      collect(Quorum, false, Np, Rp, Prepare, Rps, PrepNodes),
  742      debug(paxos, 'Set: quorum: 0x~16r, prepared by 0x~16r, gens ~p',
  743            [Quorum, PrepNodes, Rps]),
  744      majority(PrepNodes, Quorum),
  745      max_list(Rps, K),
  746      succ(K, K1),
  747      paxos_message(accept(Key,Na,K1,Ra,Value), TMO, Accept),
  748      collect(Alive, Ra == nack, Na, Ra, Accept, Ras, AcceptNodes),
  749      majority(AcceptNodes, Quorum),
  750      intersecting(PrepNodes, AcceptNodes),
  751      c_element(Ras, K, K1),
  752      broadcast(paxos(log(Key,Value,AcceptNodes,K1))),
  753      paxos_message(changed(Key,K1,Value,AcceptNodes), -, Changed),
  754      broadcast(Changed),
  755      update_failed(set, Quorum, AcceptNodes),
  756    !.
  757
  758apply_default(Var, Setting) :-
  759    var(Var),
  760    !,
  761    setting(Setting, Var).
  762apply_default(_, _).
  763
  764majority(SubSet, Set) :-
  765    popcount(SubSet) >= (popcount(Set)+2)//2.
  766
  767intersecting(Set1, Set2) :-
  768    Set1 /\ Set2 =\= 0.
 collect(+Quorum, :Stop, ?Node, ?Template, ?Message, -Result, -NodeSet) is semidet
Perform a broadcast request using Message. Node and Template share with Message and extract the replying node and the result value from Message. Result is the list of instantiations for Template received and NodeSet is the set (bitmask) of Node values that replies, i.e. |NodeSet| is length(Result). The transfer stops if all members of the set Quorum responded or the configured timeout passed.
  781collect(Quorum, Stop, Node, Template, Message, Result, NodeSet) :-
  782    State = state(0),
  783    L0 = [dummy|_],
  784    Answers = list(L0),
  785    (   broadcast_request(Message),
  786        debug(paxos(request), 'broadcast_request: ~p', [Message]),
  787        (   Stop
  788        ->  !,
  789            fail
  790        ;   true
  791        ),
  792        duplicate_term(Template, Copy),
  793        NewLastCell = [Copy|_],
  794        arg(1, Answers, LastCell),
  795        nb_linkarg(2, LastCell, NewLastCell),
  796        nb_linkarg(1, Answers, NewLastCell),
  797        arg(1, State, Replied0),
  798        Replied is Replied0 \/ (1<<Node),
  799        nb_setarg(1, State, Replied),
  800        Quorum /\ Replied =:= Quorum
  801    ->  true
  802    ;   true
  803    ),
  804    arg(1, State, NodeSet),
  805    arg(1, Answers, [_]),               % close the answer list
  806    L0 = [_|Result].
 paxos_quorum_ask(?Template, +Message, -Result, +Options)
Ask the paxos forum for their opinion. This predicate is not really part of the paxos protocol, but reuses notably the quorum maintenance mechanism of this library for asking questions to the quorum (cluster). Message is the message to be asked. Result is a list of copies of Template from the quorum. Options:
timeout(+Seconds)
Max time to wait for a reply. Default is the setting response_timeout.
node(?Node)
Can be used to include the replying node into Template.
quorum(?Quorum)
Set/query the interviewed quorum as a bitmask
  824paxos_quorum_ask(Template, Message, Result, Options) :-
  825    option(timeout(TMO), Options, TMO),
  826    option(node(Node), Options, _),
  827    option(quorum(Quorum), Options, Quorum),
  828    apply_default(TMO, response_timeout),
  829    (   var(Quorum)
  830    ->  life_quorum(Quorum, _Alive)
  831    ;   true
  832    ),
  833    paxos_message(ask(Node, Message), TMO, BroadcastMessage),
  834    collect(Quorum, false, Node, Template, BroadcastMessage, Result, _PrepNodes).
 paxos_get(?Term) is semidet
Equivalent to paxos_key(Term,Key), pasox_get(Key,Term). I.e., Term is a compound term and its key is the name/arity pair. This version provides compatibility with older versions of this library.
 paxos_get(+Key, +Value) is semidet
 paxos_get(+Key, +Value, +Options) is semidet
unifies Term with the entry retrieved from the Paxon's ledger. If no such entry exists in the member's local cache, then the quorum is asked to provide a value, which is verified for consistency. An implied paxos_set/1 follows. This predicate succeeds if a term with the same functor and arity exists in the Paxon's ledger, and fails otherwise.

Options processed:

retry(Retries)
is a non-negative integer specifying the number of retries that will be performed before a set is abandoned. Defaults to the setting max_gets (5).
timeout(+Seconds)
Max time to wait for the forum to reply. Defaults to the setting response_timeout (0.020, 20ms).
Arguments:
Term- is a compound. Any unbound variables are unified with those provided in the ledger entry.
  865paxos_get(Term) :-
  866    paxos_key(Term, Key),
  867    paxos_get(Key, Term, []).
  868paxos_get(Key, Value) :-
  869    paxos_get(Key, Value, []).
  870
  871paxos_get(Key, Value, _) :-
  872    ledger(Key, _Line, Value),
  873    !.
  874paxos_get(Key, Value, Options) :-
  875    paxos_initialize,
  876    option(retry(Retries), Options, Retries),
  877    option(timeout(TMO), Options, TMO),
  878    apply_default(Retries, max_gets),
  879    apply_default(TMO, response_timeout),
  880    Msg = Line-Value,
  881    paxos_message(retrieve(Key,Nr,Line,Value), TMO, Retrieve),
  882    node(Node),
  883    between(0, Retries, _),
  884      life_quorum(Quorum, Alive),
  885      QuorumA is Alive /\ \(1<<Node),
  886      collect(QuorumA, false, Nr, Msg, Retrieve, Terms, RetrievedNodes),
  887      debug(paxos, 'Retrieved: ~p from 0x~16r', [Terms, RetrievedNodes]),
  888      highest_vote(Terms, _Line-MajorityValue, Count),
  889      debug(paxos, 'Best: ~p with ~d votes', [MajorityValue, Count]),
  890      Count >= (popcount(QuorumA)+2)//2,
  891      debug(paxos, 'Retrieve: accept ~p', [MajorityValue]),
  892      update_failed(get, Quorum, RetrievedNodes),
  893      paxos_set(Key, MajorityValue),    % Is this needed?
  894    !.
  895
  896highest_vote(Terms, Term, Count) :-
  897    msort(Terms, Sorted),
  898    count_votes(Sorted, Counted),
  899    sort(1, >, Counted, [Count-Term|_]).
  900
  901count_votes([], []).
  902count_votes([H|T0], [N-H|T]) :-
  903    count_same(H, T0, 1, N, R),
  904    count_votes(R, T).
  905
  906count_same(H, [Hc|T0], C0, C, R) :-
  907    H == Hc,
  908    !,
  909    C1 is C0+1,
  910    count_same(H, T0, C1, C, R).
  911count_same(_, R, C, C, R).
 paxos_key(+Term, -Key) is det
Compatibility to allow for paxos_get/1, paxos_set/1, etc. The key of a compound term is a term '$c'(Name,Arity). Note that we do not use Name/Arity and X/Y is naturally used to organize keys as hierachical paths.
  920paxos_key(Compound, '$c'(Name,Arity)) :-
  921    compound(Compound), !,
  922    compound_name_arity(Compound, Name, Arity).
  923paxos_key(Compound, _) :-
  924    must_be(compound, Compound).
  925
  926
  927		 /*******************************
  928		 *          REPLICATION		*
  929		 *******************************/
 start_replicator
Start or signal the replicator thread that there may be outstanding replication work. This is the case if
  940start_replicator :-
  941    catch(thread_send_message(paxos_replicator, run),
  942          error(existence_error(_,_),_),
  943          fail),
  944    !.
  945start_replicator :-
  946    catch(thread_create(replicator, _,
  947                        [ alias(paxos_replicator),
  948                          detached(true)
  949                        ]),
  950          error(permission_error(_,_,_),_),
  951          true).
  952
  953replicator :-
  954    setting(replication_rate, ReplRate),
  955    ReplSleep is 1/ReplRate,
  956    node(Node),
  957    debug(paxos(replicate), 'Starting replicator', []),
  958    State = state(idle),
  959    repeat,
  960      quorum(Quorum),
  961      dead(Dead),
  962      LifeQuorum is Quorum /\ \Dead,
  963      (   LifeQuorum /\ \(1<<Node) =:= 0
  964      ->  debug(paxos(replicate),
  965                'Me: ~d, Quorum: 0x~16r, Dead: 0x~16r: I''m alone, waiting ...',
  966                [Node, Quorum, Dead]),
  967          thread_get_message(_)
  968      ;   (   paxos_replicate_key(LifeQuorum, Key, [])
  969          ->  replicated(State, key(Key)),
  970              thread_self(Me),
  971              thread_get_message(Me, _, [timeout(ReplSleep)])
  972          ;   replicated(State, idle),
  973              thread_get_message(_)
  974          )
  975      ),
  976      fail.
  977
  978replicated(State, key(_Key)) :-
  979    arg(1, State, idle),
  980    !,
  981    debug(paxos(replicate), 'Start replicating ...', []),
  982    nb_setarg(1, State, 1).
  983replicated(State, key(_Key)) :-
  984    !,
  985    arg(1, State, C0),
  986    C is C0+1,
  987    nb_setarg(1, State, C).
  988replicated(State, idle) :-
  989    arg(1, State, idle),
  990    !.
  991replicated(State, idle) :-
  992    arg(1, State, Count),
  993    debug(paxos(replicate), 'Replicated ~D keys', [Count]),
  994    nb_setarg(1, State, idle).
 paxos_replicate_key(+Nodes:bitmap, ?Key, +Options) is det
Replicate a Key to Nodes. If Key is unbound, a random key is selected.
timeout(+Seconds)
Max time to wait for the forum to reply. Defaults to the setting response_timeout (0.020, 20ms).
 1006paxos_replicate_key(Nodes, Key, Options) :-
 1007    replication_key(Nodes, Key),
 1008    option(timeout(TMO), Options, TMO),
 1009    apply_default(TMO, response_timeout),
 1010    ledger_current(Key, Gen, Value, Holders),
 1011    paxos_message(learn(Key,Na,Gen,Ga,Value), TMO, Learn),
 1012    collect(Nodes, Ga == nack, Na, Ga, Learn, _Gas, LearnedNodes),
 1013    NewHolders is Holders \/ LearnedNodes,
 1014    paxos_message(learned(Key,Gen,Value,NewHolders), -, Learned),
 1015    broadcast(Learned),
 1016    update_failed(replicate, Nodes, LearnedNodes).
 1017
 1018replication_key(_Nodes, Key) :-
 1019    ground(Key),
 1020    !.
 1021replication_key(Nodes, Key) :-
 1022    (   Nth is 1+random(popcount(Nodes))
 1023    ;   Nth = 1
 1024    ),
 1025    call_nth(needs_replicate(Nodes, Key), Nth),
 1026    !.
 1027
 1028needs_replicate(Nodes, Key) :-
 1029    ledger_current(Key, _Gen, _Value, Holders),
 1030    Nodes /\ \Holders =\= 0,
 1031    \+ paxos_admin_key(_, Key).
 1032
 1033
 1034		 /*******************************
 1035		 *      KEY CHANGE EVENTS	*
 1036		 *******************************/
 paxos_on_change(?Term, :Goal) is det
 paxos_on_change(?Key, ?Value, :Goal) is det
Executes the specified Goal when Key changes. paxos_on_change/2 listens for paxos_changed(Key,Value) notifications for Key, which are emitted as the result of successful paxos_set/3 transactions. When one is received for Key, then Goal is executed in a separate thread of execution.
Arguments:
Term- is a compound, identical to that used for paxos_get/1.
Goal- is one of:
  • a callable atom or term, or
  • the atom ignore, which causes monitoring for Term to be discontinued.
 1054paxos_on_change(Term, Goal) :-
 1055    paxos_key(Term, Key),
 1056    paxos_on_change(Key, Term, Goal).
 1057
 1058paxos_on_change(Key, Value, Goal) :-
 1059    Goal = _:Plain,
 1060    must_be(callable, Plain),
 1061    (   Plain == ignore
 1062    ->  unlisten(paxos_user, paxos_changed(Key,Value))
 1063    ;   listen(paxos_user, paxos_changed(Key,Value),
 1064               key_changed(Key, Value, Goal)),
 1065        paxos_initialize
 1066    ).
 1067
 1068key_changed(_Key, _Value, Goal) :-
 1069    E = error(_,_),
 1070    catch(thread_create(Goal, _, [detached(true)]),
 1071          E, key_error(E)).
 1072
 1073key_error(error(permission_error(create, thread, _), _)) :-
 1074    !.
 1075key_error(E) :-
 1076    print_message(error, E).
 1077
 1078
 1079		 /*******************************
 1080		 *            HOOKS		*
 1081		 *******************************/
 node(-Node) is det
Get the node ID for this paxos node.
 quorum(-Quorum) is det
Get the current quorum as a bitmask
 paxos_message(+PaxOS, +TimeOut, -BroadcastMessage) is det
Transform a basic PaxOS message in a message for the broadcasting service. This predicate is hooked by paxos_message_hook/3 with the same signature.
Arguments:
TimeOut- is one of - or a time in seconds.
 1099paxos_message(Paxos:From, TMO, Message) :-
 1100    paxos_message_raw(paxos(Paxos):From, TMO, Message).
 1101paxos_message(Paxos, TMO, Message) :-
 1102    paxos_message_raw(paxos(Paxos), TMO, Message).
 1103
 1104paxos_message_raw(Message, TMO, WireMessage) :-
 1105    paxos_message_hook(Message, TMO, WireMessage),
 1106    !.
 1107paxos_message_raw(Message, TMO, WireMessage) :-
 1108    throw(error(mode_error(det, fail,
 1109                           paxos:paxos_message_hook(Message, TMO, WireMessage)), _)).
 1110
 1111
 1112		 /*******************************
 1113		 *           STORAGE		*
 1114		 *******************************/
 paxos_ledger_hook(+Action, ?Key, ?Gen, ?Value, ?Holders)
Hook called for all operations on the ledger. Defined actions are:
current
Enumerate our ledger content.
get
Get a single value from our ledger.
create
Create a new key in our ledger.
accept
Accept a new newly proposed value for a key. Failure causes the library to send a NACK message.
set
Final acceptance of Ken@Gen, providing the holders that accepted the new value.
learn
Accept new keys in a new node or node that has been offline for some time.
 1136:- dynamic
 1137    paxons_ledger/4.                    % Key, Gen, Value, Holders
 ledger_current(?Key, ?Gen, ?Value, ?Holders) is nondet
True when Key is a known key in my ledger.
 1143ledger_current(Key, Gen, Value, Holders) :-
 1144    paxos_ledger_hook(current, Key, Gen, Value, Holders).
 1145ledger_current(Key, Gen, Value, Holders) :-
 1146    paxons_ledger(Key, Gen, Value, Holders),
 1147    valid(Holders).
 ledger(+Key, -Gen, -Value) is semidet
True if the ledger has Value associated with Key at generation Gen. Note that if the value is not yet acknowledged by the leader we should not use it.
 1156ledger(Key, Gen, Value) :-
 1157    paxos_ledger_hook(get, Key, Gen, Value0, Holders),
 1158    !,
 1159    valid(Holders),
 1160    Value = Value0.
 1161ledger(Key, Gen, Value) :-
 1162    paxons_ledger(Key, Gen, Value0, Holders),
 1163    valid(Holders),
 1164    !,
 1165    Value = Value0.
 ledger_create(+Key, +Gen, +Value) is det
Create a new Key-Value pair at generation Gen. This is executed during the preparation phase.
 1172ledger_create(Key, Gen, Value) :-
 1173    paxos_ledger_hook(create, Key, Gen, Value, -),
 1174    !.
 1175ledger_create(Key, Gen, Value) :-
 1176    get_time(Now),
 1177    asserta(paxons_ledger(Key, Gen, Value, created(Now))).
 ledger_update(+Key, +Gen, +Value) is semidet
Update Key to Value if the current generation is older than Gen. This reflects the accept phase of the protocol.
 1184ledger_update(Key, Gen, Value) :-
 1185    paxos_ledger_hook(accept, Key, Gen, Value, -),
 1186    !.
 1187ledger_update(Key, Gen, Value) :-
 1188    paxons_ledger(Key, Gen0, _Value, _Holders),
 1189    !,
 1190    Gen > Gen0,
 1191    get_time(Now),
 1192    asserta(paxons_ledger(Key, Gen, Value, accepted(Now))),
 1193    (   Gen0 == 0
 1194    ->  retractall(paxons_ledger(Key, Gen0, _, _))
 1195    ;   true
 1196    ).
 ledger_update_holders(+Key, +Gen, +Holders) is det
The leader acknowledged that Key@Gen represents a valid new
 1202ledger_update_holders(Key, Gen, Holders) :-
 1203    paxos_ledger_hook(set, Key, Gen, _, Holders),
 1204    !.
 1205ledger_update_holders(Key, Gen, Holders) :-
 1206    clause(paxons_ledger(Key, Gen, Value, Holders0), true, Ref),
 1207    !,
 1208    (   Holders0 == Holders
 1209    ->  true
 1210    ;   asserta(paxons_ledger(Key, Gen, Value, Holders)),
 1211        erase(Ref)
 1212    ),
 1213    clean_key(Holders0, Key, Gen).
 1214
 1215clean_key(Holders, _Key, _Gen) :-
 1216    valid(Holders),
 1217    !.
 1218clean_key(_, Key, Gen) :-
 1219    (   clause(paxons_ledger(Key, Gen0, _Value, _Holders0), true, Ref),
 1220        Gen0 < Gen,
 1221        erase(Ref),
 1222        fail
 1223    ;   true
 1224    ).
 ledger_learn(+Key, +Gen, +Value) is semidet
We received a learn event.
 1231ledger_learn(Key,Gen,Value) :-
 1232    paxos_ledger_hook(learn, Key, Gen, Value, -),
 1233    !.
 1234ledger_learn(Key,Gen,Value) :-
 1235    paxons_ledger(Key, Gen0, Value0, _Holders),
 1236    !,
 1237    (   Gen == Gen0,
 1238        Value == Value0
 1239    ->  true
 1240    ;   Gen > Gen0
 1241    ->  get_time(Now),
 1242        asserta(paxons_ledger(Key, Gen, Value, learned(Now)))
 1243    ).
 1244ledger_learn(Key,Gen,Value) :-
 1245    get_time(Now),
 1246    asserta(paxons_ledger(Key, Gen, Value, learned(Now))).
 ledger_forget(+Nodes) is det
Remove Nodes from all ledgers. This is executed in a background thread.
 1253ledger_forget(Nodes) :-
 1254    catch(thread_create(ledger_forget_threaded(Nodes), _,
 1255                        [ detached(true)
 1256                        ]),
 1257          error(permission_error(create, thread, _), _),
 1258          true).
 1259
 1260ledger_forget_threaded(Nodes) :-
 1261    debug(paxos(node), 'Forgetting 0x~16r', [Nodes]),
 1262    forall(ledger_current(Key, Gen, _Value, Holders),
 1263           ledger_forget(Nodes, Key, Gen, Holders)),
 1264    debug(paxos(node), 'Forgotten 0x~16r', [Nodes]).
 1265
 1266ledger_forget(Nodes, Key, Gen, Holders) :-
 1267    NewHolders is Holders /\ \Nodes,
 1268    (   NewHolders \== Holders,
 1269        ledger_update_holders(Key, Gen, NewHolders)
 1270    ->  true
 1271    ;   true
 1272    ).
 1273
 1274valid(Holders) :-
 1275    integer(Holders).
 1276
 1277
 1278		 /*******************************
 1279		 *             UTIL		*
 1280		 *******************************/
 c_element(+NewList, +Old, -Value)
A Muller c-element is a logic block used in asynchronous logic. Its output assumes the value of its input iff all of its inputs are identical. Otherwise, the output retains its original value.
 1288c_element([New | More], _Old, New) :-
 1289    forall(member(N, More), N == New),
 1290    !.
 1291c_element(_List, Old, Old).
 arg_union(+Arg, +ListOfTerms, -Set) is det
Get all the nth args from ListOfTerms and do a set union on the result.
 1298arg_union(Arg, NodeStatusList, Set) :-
 1299    maplist(arg(Arg), NodeStatusList, Sets),
 1300    list_union(Sets, Set).
 1301
 1302list_union(Sets, Set) :-
 1303    list_union(Sets, 0, Set).
 1304
 1305list_union([], Set, Set).
 1306list_union([H|T], Set0, Set) :-
 1307    Set1 is Set0 \/ H,
 1308    list_union(T, Set1, Set)