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\section{library(paxos): A Replicated Data Store}

\label{sec:paxos}

\begin{tags}
    \tag{author}
Jeffrey Rosenwald (JeffRose@acm.org)
    \tag{See also}
\file{tipc_broadcast.pl}, \file{udp_broadcast.pl}
    \tag{license}
BSD-2
\end{tags}

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:

\begin{quote}
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.
\end{quote}

\begin{quote}
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 \const{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.
\end{quote}

\begin{quote}
3: Finally, the coordinator concludes the successful negotiation by
broadcasting the agreement to the quorum in the form of a
paxos(\verb$changed(Key,Value)$ event. This is the only event that
should be of interest to user programs.
\end{quote}

For practical reasons, we rely on the partially synchronous behavior
(e.g. limited upper time bound for replies) of \predref{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 \predref{broadcast_request}{1} at the process level, thus obviating the need for
external mutual exclusion mechanisms.

\textit{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.}\vspace{0.7cm}

\begin{description}
    \predicate[det]{paxos_initialize}{1}{+Options}
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 \verb$paxos_initialize([])$ is executed lazily as part of
the first paxos operation. Defined options:

\begin{description}
    \termitem{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 \arg{NodeID} is a variable it is unified
with the discovered \arg{NodeID}.

\arg{NodeID} must be a small non-negative integer as these identifiers
are used in bitmaps.
\end{description}

    \predicate[semidet]{paxos_set}{1}{+Term}
Equivalent to \verb$paxos_key(Term,Key)$, \verb$pasox_set(Key,Term)$. I.e., \arg{Term}
is a ground compound term and its key is the name/arity pair. This
version provides compatibility with older versions of this library.

    \predicate[semidet]{paxos_set}{2}{+Key, +Value}
\nodescription
    \predicate[semidet]{paxos_set}{3}{+Key, +Value, +Options}
negotiates to have \arg{Key}-\arg{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. \predref{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, \predref{paxos_set}{2} will
succeed iff the first ballot succeeds.

On success, \predref{paxos_set}{1} will also broadcast the term
paxos(\verb$changed(Key,Value)$, to the quorum.

\arg{Options} processed:

\begin{description}
    \termitem{retry}{Retries}
is a non-negative integer specifying the number of retries that
will be performed before a set is abandoned. Defaults to the
\textit{setting} \verb$max_sets$ (20).
    \termitem{timeout}{+Seconds}
Max time to wait for the forum to reply. Defaults to the
\textit{setting} \verb$response_timeout$ (0.020, 20ms).
\end{description}

\begin{arguments}
\arg{Term} & is a compound that may have unbound variables. \\
\end{arguments}

\begin{tags}
    \tag{To be done}
If the \arg{Value} is already current, should we simply do nothing?
\end{tags}

    \predicate[semidet]{paxos_get}{1}{?Term}
Equivalent to \verb$paxos_key(Term,Key)$, \verb$pasox_get(Key,Term)$. I.e., \arg{Term}
is a compound term and its key is the name/arity pair. This version
provides compatibility with older versions of this library.

    \predicate[semidet]{paxos_get}{2}{+Key, +Value}
\nodescription
    \predicate[semidet]{paxos_get}{3}{+Key, +Value, +Options}
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 \predref{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.

\arg{Options} processed:

\begin{description}
    \termitem{retry}{Retries}
is a non-negative integer specifying the number of retries that
will be performed before a set is abandoned. Defaults to the
\textit{setting} \verb$max_gets$ (5).
    \termitem{timeout}{+Seconds}
Max time to wait for the forum to reply. Defaults to the
\textit{setting} \verb$response_timeout$ (0.020, 20ms).
\end{description}

\begin{arguments}
\arg{Term} & is a compound. Any unbound variables are unified with
those provided in the ledger entry. \\
\end{arguments}

    \predicate[det]{paxos_replicate_key}{3}{+Nodes:bitmap, ?Key, +Options}
Replicate a \arg{Key} to \arg{Nodes}. If \arg{Key} is unbound, a random key is
selected.

\begin{description}
    \termitem{timeout}{+Seconds}
Max time to wait for the forum to reply. Defaults to the
\textit{setting} \verb$response_timeout$ (0.020, 20ms).
\end{description}

    \predicate[det]{paxos_on_change}{2}{?Term, :Goal}
\nodescription
    \predicate[det]{paxos_on_change}{3}{?Key, ?Value, :Goal}
executes the specified \arg{Goal} when \arg{Key} changes. \predref{paxos_on_change}{2}
listens for paxos(\verb$changed(Key,Value)$ notifications for \arg{Key}, which
are emitted as the result of successful \predref{paxos_set}{3} transactions.
When one is received for \arg{Key}, then \arg{Goal} is executed in a separate
thread of execution.

\begin{arguments}
\arg{Term} & is a compound, identical to that used for
\predref{paxos_get}{1}. \\
\arg{Goal} & is one of:

\begin{itemize}
    \item a callable atom or term, or
    \item the atom \const{ignore}, which causes monitoring for \arg{Term} to be
discontinued.
\end{itemize}

 \\
\end{arguments}
\end{description}

