From 7e9a53c47103206538377cd6e540930c15fd7b58 Mon Sep 17 00:00:00 2001
From: Julian <git@julian-haas.de>
Date: Thu, 4 Apr 2024 18:20:23 +0200
Subject: [PATCH] initial Paxos version (untested)

---
 .../scala/replication/protocols/Paxos.scala   | 102 ++++++++++++++++++
 1 file changed, 102 insertions(+)
 create mode 100644 Modules/Example Replication/jvm/src/main/scala/replication/protocols/Paxos.scala

diff --git a/Modules/Example Replication/jvm/src/main/scala/replication/protocols/Paxos.scala b/Modules/Example Replication/jvm/src/main/scala/replication/protocols/Paxos.scala
new file mode 100644
index 000000000..35f36996d
--- /dev/null
+++ b/Modules/Example Replication/jvm/src/main/scala/replication/protocols/Paxos.scala	
@@ -0,0 +1,102 @@
+package replication.protocols
+
+import rdts.base.{Bottom, Lattice, Orderings, Uid}
+import rdts.syntax.LocalReplicaId
+import rdts.syntax.LocalReplicaId.replicaId
+import rdts.datatypes.GrowOnlySet._
+import rdts.base.Lattice.setLattice
+import rdts.datatypes.GrowOnlySet
+import rdts.datatypes.GrowOnlyMap.*
+import rdts.datatypes.GrowOnlyMap
+import rdts.datatypes.LastWriterWins
+import rdts.time.Dots
+import rdts.dotted.Dotted
+
+// message types
+case class Prepare(proposalNumber: Int)
+case class Promise[A](proposalNumber: Int, value: Option[A], acceptor: Uid)
+case class Accept[A](proposalNumber: Int, value: A)
+case class Accepted[A](proposalNumber: Int, value: A, acceptor: Uid)
+
+case class Paxos[A](
+    prepares: GrowOnlySet[Prepare],
+    promises: GrowOnlySet[Promise[A]],
+    accepts: GrowOnlySet[Accept[A]],
+    accepteds: GrowOnlySet[Accepted[A]]
+) {
+  val quorum = 2
+
+  def prepare(using LocalReplicaId, Bottom[A])(): Paxos[A] =
+    val proposalNumber = prepares.map(_.proposalNumber).maxOption.getOrElse(-1) + 1
+    // val p3      = proposedValues + ((replicaId, id) -> LastWriterWins.now(value))
+    val prepare = Prepare(proposalNumber)
+
+    Paxos.unchanged.copy(
+      prepares = prepares.insert(prepare)
+    )
+
+  def promise(using LocalReplicaId)(prepareId: Int): Paxos[A] =
+    val highestProposal = prepares.maxBy(_.proposalNumber)
+    // check if I already promised for an equally high id
+    if promises.filter(_.acceptor == replicaId).maxBy(_.proposalNumber).proposalNumber >= highestProposal.proposalNumber
+    then
+      // already promised for equally high id
+      Paxos.unchanged
+    else
+      // there is a new higher proposal
+      // check if I already promised a specific value
+      val value =
+        promises.filter(p => (p.acceptor == replicaId) && p.value.isDefined).map(_.value).headOption.flatten
+      Paxos.unchanged.copy(
+        promises = promises.insert(Promise(highestProposal.proposalNumber, value, replicaId))
+      )
+
+  def accept(using LocalReplicaId)(v: A): Paxos[A] =
+    val highestProposal     = prepares.maxByOption(_.proposalNumber)
+    val promisesForProposal = promises.filter(_.proposalNumber == highestProposal.map(_.proposalNumber).getOrElse(-1))
+    // check if accepted
+    if promisesForProposal.size < quorum then
+      // is not accepted
+      Paxos.unchanged
+    else
+      // is accepted, check if promise contains value
+      val promisesWithVal = promisesForProposal.filter(_.value.isDefined)
+      val value           = promisesWithVal.map(_.value).head.getOrElse(v)
+      Paxos.unchanged.copy(
+        accepts = accepts.insert(Accept(highestProposal.get.proposalNumber, value))
+      )
+
+  def accepted(using LocalReplicaId): Paxos[A] =
+    // get highest accept message
+    val highestAccept = accepts.maxByOption(_.proposalNumber)
+    if highestAccept.isEmpty || // there are no accepts
+      // I have already promised a higher proposalNumber
+      promises.filter(_.acceptor == replicaId).map(_.proposalNumber).maxOption.getOrElse(-1) >
+      highestAccept.get.proposalNumber
+    then
+      Paxos.unchanged
+    else
+      Paxos.unchanged.copy(accepteds =
+        accepteds.insert(Accepted(
+          proposalNumber = highestAccept.get.proposalNumber,
+          value = highestAccept.get.value,
+          acceptor = replicaId
+        ))
+      )
+}
+
+object Paxos {
+  given lattice[A]: Lattice[Paxos[A]]               = Lattice.derived
+  given dottedLattice[A]: Lattice[Dotted[Paxos[A]]] = Lattice.derived
+
+//  given Bottom[Int] with
+//    override def empty: Int = Int.MinValue
+
+  def unchanged[A]: Paxos[A] =
+    Paxos[A](
+      GrowOnlySet.empty[Prepare],
+      GrowOnlySet.empty[Promise[A]],
+      GrowOnlySet.empty[Accept[A]],
+      GrowOnlySet.empty[Accepted[A]]
+    )
+}