The ACS and BA algorithms

Cargo.toml

@@ -9,13 +9,14 @@ log = "0.4.1"
 reed-solomon-erasure = "3.0"
 merkle = { git = "https://github.com/vkomenda/merkle.rs", branch = "public-proof" }
 ring = "^0.12"
-rand = "*"
 protobuf = "1.4.4"
 crossbeam = "0.3.2"
 crossbeam-channel = "0.1"
+itertools = "0.7"
 
 [build-dependencies]
 protoc-rust = "1.4.4"
 
 [dev-dependencies]
 docopt = "0.8"
+rand = "0.3"

TODO

@@ -0,0 +1,12 @@
+TODO
+====
+
+* Fix the inappropriate use of Common Coin in the Byzantine Agreement protocol
+
+   This bug is explained in https://github.com/amiller/HoneyBadgerBFT/issues/59
+   where a solution is suggested introducing an additional type of message,
+   CONF.
+
+   There may be alternative solutions, such as using a different Byzantine
+   Agreement protocol altogether, for example,
+   https://people.csail.mit.edu/silvio/Selected%20Scientific%20Papers/Distributed%20Computation/BYZANTYNE%20AGREEMENT%20MADE%20TRIVIAL.pdf

proto/message.proto

@@ -41,12 +41,12 @@ message LemmaProto {
     bytes left_sibling_hash = 3;
     bytes right_sibling_hash = 4;
   }
-
 }
 
 message AgreementProto {
+  uint32 epoch = 1;
   oneof payload {
-    bool bval = 1;
-    bool aux = 2;
+    bool bval = 2;
+    bool aux = 3;
   }
 }

src/agreement.rs

@@ -1,42 +1,286 @@
 //! Binary Byzantine agreement protocol from a common coin protocol.
 
-use proto::AgreementMessage;
-use std::collections::{BTreeSet, VecDeque};
+use itertools::Itertools;
+use std::collections::{BTreeSet, HashMap, VecDeque};
+use std::hash::Hash;
 
-#[derive(Default)]
-pub struct Agreement {
-    input: Option<bool>,
-    _bin_values: BTreeSet<bool>,
+use proto::message;
+
+/// Type of output from the Agreement message handler. The first component is
+/// the value on which the Agreement has decided, also called "output" in the
+/// HoneyadgerBFT paper. The second component is a queue of messages to be sent
+/// to remote nodes as a result of handling the incomming message.
+type AgreementOutput = (Option<bool>, VecDeque<AgreementMessage>);
+
+/// Messages sent during the binary Byzantine agreement stage.
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub enum AgreementMessage {
+    /// BVAL message with an epoch.
+    BVal((u32, bool)),
+    /// AUX message with an epoch.
+    Aux((u32, bool)),
+}
+
+impl AgreementMessage {
+    pub fn into_proto(self) -> message::AgreementProto {
+        let mut p = message::AgreementProto::new();
+        match self {
+            AgreementMessage::BVal((e, b)) => {
+                p.set_epoch(e);
+                p.set_bval(b);
+            }
+            AgreementMessage::Aux((e, b)) => {
+                p.set_epoch(e);
+                p.set_aux(b);
+            }
+        }
+        p
+    }
+
+    // TODO: Re-enable lint once implemented.
+    #[cfg_attr(feature = "cargo-clippy", allow(needless_pass_by_value))]
+    pub fn from_proto(mp: message::AgreementProto) -> Option<Self> {
+        let epoch = mp.get_epoch();
+        if mp.has_bval() {
+            Some(AgreementMessage::BVal((epoch, mp.get_bval())))
+        } else if mp.has_aux() {
+            Some(AgreementMessage::Aux((epoch, mp.get_aux())))
+        } else {
+            None
+        }
+    }
+}
+
+/// Binary Agreement instance.
+pub struct Agreement<NodeUid> {
+    /// The UID of the corresponding proposer node.
+    uid: NodeUid,
+    num_nodes: usize,
+    num_faulty_nodes: usize,
+    epoch: u32,
+    /// Bin values. Reset on every epoch update.
+    bin_values: BTreeSet<bool>,
+    /// Values received in BVAL messages. Reset on every epoch update.
+    received_bval: HashMap<NodeUid, BTreeSet<bool>>,
+    /// Sent BVAL values. Reset on every epoch update.
+    sent_bval: BTreeSet<bool>,
+    /// Values received in AUX messages. Reset on every epoch update.
+    received_aux: HashMap<NodeUid, bool>,
+    /// The estimate of the decision value in the current epoch.
+    estimated: Option<bool>,
+    /// The value output by the agreement instance. It is set once to `Some(b)`
+    /// and then never changed. That is, no instance of Binary Agreement can
+    /// decide on two different values of output.
+    output: Option<bool>,
+    /// Termination flag. The Agreement instance doesn't terminate immediately
+    /// upon deciding on the agreed value. This is done in order to help other
+    /// nodes decide despite asynchrony of communication. Once the instance
+    /// determines that all the remote nodes have reached agreement, it sets the
+    /// `terminated` flag and accepts no more incoming messages.
+    terminated: bool,
 }
 
-impl Agreement {
-    pub fn new() -> Self {
+impl<NodeUid: Clone + Eq + Hash> Agreement<NodeUid> {
+    pub fn new(uid: NodeUid, num_nodes: usize) -> Self {
+        let num_faulty_nodes = (num_nodes - 1) / 3;
+
         Agreement {
-            input: None,
-            _bin_values: BTreeSet::new(),
+            uid,
+            num_nodes,
+            num_faulty_nodes,
+            epoch: 0,
+            bin_values: BTreeSet::new(),
+            received_bval: HashMap::new(),
+            sent_bval: BTreeSet::new(),
+            received_aux: HashMap::new(),
+            estimated: None,
+            output: None,
+            terminated: false,
         }
     }
 
-    pub fn set_input(&mut self, input: bool) -> AgreementMessage {
-        self.input = Some(input);
+    /// Algorithm has terminated.
+    pub fn terminated(&self) -> bool {
+        self.terminated
+    }
+
+    /// Sets the input value for agreement.
+    pub fn set_input(&mut self, input: bool) -> Result<AgreementMessage, Error> {
+        if self.epoch != 0 {
+            return Err(Error::InputNotAccepted);
+        }
+
+        // Set the initial estimated value to the input value.
+        self.estimated = Some(input);
+        // Receive the BVAL message locally.
+        self.received_bval
+            .entry(self.uid.clone())
+            .or_insert_with(BTreeSet::new)
+            .insert(input);
         // Multicast BVAL
-        AgreementMessage::BVal(input)
+        Ok(AgreementMessage::BVal((self.epoch, input)))
     }
 
-    pub fn has_input(&self) -> bool {
-        self.input.is_some()
+    /// Acceptance check to be performed before setting the input value.
+    pub fn accepts_input(&self) -> bool {
+        self.epoch == 0 && self.estimated.is_none()
     }
 
     /// Receive input from a remote node.
-    pub fn on_input(
-        &self,
-        _message: &AgreementMessage,
-    ) -> Result<VecDeque<AgreementMessage>, Error> {
-        Err(Error::NotImplemented)
+    ///
+    /// Outputs an optional agreement result and a queue of agreement messages
+    /// to remote nodes. There can be up to 2 messages.
+    pub fn handle_agreement_message(
+        &mut self,
+        sender_id: &NodeUid,
+        message: &AgreementMessage,
+    ) -> Result<AgreementOutput, Error> {
+        match *message {
+            // The algorithm instance has already terminated.
+            _ if self.terminated => Err(Error::Terminated),
+
+            AgreementMessage::BVal((epoch, b)) if epoch == self.epoch => {
+                self.handle_bval(sender_id, b)
+            }
+
+            AgreementMessage::Aux((epoch, b)) if epoch == self.epoch => {
+                self.handle_aux(sender_id, b)
+            }
+
+            // Epoch does not match. Ignore the message.
+            _ => Ok((None, VecDeque::new())),
+        }
+    }
+
+    fn handle_bval(&mut self, sender_id: &NodeUid, b: bool) -> Result<AgreementOutput, Error> {
+        let mut outgoing = VecDeque::new();
+
+        self.received_bval
+            .entry(sender_id.clone())
+            .or_insert_with(BTreeSet::new)
+            .insert(b);
+        let count_bval = self.received_bval
+            .values()
+            .filter(|values| values.contains(&b))
+            .count();
+
+        // upon receiving BVAL_r(b) messages from 2f + 1 nodes,
+        // bin_values_r := bin_values_r ∪ {b}
+        if count_bval == 2 * self.num_faulty_nodes + 1 {
+            self.bin_values.insert(b);
+
+            // wait until bin_values_r != 0, then multicast AUX_r(w)
+            // where w ∈ bin_values_r
+            if self.bin_values.len() == 1 {
+                // Send an AUX message at most once per epoch.
+                outgoing.push_back(AgreementMessage::Aux((self.epoch, b)));
+                // Receive the AUX message locally.
+                self.received_aux.insert(self.uid.clone(), b);
+            }
+
+            outgoing.extend(self.try_coin());
+            Ok((self.output, outgoing))
+        }
+        // upon receiving BVAL_r(b) messages from f + 1 nodes, if
+        // BVAL_r(b) has not been sent, multicast BVAL_r(b)
+        else if count_bval == self.num_faulty_nodes + 1 && !self.sent_bval.contains(&b) {
+            outgoing.push_back(AgreementMessage::BVal((self.epoch, b)));
+            // Receive the BVAL message locally.
+            self.received_bval
+                .entry(self.uid.clone())
+                .or_insert_with(BTreeSet::new)
+                .insert(b);
+            Ok((None, outgoing))
+        } else {
+            Ok((None, outgoing))
+        }
+    }
+
+    fn handle_aux(&mut self, sender_id: &NodeUid, b: bool) -> Result<AgreementOutput, Error> {
+        self.received_aux.insert(sender_id.clone(), b);
+        let mut outgoing = VecDeque::new();
+        if !self.bin_values.is_empty() {
+            outgoing.extend(self.try_coin());
+            Ok((self.output, outgoing))
+        } else {
+            Ok((None, outgoing))
+        }
+    }
+
+    /// AUX_r messages such that the set of values carried by those messages is
+    /// a subset of bin_values_r. Outputs this subset.
+    ///
+    /// FIXME: Clarify whether the values of AUX messages should be the same or
+    /// not. It is assumed in `count_aux` that they can differ.
+    ///
+    /// In general, we can't expect every good node to send the same AUX value,
+    /// so waiting for N - f agreeing messages would not always terminate. We
+    /// can, however, expect every good node to send an AUX value that will
+    /// eventually end up in our bin_values.
+    fn count_aux(&self) -> (usize, BTreeSet<bool>) {
+        let (vals_cnt, vals) = self.received_aux
+            .values()
+            .filter(|b| self.bin_values.contains(b))
+            .tee();
+
+        (vals_cnt.count(), vals.cloned().collect())
+    }
+
+    /// Waits until at least (N − f) AUX_r messages have been received, such that
+    /// the set of values carried by these messages, vals, are a subset of
+    /// bin_values_r (note that bin_values_r may continue to change as BVAL_r
+    /// messages are received, thus this condition may be triggered upon arrival
+    /// of either an AUX_r or a BVAL_r message).
+    ///
+    /// Once the (N - f) messages are received, gets a common coin and uses it
+    /// to compute the next decision estimate and, optionally, sets the output
+    /// decision value. The function may start the next epoch. In that case, it
+    /// returns a message for broadcast.
+    fn try_coin(&mut self) -> VecDeque<AgreementMessage> {
+        let (count_aux, vals) = self.count_aux();
+        if count_aux < self.num_nodes - self.num_faulty_nodes {
+            // Continue waiting for the (N - f) AUX messages.
+            return VecDeque::new();
+        }
+
+        // FIXME: Implement the Common Coin algorithm. At the moment the
+        // coin value is common across different nodes but not random.
+        let coin = (self.epoch % 2) == 0;
+
+        // Check the termination condition: "continue looping until both a
+        // value b is output in some round r, and the value Coin_r' = b for
+        // some round r' > r."
+        self.terminated = self.terminated || self.output == Some(coin);
+
+        // Start the next epoch.
+        self.bin_values.clear();
+        self.received_aux.clear();
+        self.epoch += 1;
+
+        if vals.len() != 1 {
+            self.estimated = Some(coin);
+        } else {
+            // NOTE: `vals` has exactly one element due to `vals.len() == 1`
+            let v: Vec<bool> = vals.into_iter().collect();
+            let b = v[0];
+            self.estimated = Some(b);
+            // Setting the output value is allowed only once.
+            if self.output.is_none() && b == coin {
+                // Output the agreement value.
+                self.output = Some(b);
+            }
+        }
+
+        vec![AgreementMessage::BVal((
+            self.epoch,
+            self.estimated.unwrap(),
+        ))].into_iter()
+            .collect()
     }
 }
 
 #[derive(Clone, Debug)]
 pub enum Error {
-    NotImplemented,
+    Terminated,
+    InputNotAccepted,
 }