fix(blend): ensure all nodes are connected and have the same peering degree (#64)

* fix(blendnet): ensure all nodes are connected and have the same peering degree

* cover the case where it is impossible to ensure all nodes have the same num of conns

simlib/blendnet-sims/src/main.rs

@@ -16,12 +16,12 @@ use netrunner::node::{NodeId, NodeIdExt};
 use netrunner::output_processors::Record;
 use netrunner::runner::{BoxedNode, SimulationRunnerHandle};
 use netrunner::streaming::{io::IOSubscriber, naive::NaiveSubscriber, StreamType};
+use node::blend::topology::build_topology;
 use nomos_blend::cover_traffic::CoverTrafficSettings;
 use nomos_blend::message_blend::{
     CryptographicProcessorSettings, MessageBlendSettings, TemporalSchedulerSettings,
 };
 use parking_lot::Mutex;
-use rand::prelude::IteratorRandom;
 use rand::seq::SliceRandom;
 use rand::{RngCore, SeedableRng};
 use rand_chacha::ChaCha12Rng;
@@ -87,9 +87,10 @@ impl SimulationApp {
         let behaviours = create_behaviours(&settings.simulation_settings.network_settings);
         let regions_data = RegionsData::new(regions, behaviours);
 
-        let ids = node_ids.clone();
         let network = Arc::new(Mutex::new(Network::<BlendMessage>::new(regions_data, seed)));
 
+        let topology = build_topology(&node_ids, settings.connected_peers_count, &mut rng);
+
         let nodes: Vec<_> = node_ids
             .iter()
             .copied()
@@ -101,11 +102,7 @@ impl SimulationApp {
                     settings.simulation_settings.clone(),
                     no_netcap,
                     BlendnodeSettings {
-                        connected_peers: ids
+                        connected_peers: topology.get(&node_id).unwrap().iter().copied().collect(),
-                            .iter()
-                            .filter(|&id| id != &node_id)
-                            .copied()
-                            .choose_multiple(&mut rng, settings.connected_peers_count),
                         data_message_lottery_interval: settings.data_message_lottery_interval,
                         stake_proportion: settings.stake_proportion / node_ids.len() as f64,
                         seed: rng.next_u64(),
@@ -174,7 +171,11 @@ fn create_boxed_blendnode(
         node_message_sender,
         network_message_receiver,
     );
-    Box::new(BlendNode::new(node_id, blendnode_settings, network_interface))
+    Box::new(BlendNode::new(
+        node_id,
+        blendnode_settings,
+        network_interface,
+    ))
 }
 
 fn run<M, S, T>(

simlib/blendnet-sims/src/node/blend/mod.rs

@@ -4,6 +4,7 @@ mod message;
 pub mod scheduler;
 pub mod state;
 pub mod stream_wrapper;
+pub mod topology;
 
 use crate::node::blend::consensus_streams::{Epoch, Slot};
 use cached::{Cached, TimedCache};

simlib/blendnet-sims/src/node/blend/topology.rs

@@ -0,0 +1,127 @@
+use std::collections::{HashMap, HashSet};
+
+use netrunner::node::NodeId;
+use rand::{seq::SliceRandom, RngCore};
+
+pub type Topology = HashMap<NodeId, HashSet<NodeId>>;
+
+/// Builds a topology with the given nodes and peering degree
+/// by ensuring that all nodes are connected (no partition)
+/// and all nodes have the same number of connections (only if possible).
+pub fn build_topology<R: RngCore>(nodes: &[NodeId], peering_degree: usize, mut rng: R) -> Topology {
+    tracing::info!("Building topology: peering_degree:{}", peering_degree);
+    loop {
+        let mut topology = nodes
+            .iter()
+            .map(|&node| (node, HashSet::new()))
+            .collect::<HashMap<_, _>>();
+
+        for node in nodes.iter() {
+            // Collect peer candidates
+            let mut others = nodes
+                .iter()
+                .filter(|&other| {
+                    // Check if the other node is not already connected to the current node
+                    // and the other node has not reached the peering degree.
+                    other != node
+                        && !topology.get(node).unwrap().contains(other)
+                        && topology.get(other).unwrap().len() < peering_degree
+                })
+                .copied()
+                .collect::<Vec<_>>();
+
+            // How many more connections the current node needs
+            let num_needs = peering_degree - topology.get(node).unwrap().len();
+            // Sample peers as many as possible and connect them to the current node
+            let k = std::cmp::min(num_needs, others.len());
+            others.as_mut_slice().shuffle(&mut rng);
+            others.into_iter().take(k).for_each(|peer| {
+                topology.get_mut(node).unwrap().insert(peer);
+                topology.get_mut(&peer).unwrap().insert(*node);
+            });
+        }
+
+        // Check constraints:
+        // - All nodes are connected (no partition)
+        // - All nodes have the same number of connections (if possible)
+        let can_have_equal_conns = (nodes.len() * peering_degree) % 2 == 0;
+        if check_all_connected(&topology)
+            && (!can_have_equal_conns || check_equal_conns(&topology, peering_degree))
+        {
+            return topology;
+        }
+        tracing::info!("Topology doesn't meet constraints. Retrying...");
+    }
+}
+
+/// Checks if all nodes are connected (no partition) in the topology.
+fn check_all_connected(topology: &Topology) -> bool {
+    let visited = dfs(topology, *topology.keys().next().unwrap());
+    visited.len() == topology.len()
+}
+
+/// Depth-first search to visit nodes in the topology.
+fn dfs(topology: &Topology, start_node: NodeId) -> HashSet<NodeId> {
+    let mut visited: HashSet<NodeId> = HashSet::new();
+    let mut stack: Vec<NodeId> = Vec::new();
+    stack.push(start_node);
+    while let Some(node) = stack.pop() {
+        visited.insert(node);
+        for peer in topology.get(&node).unwrap().iter() {
+            if !visited.contains(peer) {
+                stack.push(*peer);
+            }
+        }
+    }
+    visited
+}
+
+/// Checks if all nodes have the same number of connections.
+fn check_equal_conns(topology: &Topology, peering_degree: usize) -> bool {
+    topology
+        .iter()
+        .all(|(_, peers)| peers.len() == peering_degree)
+}
+
+#[cfg(test)]
+mod tests {
+    use netrunner::node::NodeIdExt;
+
+    use super::*;
+
+    #[test]
+    fn test_build_topology_with_equal_conns() {
+        // If num_nodes * peering_degree is even,
+        // it is possible that all nodes can have the same number of connections
+        let nodes = (0..7).map(NodeId::from_index).collect::<Vec<_>>();
+        let peering_degree = 4;
+
+        let mut rng = rand::rngs::OsRng;
+        let topology = build_topology(&nodes, peering_degree, &mut rng);
+        assert_eq!(topology.len(), nodes.len());
+        for (node, peers) in topology.iter() {
+            assert!(peers.len() == peering_degree);
+            for peer in peers.iter() {
+                assert!(topology.get(peer).unwrap().contains(node));
+            }
+        }
+    }
+
+    #[test]
+    fn test_build_topology_with_inequal_conns() {
+        // If num_nodes * peering_degree is odd,
+        // it is impossible that all nodes can have the same number of connections
+        let nodes = (0..7).map(NodeId::from_index).collect::<Vec<_>>();
+        let peering_degree = 3;
+
+        let mut rng = rand::rngs::OsRng;
+        let topology = build_topology(&nodes, peering_degree, &mut rng);
+        assert_eq!(topology.len(), nodes.len());
+        for (node, peers) in topology.iter() {
+            assert!(peers.len() <= peering_degree);
+            for peer in peers.iter() {
+                assert!(topology.get(peer).unwrap().contains(node));
+            }
+        }
+    }
+}