added remote table to update respones

Cargo.toml

@@ -53,6 +53,7 @@ unstable = []
 ipv6 = []
 cuda = []
 erasure = []
+gossip_choose_weighted_peer = []
 
 [dependencies]
 rayon = "1.0.0"

src/choose_gossip_peer_strategy.rs

@@ -0,0 +1,323 @@
+use crdt::ReplicatedData;
+use rand::thread_rng;
+use rand::distributions::{IndependentSample, Weighted, WeightedChoice};
+use result::{Error, Result};
+use signature::PublicKey;
+use std;
+use std::collections::HashMap;
+
+pub const DEFAULT_WEIGHT: u32 = 1;
+
+pub trait ChooseGossipPeerStrategy {
+    fn choose_peer(&self, options: Vec<&ReplicatedData>) -> 
+        Result<ReplicatedData>;
+}
+
+pub struct ChooseRandomPeerStrategy<'a> {
+    random: &'a Fn() -> u64,
+}
+
+impl<'a> ChooseRandomPeerStrategy<'a> {
+    pub fn new(random: &'a Fn() -> u64,) -> Self {
+        ChooseRandomPeerStrategy { random }
+    }
+}
+
+impl<'a> ChooseGossipPeerStrategy for ChooseRandomPeerStrategy<'a> {
+    fn choose_peer(&self, options: Vec<&ReplicatedData>) -> Result<ReplicatedData> {
+        if options.len() < 1 {
+            return Err(Error::CrdtTooSmall);
+        }
+
+        let n = ((self.random)() as usize) % options.len();
+        Ok(options[n].clone())
+    }
+}
+
+pub struct ChooseWeightedPeerStrategy<'a> {
+    remote: &'a HashMap<PublicKey, u64>,
+    external_liveness: &'a HashMap<PublicKey, HashMap<PublicKey, u64>>,
+    get_stake: &'a Fn(PublicKey) -> f64,
+}
+
+impl<'a> ChooseWeightedPeerStrategy<'a> {
+    pub fn new(
+        remote: &'a HashMap<PublicKey, u64>,
+        external_liveness: &'a HashMap<PublicKey, HashMap<PublicKey, u64>>,
+        get_stake: &'a Fn(PublicKey) -> f64,
+    ) -> Self
+    {
+        ChooseWeightedPeerStrategy { remote, external_liveness, get_stake }
+    }
+
+    fn calculate_weighted_remote_index(&self, peer_id: PublicKey) -> u32 {
+        let mut last_seen_index = 0;
+        // If the peer is not in our remote table, then we leave last_seen_index as zero.
+        // Only happens when a peer appears in our crdt.table but not in our crdt.remote,
+        // which means a validator was directly injected into our crdt.table
+        if let Some(index) = self.remote.get(&peer_id) {
+            last_seen_index = *index;
+        }
+
+        let liveness_entry = self.external_liveness.get(&peer_id);
+        if liveness_entry.is_none(){
+            return DEFAULT_WEIGHT;
+        }
+
+        let votes = liveness_entry.unwrap();
+
+        if votes.is_empty() {
+            return DEFAULT_WEIGHT;
+        }
+
+        // Calculate the weighted average of the rumors
+        let mut relevant_votes = vec![];
+
+        let total_stake = votes.iter().fold(
+            0.0, 
+            |total_stake, (&id, &vote)| {
+                let stake = (self.get_stake)(id);
+                // If the total stake is going to overflow u64, pick
+                // the larger of either the current total_stake, or the 
+                // new stake, this way we are guaranteed to get at least u64/2 
+                // sample of stake in our weighted calculation
+                if std::f64::MAX - total_stake < stake {
+                    if stake > total_stake {
+                        relevant_votes = vec![(stake, vote)];
+                        stake
+                    } else {
+                        total_stake
+                    }
+                } else {
+                    relevant_votes.push((stake, vote));
+                    total_stake + stake
+                }
+            }
+        );
+
+        let weighted_vote = relevant_votes.iter().fold(
+            0.0,
+            |sum, &(stake, vote)| {
+                if vote < last_seen_index {
+                    // This should never happen b/c we maintain the invariant that the indexes
+                    // in the external_liveness table are always greater than the corresponding 
+                    // indexes in the remote table, if the index exists in the remote table at all.
+                    
+                    // Case 1: Attempt to insert bigger index into the "external_liveness" table
+                    // happens after an insertion into the "remote" table. In this case, 
+                    // (see apply_updates()) function, we prevent the insertion if the entry
+                    // in the remote table >= the atempted insertion into the "external" liveness
+                    // table.
+                    
+                    // Case 2: Bigger index in the "external_liveness" table inserted before
+                    // a smaller insertion into the "remote" table. We clear the corresponding 
+                    // "external_liveness" table entry on all insertions into the  "remote" table
+                    // See apply_updates() function.
+
+                    warn!("weighted peer index was smaller than local entry in remote table");
+                    return sum;
+                }
+
+                let vote_difference = (vote - last_seen_index) as f64;
+                let new_weight = vote_difference * (stake / total_stake);
+
+                if std::f64::MAX - sum < new_weight {
+                    return f64::max(new_weight, sum);
+                }
+
+                sum + new_weight
+            },
+        );
+
+        // Return u32 b/c the weighted sampling API from rand::distributions
+        // only takes u32 for weights
+        if weighted_vote >= std::u32::MAX as f64 {
+            return std::u32::MAX;
+        }
+
+        // If the weighted rumors we've heard about aren't any greater than
+        // what we've directly learned from the last time we communicated with the 
+        // peer (i.e. weighted_vote == 0), then return a weight of 1. 
+        // Otherwise, return the calculated weight.
+        weighted_vote as u32 + DEFAULT_WEIGHT
+    }
+}
+
+impl<'a> ChooseGossipPeerStrategy for ChooseWeightedPeerStrategy<'a> {
+    fn choose_peer(&self, options: Vec<&ReplicatedData>) -> Result<ReplicatedData> {
+        if options.len() < 1 {
+            return Err(Error::CrdtTooSmall);
+        }
+
+        let mut weighted_peers = vec![];
+        for peer in options {
+            let weight = self.calculate_weighted_remote_index(peer.id);
+            weighted_peers.push(Weighted {
+                weight: weight,
+                item: peer,
+            });
+        }
+
+        let mut rng = thread_rng();
+        Ok(WeightedChoice::new(&mut weighted_peers).ind_sample(&mut rng).clone())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use logger;
+    use signature::{KeyPair, KeyPairUtil, PublicKey};
+    use std;
+    use std::collections::HashMap;
+    use choose_gossip_peer_strategy::{ChooseWeightedPeerStrategy, DEFAULT_WEIGHT};
+
+    fn get_stake(id: PublicKey) -> f64 {
+        return 1.0;
+    }
+
+    #[test]
+    fn test_default() {
+        logger::setup();
+
+        // Initialize the filler keys
+        let key1 = KeyPair::new().pubkey();
+
+        let remote: HashMap<PublicKey, u64> = HashMap::new();
+        let external_liveness: HashMap<PublicKey, HashMap<PublicKey, u64>> = HashMap::new();
+
+        let weighted_strategy = ChooseWeightedPeerStrategy::new(
+            &remote,
+            &external_liveness,
+            &get_stake,
+        );
+
+        // If external_liveness table doesn't contain this entry, 
+        // return the default weight
+        let result = weighted_strategy.calculate_weighted_remote_index(key1);
+        assert_eq!(result, DEFAULT_WEIGHT);
+    }
+
+    #[test]
+    fn test_only_external_liveness() {
+        logger::setup();
+
+        // Initialize the filler keys
+        let key1 = KeyPair::new().pubkey();
+        let key2 = KeyPair::new().pubkey();
+
+        let remote: HashMap<PublicKey, u64> = HashMap::new();
+        let mut external_liveness: HashMap<PublicKey, HashMap<PublicKey, u64>> = HashMap::new();
+
+        // If only the liveness table contains the entry, should return the
+        // weighted liveness entries 
+        let test_value : u32 = 5;
+        let mut rumors: HashMap<PublicKey, u64> = HashMap::new();
+        rumors.insert(key2, test_value as u64);
+        external_liveness.insert(key1, rumors);
+
+        let weighted_strategy = ChooseWeightedPeerStrategy::new(
+            &remote,
+            &external_liveness,
+            &get_stake,
+        );
+
+        let result = weighted_strategy.calculate_weighted_remote_index(key1);
+        assert_eq!(result, test_value + DEFAULT_WEIGHT);
+    }
+
+    #[test]
+    fn test_overflow_votes() {
+        logger::setup();
+
+        // Initialize the filler keys
+        let key1 = KeyPair::new().pubkey();
+        let key2 = KeyPair::new().pubkey();
+
+        let remote: HashMap<PublicKey, u64> = HashMap::new();
+        let mut external_liveness: HashMap<PublicKey, HashMap<PublicKey, u64>> = HashMap::new();
+
+        // If the vote index is greater than u32::MAX, default to u32::MAX
+        let test_value = (std::u32::MAX as u64) + 10;
+        let mut rumors: HashMap<PublicKey, u64> = HashMap::new();
+        rumors.insert(key2, test_value);
+        external_liveness.insert(key1, rumors);
+
+        let weighted_strategy = ChooseWeightedPeerStrategy::new(
+            &remote,
+            &external_liveness,
+            &get_stake,
+        );
+
+        let result = weighted_strategy.calculate_weighted_remote_index(key1);
+        assert_eq!(result, std::u32::MAX);
+    }
+
+    #[test]
+    fn test_many_validators() {
+        logger::setup();
+
+        // Initialize the filler keys
+        let key1 = KeyPair::new().pubkey();
+
+        let mut remote: HashMap<PublicKey, u64> = HashMap::new();
+        let mut external_liveness: HashMap<PublicKey, HashMap<PublicKey, u64>> = HashMap::new();
+
+        // Test many validators' rumors in external_liveness 
+        let num_peers = 10;
+        let mut rumors: HashMap<PublicKey, u64> = HashMap::new();
+
+        remote.insert(key1, 0);
+
+        for i in 0..num_peers {
+            let pk = KeyPair::new().pubkey();
+            rumors.insert(pk, i);
+        }
+
+        external_liveness.insert(key1, rumors);
+
+        let weighted_strategy = ChooseWeightedPeerStrategy::new(
+            &remote,
+            &external_liveness,
+            &get_stake,
+        );
+
+        let result = weighted_strategy.calculate_weighted_remote_index(key1);
+        assert_eq!(result, (num_peers/2) as u32);
+    }
+
+    #[test]
+    fn test_many_validators2() {
+        logger::setup();
+
+        // Initialize the filler keys
+        let key1 = KeyPair::new().pubkey();
+
+        let mut remote: HashMap<PublicKey, u64> = HashMap::new();
+        let mut external_liveness: HashMap<PublicKey, HashMap<PublicKey, u64>> = HashMap::new();
+
+        // Test many validators' rumors in external_liveness 
+        let num_peers = 10;
+        let old_index = 20;
+        let mut rumors: HashMap<PublicKey, u64> = HashMap::new();
+
+        remote.insert(key1, old_index);
+
+        for i in 0..num_peers {
+            let pk = KeyPair::new().pubkey();
+            rumors.insert(pk, old_index);
+        }
+
+        external_liveness.insert(key1, rumors);
+
+        let weighted_strategy = ChooseWeightedPeerStrategy::new(
+            &remote,
+            &external_liveness,
+            &get_stake,
+        );
+
+        let result = weighted_strategy.calculate_weighted_remote_index(key1);
+
+        // If nobody has seen a newer update then rever to default
+        assert_eq!(result, DEFAULT_WEIGHT);
+    }
+}

src/crdt.rs

@@ -15,6 +15,11 @@
 
 use bincode::{deserialize, serialize};
 use byteorder::{LittleEndian, ReadBytesExt};
+use choose_gossip_peer_strategy::{
+    ChooseGossipPeerStrategy,
+    ChooseRandomPeerStrategy,
+    ChooseWeightedPeerStrategy,
+};
 use hash::Hash;
 use packet::{to_blob, Blob, BlobRecycler, SharedBlob, BLOB_SIZE};
 use pnet_datalink as datalink;
@@ -190,6 +195,7 @@ pub struct Crdt {
     pub alive: HashMap<PublicKey, u64>,
     pub update_index: u64,
     pub me: PublicKey,
+    external_liveness: HashMap<PublicKey, HashMap<PublicKey, u64>>,
 }
 // TODO These messages should be signed, and go through the gpu pipeline for spam filtering
 #[derive(Serialize, Deserialize, Debug)]
@@ -200,7 +206,7 @@ enum Protocol {
     RequestUpdates(u64, ReplicatedData),
     //TODO might need a since?
     /// from id, form's last update index, ReplicatedData
-    ReceiveUpdates(PublicKey, u64, Vec<ReplicatedData>),
+    ReceiveUpdates(PublicKey, u64, Vec<ReplicatedData>, Vec<(PublicKey, u64)>),
     /// ask for a missing index
     RequestWindowIndex(ReplicatedData, u64),
 }
@@ -213,6 +219,7 @@ impl Crdt {
             local: HashMap::new(),
             remote: HashMap::new(),
             alive: HashMap::new(),
+            external_liveness: HashMap::new(),
             me: me.id,
             update_index: 1,
         };
@@ -234,6 +241,14 @@ impl Crdt {
         self.insert(&me);
     }
 
+    pub fn get_external_liveness_entry(
+        &self,
+        key: &PublicKey,
+    ) -> Option<&HashMap<PublicKey, u64>>
+    {
+        self.external_liveness.get(key)
+    }
+
     pub fn insert(&mut self, v: &ReplicatedData) {
         // TODO check that last_verified types are always increasing
         if self.table.get(&v.id).is_none() || (v.version > self.table[&v.id].version) {
@@ -270,9 +285,11 @@ impl Crdt {
         if self.table.len() <= MIN_TABLE_SIZE {
             return;
         }
+
         //wait for 4x as long as it would randomly take to reach our node
         //assuming everyone is waiting the same amount of time as this node
         let limit = self.table.len() as u64 * GOSSIP_SLEEP_MILLIS * 4;
+
         let dead_ids: Vec<PublicKey> = self.alive
             .iter()
             .filter_map(|(&k, v)| {
@@ -285,11 +302,13 @@ impl Crdt {
                 }
             })
             .collect();
+
         for id in dead_ids.iter() {
             self.alive.remove(id);
             self.table.remove(id);
             self.remote.remove(id);
             self.local.remove(id);
+            self.external_liveness.remove(id);
         }
     }
 
@@ -473,6 +492,12 @@ impl Crdt {
         rdr.read_u64::<LittleEndian>()
             .expect("rdr.read_u64 in fn random")
     }
+
+    // TODO: fill in with real implmentation wonce staking is implemented
+    fn get_stake(id: PublicKey) -> f64 {
+        return 1.0;
+    }
+
     fn get_updates_since(&self, v: u64) -> (PublicKey, u64, Vec<ReplicatedData>) {
         //trace!("get updates since {}", v);
         let data = self.table
@@ -508,16 +533,32 @@ impl Crdt {
     /// * B - RequestUpdates protocol message
     fn gossip_request(&self) -> Result<(SocketAddr, Protocol)> {
         let options: Vec<_> = self.table.values().filter(|v| v.id != self.me).collect();
-        if options.len() < 1 {
+
+        #[cfg(not(feature = "choose_gossip_peer_strategy"))]
+        let choose_peer_strategy = ChooseRandomPeerStrategy::new(&Self::random);
+
+        #[cfg(feature = "choose_gossip_peer_strategy")]
+        let choose_peer_strategy = ChooseWeightedPeerStrategy::new(
+            &self.remote,
+            &self.external_liveness,
+            &Self::get_stake,
+        );
+
+        let choose_peer_result = choose_peer_strategy.choose_peer(options);
+
+        let v = match choose_peer_result {
+            Ok(peer) => peer,
+            Err(Error::CrdtTooSmall) => {
                 trace!(
                     "crdt too small for gossip {:?} {}",
                     &self.me[..4],
                     self.table.len()
                 );
                 return Err(Error::CrdtTooSmall);
-        }
+            },
-        let n = (Self::random() as usize) % options.len();
+            Err(e) => return Err(e),
-        let v = options[n].clone();
+        };
+
         let remote_update_index = *self.remote.get(&v.id).unwrap_or(&0);
         let req = Protocol::RequestUpdates(remote_update_index, self.table[&self.me].clone());
         trace!(
@@ -526,6 +567,7 @@ impl Crdt {
             &v.id[..4],
             v.gossip_addr
         );
+
         Ok((v.gossip_addr, req))
     }
 
@@ -543,6 +585,7 @@ impl Crdt {
         let (remote_gossip_addr, req) = obj.read()
             .expect("'obj' read lock in fn run_gossip")
             .gossip_request()?;
+
         // TODO this will get chatty, so we need to first ask for number of updates since
         // then only ask for specific data that we dont have
         let blob = to_blob(req, remote_gossip_addr, blob_recycler)?;
@@ -583,14 +626,43 @@ impl Crdt {
     /// * `from` - identity of the sender of the updates
     /// * `update_index` - the number of updates that `from` has completed and this set of `data` represents
     /// * `data` - the update data
-    fn apply_updates(&mut self, from: PublicKey, update_index: u64, data: &[ReplicatedData]) {
+    fn apply_updates(
+        &mut self, from: PublicKey,
+        update_index: u64,
+        data: &[ReplicatedData],
+        external_liveness: &[(PublicKey, u64)],
+    ){
         trace!("got updates {}", data.len());
         // TODO we need to punish/spam resist here
         // sig verify the whole update and slash anyone who sends a bad update
         for v in data {
             self.insert(&v);
         }
+
+        for (pk, external_remote_index) in external_liveness.iter() {
+            let remote_entry = 
+                if let Some(v) = self.remote.get(pk) {
+                    *v
+                } else {
+                    0
+                };
+
+            if remote_entry >= *external_remote_index {
+                continue;
+            }
+
+            let liveness_entry = self.external_liveness.entry(*pk).or_insert(HashMap::new());
+            let peer_index = *liveness_entry.entry(from).or_insert(*external_remote_index);
+            if *external_remote_index > peer_index {
+                liveness_entry.insert(from, *external_remote_index);
+            }
+        }
+
         *self.remote.entry(from).or_insert(update_index) = update_index;
+
+        // Clear the remote liveness table for this node, b/c we've heard directly from them
+        // so we don't need to rely on rumors
+        self.external_liveness.remove(&from);
     }
 
     /// randomly pick a node and ask them for updates asynchronously
@@ -682,13 +754,14 @@ impl Crdt {
             Ok(Protocol::RequestUpdates(v, from_rd)) => {
                 trace!("RequestUpdates {}", v);
                 let addr = from_rd.gossip_addr;
-                // only lock for this call, dont lock during IO `sock.send_to` or `sock.recv_from`
+                let me = obj.read().unwrap();
-                let (from, ups, data) = obj.read()
+                // only lock for these two calls, dont lock during IO `sock.send_to` or `sock.recv_from`
-                    .expect("'obj' read lock in RequestUpdates")
+                let (from, ups, data) = me.get_updates_since(v);
-                    .get_updates_since(v);
+                let external_liveness = me.remote.iter().map(|(k, v)| (k.clone(), v.clone())).collect();
+                drop(me);
                 trace!("get updates since response {} {}", v, data.len());
                 let len = data.len();
-                let rsp = Protocol::ReceiveUpdates(from, ups, data);
+                let rsp = Protocol::ReceiveUpdates(from, ups, data, external_liveness);
                 obj.write().unwrap().insert(&from_rd);
                 if len < 1 {
                     let me = obj.read().unwrap();
@@ -713,11 +786,11 @@ impl Crdt {
                     None
                 }
             }
-            Ok(Protocol::ReceiveUpdates(from, ups, data)) => {
+            Ok(Protocol::ReceiveUpdates(from, ups, data, external_liveness)) => {
                 trace!("ReceivedUpdates {:?} {} {}", &from[0..4], ups, data.len());
                 obj.write()
                     .expect("'obj' write lock in ReceiveUpdates")
-                    .apply_updates(from, ups, &data);
+                    .apply_updates(from, ups, &data, &external_liveness);
                 None
             }
             Ok(Protocol::RequestWindowIndex(from, ix)) => {
@@ -956,7 +1029,7 @@ mod tests {
             sorted(&vec![d1.clone(), d2.clone(), d3.clone()])
         );
         let mut crdt2 = Crdt::new(d2.clone());
-        crdt2.apply_updates(key, ix, &ups);
+        crdt2.apply_updates(key, ix, &ups, &vec![]);
         assert_eq!(crdt2.table.values().len(), 3);
         assert_eq!(
             sorted(&crdt2.table.values().map(|x| x.clone()).collect()),

src/lib.rs

@@ -47,6 +47,7 @@ pub mod transaction;
 pub mod tvu;
 pub mod window_stage;
 pub mod write_stage;
+mod choose_gossip_peer_strategy;
 extern crate bincode;
 extern crate byteorder;
 extern crate chrono;

tests/data_replicator.rs

@@ -183,3 +183,105 @@ pub fn crdt_retransmit() {
         t.join().unwrap();
     }
 }
+
+#[test]
+fn check_external_liveness_table() {
+    logger::setup();
+    let c1_c4_exit = Arc::new(AtomicBool::new(false));
+    let c2_c3_exit = Arc::new(AtomicBool::new(false));
+
+    trace!("c1:");
+    let (c1, dr1, _) = test_node(c1_c4_exit.clone());
+    trace!("c2:");
+    let (c2, dr2, _) = test_node(c2_c3_exit.clone());
+    trace!("c3:");
+    let (c3, dr3, _) = test_node(c2_c3_exit.clone());
+    trace!("c4:");
+    let (c4, dr4, _) = test_node(c1_c4_exit.clone());
+
+    let c1_data = c1.read().unwrap().my_data().clone();
+    c1.write().unwrap().set_leader(c1_data.id);
+
+    let c2_id = c2.read().unwrap().me;
+    let c3_id = c3.read().unwrap().me;
+    let c4_id = c4.read().unwrap().me;
+
+    // Insert the remote data about c4
+    let c2_index_for_c4 = 10;
+    c2.write().unwrap().remote.insert(c4_id, c2_index_for_c4);
+    let c3_index_for_c4 = 20;
+    c3.write().unwrap().remote.insert(c4_id, c3_index_for_c4);
+
+    // Set up the initial network topology
+    c2.write().unwrap().insert(&c1_data);
+    c3.write().unwrap().insert(&c1_data);
+
+    c2.write().unwrap().set_leader(c1_data.id);
+    c3.write().unwrap().set_leader(c1_data.id);
+
+    // Wait to converge
+    trace!("waiting to converge:");
+    let mut done = false;
+    for _ in 0..30 {
+        done = c1.read().unwrap().table.len() == 3 && c2.read().unwrap().table.len() == 3
+            && c3.read().unwrap().table.len() == 3;
+        if done {
+            break;
+        }
+        sleep(Duration::new(1, 0));
+    }
+    assert!(done);
+
+    // Validate c1's external liveness table, then release lock rc1
+    {
+        let rc1 = c1.read().unwrap();
+        let el = rc1.get_external_liveness_entry(&c4.read().unwrap().me);
+
+        // Make sure liveness table entry for c4 exists on node c1
+        assert!(el.is_some());
+        let liveness_map = el.unwrap();
+
+        // Make sure liveness table entry contains correct result for c2
+        let c2_index_result_for_c4 = liveness_map.get(&c2_id);
+        assert!(c2_index_result_for_c4.is_some());
+        assert!(*(c2_index_result_for_c4.unwrap()) == c2_index_for_c4);
+
+        // Make sure liveness table entry contains correct result for c3
+        let c3_index_result_for_c4 = liveness_map.get(&c3_id);
+        assert!(c3_index_result_for_c4.is_some());
+        assert!(*(c3_index_result_for_c4.unwrap()) == c3_index_for_c4);
+    }
+
+    // Shutdown validators c2 and c3
+    c2_c3_exit.store(true, Ordering::Relaxed);
+    let mut threads = vec![];
+    threads.extend(dr2.thread_hdls.into_iter());
+    threads.extend(dr3.thread_hdls.into_iter());
+
+    for t in threads.into_iter() {
+        t.join().unwrap();
+    }
+
+    // Allow communication between c1 and c4, make sure that c1's external_liveness table
+    // entry for c4 gets cleared
+    c4.write().unwrap().insert(&c1_data);
+    c4.write().unwrap().set_leader(c1_data.id);
+    for _ in 0..30 {
+        done = c1.read().unwrap().get_external_liveness_entry(&c4_id).is_none();
+        if done {
+            break;
+        }
+        sleep(Duration::new(1, 0));
+    }
+    assert!(done);
+
+    // Shutdown validators c1 and c4
+    c1_c4_exit.store(true, Ordering::Relaxed);
+    let mut threads = vec![];
+    threads.extend(dr1.thread_hdls.into_iter());
+    threads.extend(dr4.thread_hdls.into_iter());
+
+    for t in threads.into_iter() {
+        t.join().unwrap();
+    }
+}