Key deletion logic bugfix

See README and issue #121.

Closes #121

README.md

@@ -56,23 +56,25 @@ associated with a version, and replicated using the same mechanism as a KV updat
 The library will then interpret this versioned tombstone before exposing kv to
 the user.
 
-To avoid keeping deleted KV indefinitely, the library includes a GC mechanism. Any nodes containing a tombstone older than a given `grace period threshold`
-(age is measured in ticks of heartbeat), it is safe to be deleted.
+To avoid keeping deleted KV indefinitely, the library includes a GC mechanism.
+Every tombstone is associated with a monotonic timestamp.
+It is local in the sense that it is computed locally to the given node, and never shared with other servers.
+
+All KV with a timestamp older than a given `grace_period_threshold` will be deleted upon delete operations. (Note for a given KV, GC can happen at different
+times on different nodes.)
 
 This yields the following problem. If a node was disconnected for more than
 `marked_for_deletion_grace_period`, they could have missed the deletion of a KV and never be aware of it.
 
-To address this problem, nodes that are too outdated have to reset their state.
+To address this problem, nodes keep a record of the version of the last KV they
+have GCed. Here is how it works:
+
+Let's assume a Node A sends a Syn message to a Node B. The digest expresses that A want for updates about Node N with a version stricly greater than `V`.
+Node B will compare the version `V` of the digest with its `max_gc_version` for the node N.
 
-More accurately, let's assume a Node A sends a Syn message to Node B with a digest with an outdated version V for a node N.
-Node B will compare the version of the digest with its own version.
+If `V > max_gc_version`, Node B knows that no GC has impacted Key values with a version above V. It can safely emit a normal delta to A.
 
-If V is fresher than `own version - marked_for_deletion_grace_period`,
-Node B knows that no GC has impacted Key values with a version above V. It can
-safely emit a normal delta to A.
-If however V is older than `own version - marked_for_deletion_grace_period`,
-a GC could have been executed. Instead of sending a delta to Node A, Node B will
-instruct A to reset its state.
+If however V is older, a GC could have been executed. Instead of sending a delta to Node A, Node B will instruct A to reset its state.
 
 Node A will then wipe-off whatever information it has about N, and will start syncing from a blank state.
 

chitchat-test/src/main.rs

@@ -99,7 +99,7 @@ async fn main() -> anyhow::Result<()> {
             dead_node_grace_period: Duration::from_secs(10),
             ..FailureDetectorConfig::default()
         },
-        marked_for_deletion_grace_period: 60,
+        marked_for_deletion_grace_period: Duration::from_secs(60),
     };
     let chitchat_handler = spawn_chitchat(config, Vec::new(), &UdpTransport).await?;
     let chitchat = chitchat_handler.chitchat();

chitchat/src/configuration.rs

@@ -15,14 +15,14 @@ pub struct ChitchatConfig {
     pub failure_detector_config: FailureDetectorConfig,
     // Marked for deletion grace period expressed as a number of hearbeats.
     // Chitchat ensures a key marked for deletion is eventually deleted by three mechanisms:
-    // - Garbage collection: each heartbeat, marked for deletion keys with `tombstone +
-    //   marked_for_deletion_grace_period > node.heartbeat` are deleted.
+    // - Garbage collection: each heartbeat, marked for deletion keys with `deletion now > instant
+    //   + marked_for_deletion_grace_period` are deleted.
     // - Compute delta: for a given node digest, if `node_digest.heartbeat +
     //   marked_for_deletion_grace_period < node_state.heartbeat` the node is flagged "to be reset"
     //   and the delta is populated with all keys and values.
     // - Apply delta: for a node flagged "to be reset", Chitchat will remove the node state and
     //   populate a fresh new node state with the keys and values present in the delta.
-    pub marked_for_deletion_grace_period: usize,
+    pub marked_for_deletion_grace_period: Duration,
 }
 
 impl ChitchatConfig {
@@ -37,7 +37,7 @@ impl ChitchatConfig {
             listen_addr,
             seed_nodes: Vec::new(),
             failure_detector_config: Default::default(),
-            marked_for_deletion_grace_period: 10_000,
+            marked_for_deletion_grace_period: Duration::from_secs(10_000),
         }
     }
 }
@@ -56,7 +56,8 @@ impl Default for ChitchatConfig {
             failure_detector_config: Default::default(),
             // Each heartbeat increments the version, with one heartbeat each second
             // 86400 ~ 24h.
-            marked_for_deletion_grace_period: 86400,
+            // TODO set that to something much lower.
+            marked_for_deletion_grace_period: Duration::from_secs(86_400),
         }
     }
 }

chitchat/src/delta.rs

@@ -1,5 +1,7 @@
 use std::collections::HashSet;
 
+use tokio::time::Instant;
+
 use crate::serialize::*;
 use crate::{ChitchatId, Heartbeat, VersionedValue};
 
@@ -116,7 +118,8 @@ impl Deserializable for DeltaOp {
                 let key = String::deserialize(buf)?;
                 let value = String::deserialize(buf)?;
                 let version = u64::deserialize(buf)?;
-                let tombstone = Option::<u64>::deserialize(buf)?;
+                let deleted = bool::deserialize(buf)?;
+                let tombstone = if deleted { Some(Instant::now()) } else { None };
                 let versioned_value: VersionedValue = VersionedValue {
                     value,
                     version,
@@ -182,7 +185,7 @@ impl<'a> Serializable for DeltaOpRef<'a> {
                 key.serialize(buf);
                 versioned_value.value.serialize(buf);
                 versioned_value.version.serialize(buf);
-                versioned_value.tombstone.serialize(buf);
+                versioned_value.tombstone.is_some().serialize(buf);
             }
             Self::NodeToReset(chitchat_id) => {
                 buf.push(DeltaOpTag::NodeToReset.into());
@@ -204,7 +207,7 @@ impl<'a> Serializable for DeltaOpRef<'a> {
                 key.serialized_len()
                     + versioned_value.value.serialized_len()
                     + versioned_value.version.serialized_len()
-                    + versioned_value.tombstone.serialized_len()
+                    + 1
             }
             Self::NodeToReset(chitchat_id) => chitchat_id.serialized_len(),
         }
@@ -267,13 +270,14 @@ impl Delta {
         key: &str,
         value: &str,
         version: crate::Version,
-        tombstone: Option<u64>,
+        deleted: bool,
     ) {
         let node_delta = self
             .node_deltas
             .iter_mut()
             .find(|node_delta| &node_delta.chitchat_id == chitchat_id)
             .unwrap();
+        let tombstone = if deleted { Some(Instant::now()) } else { None };
         node_delta.key_values.push((
             key.to_string(),
             VersionedValue {
@@ -488,7 +492,7 @@ mod tests {
             VersionedValue {
                 value: "".to_string(),
                 version: 2,
-                tombstone: Some(0),
+                tombstone: Some(Instant::now()),
             },
         ));
 
@@ -515,7 +519,7 @@ mod tests {
                 tombstone: None,
             },
         ));
-        test_aux_delta_writer(delta_writer, 99);
+        test_aux_delta_writer(delta_writer, 98);
     }
 
     #[test]

chitchat/src/lib.rs

@@ -122,7 +122,6 @@ impl Chitchat {
                     &digest,
                     delta_mtu,
                     &scheduled_for_deletion,
-                    self.config.marked_for_deletion_grace_period as u64,
                 );
                 Some(ChitchatMessage::SynAck {
                     digest: self_digest,
@@ -137,7 +136,6 @@ impl Chitchat {
                     &digest,
                     MAX_UDP_DATAGRAM_PAYLOAD_SIZE - 1,
                     &scheduled_for_deletion,
-                    self.config.marked_for_deletion_grace_period as u64,
                 );
                 Some(ChitchatMessage::Ack { delta })
             }
@@ -153,11 +151,8 @@ impl Chitchat {
     }
 
     fn gc_keys_marked_for_deletion(&mut self) {
-        let dead_nodes = self.dead_nodes().cloned().collect::<HashSet<_>>();
-        self.cluster_state.gc_keys_marked_for_deletion(
-            self.config.marked_for_deletion_grace_period as u64,
-            &dead_nodes,
-        );
+        self.cluster_state
+            .gc_keys_marked_for_deletion(self.config.marked_for_deletion_grace_period);
     }
 
     /// Reports heartbeats to the failure detector for nodes in the delta for which we received an
@@ -402,7 +397,7 @@ mod tests {
                 initial_interval: Duration::from_millis(100),
                 ..Default::default()
             },
-            marked_for_deletion_grace_period: 10_000,
+            marked_for_deletion_grace_period: Duration::from_secs(3_600),
         };
         let initial_kvs: Vec<(String, String)> = Vec::new();
         spawn_chitchat(config, initial_kvs, transport)

chitchat/src/message.rs

@@ -165,12 +165,12 @@ mod tests {
             // +37 bytes = 27 bytes (ChitchatId) + 2 bytes (node delta len) + 8 bytes (heartbeat).
             delta.add_node(node.clone(), Heartbeat(0));
             // +29 bytes.
-            delta.add_kv(&node, "key", "value", 0, Some(5));
-            delta.set_serialized_len(70);
+            delta.add_kv(&node, "key", "value", 0, true);
+            delta.set_serialized_len(62);
 
             let syn_ack = ChitchatMessage::SynAck { digest, delta };
             // 1 bytes (syn ack message) + 45 bytes (digest) + 69 bytes (delta).
-            test_serdeser_aux(&syn_ack, 116);
+            test_serdeser_aux(&syn_ack, 108);
         }
     }
 
@@ -188,10 +188,10 @@ mod tests {
             // +37 bytes = 27 bytes (ChitchatId) + 2 bytes (node delta len) + 8 bytes (heartbeat).
             delta.add_node(node.clone(), Heartbeat(0));
             // +29 bytes.
-            delta.add_kv(&node, "key", "value", 0, Some(5));
-            delta.set_serialized_len(70);
+            delta.add_kv(&node, "key", "value", 0, true);
+            delta.set_serialized_len(62);
             let ack = ChitchatMessage::Ack { delta };
-            test_serdeser_aux(&ack, 71);
+            test_serdeser_aux(&ack, 63);
         }
     }
 

chitchat/src/serialize.rs

@@ -93,33 +93,6 @@ impl Deserializable for u64 {
     }
 }
 
-impl Serializable for Option<u64> {
-    fn serialize(&self, buf: &mut Vec<u8>) {
-        self.is_some().serialize(buf);
-        if let Some(tombstone) = &self {
-            tombstone.serialize(buf);
-        }
-    }
-    fn serialized_len(&self) -> usize {
-        if self.is_some() {
-            9
-        } else {
-            1
-        }
-    }
-}
-
-impl Deserializable for Option<u64> {
-    fn deserialize(buf: &mut &[u8]) -> anyhow::Result<Self> {
-        let is_some: bool = Deserializable::deserialize(buf)?;
-        if is_some {
-            let u64_value = Deserializable::deserialize(buf)?;
-            return Ok(Some(u64_value));
-        }
-        Ok(None)
-    }
-}
-
 impl Serializable for bool {
     fn serialize(&self, buf: &mut Vec<u8>) {
         buf.push(*self as u8);
@@ -525,12 +498,6 @@ mod tests {
         test_serdeser_aux(&ipv6, 17);
     }
 
-    #[test]
-    fn test_serialize_option_u64() {
-        test_serdeser_aux(&Some(1), 9);
-        test_serdeser_aux(&None, 1);
-    }
-
     #[test]
     fn test_serialize_block_type() {
         let mut valid_vals_count = 0;