diff --git a/Cargo.lock b/Cargo.lock
index d6846dd..d019fd4 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -176,6 +176,7 @@ name = "atuin-common"
 version = "15.0.0"
 dependencies = [
  "chrono",
+ "pretty_assertions",
  "rand",
  "serde",
  "typed-builder",
@@ -600,6 +601,22 @@ dependencies = [
  "typenum",
 ]
 
+[[package]]
+name = "ctor"
+version = "0.1.26"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6d2301688392eb071b0bf1a37be05c469d3cc4dbbd95df672fe28ab021e6a096"
+dependencies = [
+ "quote",
+ "syn 1.0.99",
+]
+
+[[package]]
+name = "diff"
+version = "0.1.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "56254986775e3233ffa9c4d7d3faaf6d36a2c09d30b20687e9f88bc8bafc16c8"
+
 [[package]]
 name = "digest"
 version = "0.10.5"
@@ -1434,6 +1451,15 @@ version = "0.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ff011a302c396a5197692431fc1948019154afc178baf7d8e37367442a4601cf"
 
+[[package]]
+name = "output_vt100"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "628223faebab4e3e40667ee0b2336d34a5b960ff60ea743ddfdbcf7770bcfb66"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "overload"
 version = "0.1.1"
@@ -1598,6 +1624,18 @@ version = "0.2.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "eb9f9e6e233e5c4a35559a617bf40a4ec447db2e84c20b55a6f83167b7e57872"
 
+[[package]]
+name = "pretty_assertions"
+version = "1.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a25e9bcb20aa780fd0bb16b72403a9064d6b3f22f026946029acb941a50af755"
+dependencies = [
+ "ctor",
+ "diff",
+ "output_vt100",
+ "yansi",
+]
+
 [[package]]
 name = "proc-macro2"
 version = "1.0.56"
@@ -2976,6 +3014,12 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "yansi"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "09041cd90cf85f7f8b2df60c646f853b7f535ce68f85244eb6731cf89fa498ec"
+
 [[package]]
 name = "zeroize"
 version = "1.6.0"
diff --git a/atuin-common/Cargo.toml b/atuin-common/Cargo.toml
index 918b5b5..b693a46 100644
--- a/atuin-common/Cargo.toml
+++ b/atuin-common/Cargo.toml
@@ -17,3 +17,4 @@ serde = { workspace = true }
 uuid = { workspace = true }
 rand = { workspace = true }
 typed-builder = { workspace = true }
+pretty_assertions = "1.3.0"
diff --git a/atuin-common/src/record.rs b/atuin-common/src/record.rs
index 1fb60e5..a9c177c 100644
--- a/atuin-common/src/record.rs
+++ b/atuin-common/src/record.rs
@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use serde::{Deserialize, Serialize};
 use typed_builder::TypedBuilder;
 
@@ -47,3 +49,215 @@ impl Record {
             .build()
     }
 }
+
+/// An index representing the current state of the record stores
+/// This can be both remote, or local, and compared in either direction
+pub struct RecordIndex {
+    // A map of host -> tag -> tail
+    pub hosts: HashMap<String, HashMap<String, String>>,
+}
+
+impl Default for RecordIndex {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl RecordIndex {
+    pub fn new() -> RecordIndex {
+        RecordIndex {
+            hosts: HashMap::new(),
+        }
+    }
+
+    /// Insert a new tail record into the store
+    pub fn set(&mut self, tail: Record) {
+        self.hosts
+            .entry(tail.host)
+            .or_default()
+            .insert(tail.tag, tail.id);
+    }
+
+    pub fn get(&self, host: String, tag: String) -> Option<String> {
+        self.hosts.get(&host).and_then(|v| v.get(&tag)).cloned()
+    }
+
+    /// Diff this index with another, likely remote index.
+    /// The two diffs can then be reconciled, and the optimal change set calculated
+    /// Returns a tuple, with (host, tag, Option(OTHER))
+    /// OTHER is set to the value of the tail on the other machine. For example, if the
+    /// other machine has a different tail, it will be the differing tail. This is useful to
+    /// check if the other index is ahead of us, or behind.
+    /// If the other index does not have the (host, tag) pair, then the other value will be None.
+    pub fn diff(&self, other: &Self) -> Vec<(String, String, Option<String>)> {
+        let mut ret = Vec::new();
+
+        // First, we check if other has everything that self has
+        for (host, tag_map) in self.hosts.iter() {
+            for (tag, tail) in tag_map.iter() {
+                match other.get(host.clone(), tag.clone()) {
+                    // The other store is all up to date! No diff.
+                    Some(t) if t.eq(tail) => continue,
+
+                    // The other store does exist, but it is either ahead or behind us. A diff regardless
+                    Some(t) => ret.push((host.clone(), tag.clone(), Some(t))),
+
+                    // The other store does not exist :O
+                    None => ret.push((host.clone(), tag.clone(), None)),
+                };
+            }
+        }
+
+        // At this point, there is a single case we have not yet considered.
+        // If the other store knows of a tag that we are not yet aware of, then the diff will be missed
+
+        // account for that!
+        for (host, tag_map) in other.hosts.iter() {
+            for (tag, tail) in tag_map.iter() {
+                match self.get(host.clone(), tag.clone()) {
+                    // If we have this host/tag combo, the comparison and diff will have already happened above
+                    Some(_) => continue,
+
+                    None => ret.push((host.clone(), tag.clone(), Some(tail.clone()))),
+                };
+            }
+        }
+
+        ret.sort();
+        ret
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{Record, RecordIndex};
+    use pretty_assertions::{assert_eq, assert_ne};
+
+    fn test_record() -> Record {
+        Record::builder()
+            .host(crate::utils::uuid_v7().simple().to_string())
+            .version("v1".into())
+            .tag(crate::utils::uuid_v7().simple().to_string())
+            .data(vec![0, 1, 2, 3])
+            .build()
+    }
+
+    #[test]
+    fn record_index() {
+        let mut index = RecordIndex::new();
+        let record = test_record();
+
+        index.set(record.clone());
+
+        let tail = index.get(record.host, record.tag);
+
+        assert_eq!(
+            record.id,
+            tail.expect("tail not in store"),
+            "tail in store did not match"
+        );
+    }
+
+    #[test]
+    fn record_index_overwrite() {
+        let mut index = RecordIndex::new();
+        let record = test_record();
+        let child = record.new_child(vec![1, 2, 3]);
+
+        index.set(record.clone());
+        index.set(child.clone());
+
+        let tail = index.get(record.host, record.tag);
+
+        assert_eq!(
+            child.id,
+            tail.expect("tail not in store"),
+            "tail in store did not match"
+        );
+    }
+
+    #[test]
+    fn record_index_no_diff() {
+        // Here, they both have the same version and should have no diff
+
+        let mut index1 = RecordIndex::new();
+        let mut index2 = RecordIndex::new();
+
+        let record1 = test_record();
+
+        index1.set(record1.clone());
+        index2.set(record1);
+
+        let diff = index1.diff(&index2);
+
+        assert_eq!(0, diff.len(), "expected empty diff");
+    }
+
+    #[test]
+    fn record_index_single_diff() {
+        // Here, they both have the same stores, but one is ahead by a single record
+
+        let mut index1 = RecordIndex::new();
+        let mut index2 = RecordIndex::new();
+
+        let record1 = test_record();
+        let record2 = record1.new_child(vec![1, 2, 3]);
+
+        index1.set(record1);
+        index2.set(record2.clone());
+
+        let diff = index1.diff(&index2);
+
+        assert_eq!(1, diff.len(), "expected single diff");
+        assert_eq!(diff[0], (record2.host, record2.tag, Some(record2.id)));
+    }
+
+    #[test]
+    fn record_index_multi_diff() {
+        // A much more complex case, with a bunch more checks
+        let mut index1 = RecordIndex::new();
+        let mut index2 = RecordIndex::new();
+
+        let store1record1 = test_record();
+        let store1record2 = store1record1.new_child(vec![1, 2, 3]);
+
+        let store2record1 = test_record();
+        let store2record2 = store2record1.new_child(vec![1, 2, 3]);
+
+        let store3record1 = test_record();
+
+        let store4record1 = test_record();
+
+        // index1 only knows about the first two entries of the first two stores
+        index1.set(store1record1);
+        index1.set(store2record1);
+
+        // index2 is fully up to date with the first two stores, and knows of a third
+        index2.set(store1record2);
+        index2.set(store2record2);
+        index2.set(store3record1);
+
+        // index1 knows of a 4th store
+        index1.set(store4record1);
+
+        let diff1 = index1.diff(&index2);
+        let diff2 = index2.diff(&index1);
+
+        // both diffs the same length
+        assert_eq!(4, diff1.len());
+        assert_eq!(4, diff2.len());
+
+        // both diffs should be ALMOST the same. They will agree on which hosts and tags
+        // require updating, but the "other" value will not be the same.
+        let smol_diff_1: Vec<(String, String)> =
+            diff1.iter().map(|v| (v.0.clone(), v.1.clone())).collect();
+        let smol_diff_2: Vec<(String, String)> =
+            diff1.iter().map(|v| (v.0.clone(), v.1.clone())).collect();
+
+        assert_eq!(smol_diff_1, smol_diff_2);
+
+        // diffing with yourself = no diff
+        assert_eq!(index1.diff(&index1).len(), 0);
+        assert_eq!(index2.diff(&index2).len(), 0);
+    }
+}