diff --git a/libs/layer_map/src/inmem_imm_historic_imm.rs b/libs/layer_map/src/inmem_imm_historic_imm.rs
new file mode 100644
index 0000000000..23c7b685b1
--- /dev/null
+++ b/libs/layer_map/src/inmem_imm_historic_imm.rs
@@ -0,0 +1,410 @@
+use std::{marker::PhantomData, time::Duration};
+
+use utils::seqwait::{self, Advance, SeqWait, Wait};
+
+pub trait Types {
+    type Key: Copy;
+    type Lsn: Ord + Copy;
+    type LsnCounter: seqwait::MonotonicCounter<Self::Lsn> + Copy;
+    type DeltaRecord;
+    type HistoricLayer;
+    type InMemoryLayer: InMemoryLayer<Types = Self> + Clone;
+    type HistoricStuff: HistoricStuff<Types = Self> + Clone;
+}
+
+#[derive(thiserror::Error)]
+pub struct InMemoryLayerPutError<DeltaRecord> {
+    delta: DeltaRecord,
+    kind: InMemoryLayerPutErrorKind,
+}
+
+#[derive(Debug)]
+pub enum InMemoryLayerPutErrorKind {
+    LayerFull,
+    AlreadyHaveRecordForKeyAndLsn,
+}
+
+impl<DeltaRecord> std::fmt::Debug for InMemoryLayerPutError<DeltaRecord> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("InMemoryLayerPutError")
+            // would require DeltaRecord to impl Debug
+            //         .field("delta", &self.delta)
+            .field("kind", &self.kind)
+            .finish()
+    }
+}
+
+pub trait InMemoryLayer: std::fmt::Debug + Default + Clone {
+    type Types: Types;
+    fn put(
+        &mut self,
+        key: <Self::Types as Types>::Key,
+        lsn: <Self::Types as Types>::Lsn,
+        delta: <Self::Types as Types>::DeltaRecord,
+    ) -> Result<Self, InMemoryLayerPutError<<Self::Types as Types>::DeltaRecord>>;
+    fn get(
+        &self,
+        key: <Self::Types as Types>::Key,
+        lsn: <Self::Types as Types>::Lsn,
+    ) -> Vec<<Self::Types as Types>::DeltaRecord>;
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum GetReconstructPathError {}
+
+pub trait HistoricStuff {
+    type Types: Types;
+    fn get_reconstruct_path(
+        &self,
+        key: <Self::Types as Types>::Key,
+        lsn: <Self::Types as Types>::Lsn,
+    ) -> Result<Vec<<Self::Types as Types>::HistoricLayer>, GetReconstructPathError>;
+    /// Produce a new version of `self` that includes the given inmem layer.
+    fn make_historic(&self, inmem: <Self::Types as Types>::InMemoryLayer) -> Self;
+}
+
+struct Snapshot<T: Types> {
+    _types: PhantomData<T>,
+    inmem: Option<T::InMemoryLayer>,
+    historic: T::HistoricStuff,
+}
+
+impl<T: Types> Clone for Snapshot<T> {
+    fn clone(&self) -> Self {
+        Self {
+            _types: self._types.clone(),
+            inmem: self.inmem.clone(),
+            historic: self.historic.clone(),
+        }
+    }
+}
+
+pub struct Reader<T: Types> {
+    wait: Wait<T::LsnCounter, T::Lsn, Snapshot<T>>,
+}
+
+pub struct ReadWriter<T: Types> {
+    advance: Advance<T::LsnCounter, T::Lsn, Snapshot<T>>,
+}
+
+pub fn empty<T: Types>(
+    lsn: T::LsnCounter,
+    historic: T::HistoricStuff,
+) -> (Reader<T>, ReadWriter<T>) {
+    let state = Snapshot {
+        _types: PhantomData::<T>::default(),
+        inmem: None,
+        historic: historic,
+    };
+    let (wait, advance) = SeqWait::new(lsn, state).split_spmc();
+    let reader = Reader { wait };
+    let read_writer = ReadWriter { advance };
+    (reader, read_writer)
+}
+
+#[derive(Debug, thiserror::Error)]
+pub enum GetError {
+    #[error(transparent)]
+    SeqWait(#[from] seqwait::SeqWaitError),
+    #[error(transparent)]
+    GetReconstructPath(#[from] GetReconstructPathError),
+}
+
+pub struct ReconstructWork<T: Types> {
+    pub key: T::Key,
+    pub lsn: T::Lsn,
+    pub inmem_records: Vec<T::DeltaRecord>,
+    pub historic_path: Vec<T::HistoricLayer>,
+}
+
+impl<T: Types> Reader<T> {
+    pub async fn get(&self, key: T::Key, lsn: T::Lsn) -> Result<ReconstructWork<T>, GetError> {
+        // XXX dedup with ReadWriter::get_nowait
+        let state = self.wait.wait_for(lsn).await?;
+        let inmem_records = state
+            .inmem
+            .as_ref()
+            .map(|iml| iml.get(key, lsn))
+            .unwrap_or_default();
+        let historic_path = state.historic.get_reconstruct_path(key, lsn)?;
+        Ok(ReconstructWork {
+            key,
+            lsn,
+            inmem_records,
+            historic_path,
+        })
+    }
+}
+
+#[derive(thiserror::Error)]
+pub struct PutError<T: Types> {
+    pub delta: T::DeltaRecord,
+    pub kind: PutErrorKind,
+}
+#[derive(Debug)]
+pub enum PutErrorKind {
+    AlreadyHaveInMemoryRecordForKeyAndLsn,
+}
+
+impl<T: Types> std::fmt::Debug for PutError<T> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("PutError")
+            // would need to require Debug for DeltaRecord
+            // .field("delta", &self.delta)
+            .field("kind", &self.kind)
+            .finish()
+    }
+}
+
+impl<T: Types> ReadWriter<T> {
+    pub async fn put(
+        &mut self,
+        key: T::Key,
+        lsn: T::Lsn,
+        delta: T::DeltaRecord,
+    ) -> Result<(), PutError<T>> {
+        let (snapshot_lsn, snapshot) = self.advance.get_current_data();
+        // TODO ensure snapshot_lsn <= lsn?
+        let mut inmem = snapshot
+            .inmem
+            .unwrap_or_else(|| T::InMemoryLayer::default());
+        // XXX: use the Advance as witness and only allow witness to access inmem in write mode
+        match inmem.put(key, lsn, delta) {
+            Ok(new_inmem) => {
+                let new_snapshot = Snapshot {
+                    _types: PhantomData,
+                    inmem: Some(new_inmem),
+                    historic: snapshot.historic,
+                };
+                self.advance.advance(lsn, Some(new_snapshot));
+            }
+            Err(InMemoryLayerPutError {
+                delta,
+                kind: InMemoryLayerPutErrorKind::AlreadyHaveRecordForKeyAndLsn,
+            }) => {
+                return Err(PutError {
+                    delta,
+                    kind: PutErrorKind::AlreadyHaveInMemoryRecordForKeyAndLsn,
+                });
+            }
+            Err(InMemoryLayerPutError {
+                delta,
+                kind: InMemoryLayerPutErrorKind::LayerFull,
+            }) => {
+                let new_historic = snapshot.historic.make_historic(inmem);
+                let mut new_inmem = T::InMemoryLayer::default();
+                let new_inmem = new_inmem
+                    .put(key, lsn, delta)
+                    .expect("put into default inmem layer must not fail");
+                let new_state = Snapshot {
+                    _types: PhantomData::<T>::default(),
+                    inmem: Some(new_inmem),
+                    historic: new_historic,
+                };
+                self.advance.advance(lsn, Some(new_state));
+            }
+        }
+        Ok(())
+    }
+
+    pub async fn force_flush(&mut self) -> tokio::io::Result<()> {
+        let (snapshot_lsn, snapshot) = self.advance.get_current_data();
+        let Snapshot {
+            _types,
+            inmem,
+            historic,
+        } = snapshot;
+        // XXX: use the Advance as witness and only allow witness to access inmem in "write" mode
+        let Some(inmem) = inmem else {
+            // nothing to do
+            return Ok(());
+        };
+        let new_historic = historic.make_historic(inmem);
+        let new_snapshot = Snapshot {
+            _types: PhantomData::<T>::default(),
+            inmem: None,
+            historic: new_historic,
+        };
+        self.advance.advance(snapshot_lsn, Some(new_snapshot)); // TODO: should fail if we're past snapshot_lsn
+        Ok(())
+    }
+
+    pub async fn get_nowait(
+        &self,
+        key: T::Key,
+        lsn: T::Lsn,
+    ) -> Result<ReconstructWork<T>, GetError> {
+        // XXX dedup with Reader::get
+        let state = self
+            .advance
+            .wait_for_timeout(lsn, Duration::from_secs(0))
+            // The await is never going to block because we pass from_secs(0).
+            .await?;
+        let inmem_records = state
+            .inmem
+            .as_ref()
+            .map(|iml| iml.get(key, lsn))
+            .unwrap_or_default();
+        let historic_path = state.historic.get_reconstruct_path(key, lsn)?;
+        Ok(ReconstructWork {
+            key,
+            lsn,
+            inmem_records,
+            historic_path,
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::{btree_map::Entry, BTreeMap};
+    use std::sync::Arc;
+
+    use crate::tests_common::UsizeCounter;
+
+    /// The ZST for which we impl the `super::Types` type collection trait.
+    struct TestTypes;
+
+    impl super::Types for TestTypes {
+        type Key = usize;
+
+        type Lsn = usize;
+
+        type LsnCounter = UsizeCounter;
+
+        type DeltaRecord = &'static str;
+
+        type HistoricLayer = Arc<TestHistoricLayer>;
+
+        type InMemoryLayer = TestInMemoryLayer;
+
+        type HistoricStuff = TestHistoricStuff;
+    }
+
+    /// For testing, our in-memory layer is a simple hashmap.
+    #[derive(Clone, Default, Debug)]
+    struct TestInMemoryLayer {
+        by_key: BTreeMap<usize, BTreeMap<usize, &'static str>>,
+    }
+
+    /// For testing, our historic layers are just in-memory layer objects with `frozen==true`.
+    struct TestHistoricLayer(TestInMemoryLayer);
+
+    /// This is the data structure that impls the `HistoricStuff` trait.
+    #[derive(Default, Clone)]
+    struct TestHistoricStuff {
+        by_key: BTreeMap<usize, BTreeMap<usize, Arc<TestHistoricLayer>>>,
+    }
+
+    // Our testing impl of HistoricStuff references the frozen InMemoryLayer objects
+    // from all the (key,lsn) entries that it covers.
+    // This mimics the (much more efficient) search tree in the real impl.
+    impl super::HistoricStuff for TestHistoricStuff {
+        type Types = TestTypes;
+        fn get_reconstruct_path(
+            &self,
+            key: usize,
+            lsn: usize,
+        ) -> Result<Vec<Arc<TestHistoricLayer>>, super::GetReconstructPathError> {
+            let Some(bk) = self.by_key.get(&key) else {
+                return Ok(vec![]);
+            };
+            Ok(bk.range(..=lsn).rev().map(|(_, l)| Arc::clone(l)).collect())
+        }
+
+        fn make_historic(&self, inmem: TestInMemoryLayer) -> Self {
+            // For the purposes of testing, just turn the inmemory layer historic through the type system
+            let historic = Arc::new(TestHistoricLayer(inmem));
+            // Deep-copy
+            let mut copy = self.by_key.clone();
+            // Add the references to `inmem` to the deep-copied struct
+            for (k, v) in historic.0.by_key.iter() {
+                for (lsn, _deltas) in v.into_iter() {
+                    let by_key = copy.entry(*k).or_default();
+                    let overwritten = by_key.insert(*lsn, historic.clone());
+                    assert!(matches!(overwritten, None), "layers must not overlap");
+                }
+            }
+            Self { by_key: copy }
+        }
+    }
+
+    impl super::InMemoryLayer for TestInMemoryLayer {
+        type Types = TestTypes;
+
+        fn put(
+            &mut self,
+            key: usize,
+            lsn: usize,
+            delta: &'static str,
+        ) -> Result<Self, super::InMemoryLayerPutError<&'static str>> {
+            let mut clone = self.clone();
+            drop(self);
+            let by_key = clone.by_key.entry(key).or_default();
+            match by_key.entry(lsn) {
+                Entry::Occupied(_record) => {
+                    return Err(super::InMemoryLayerPutError {
+                        delta,
+                        kind: super::InMemoryLayerPutErrorKind::AlreadyHaveRecordForKeyAndLsn,
+                    });
+                }
+                Entry::Vacant(vacant) => vacant.insert(delta),
+            };
+            Ok(clone)
+        }
+
+        fn get(&self, key: usize, lsn: usize) -> Vec<&'static str> {
+            let by_key = match self.by_key.get(&key) {
+                Some(by_key) => by_key,
+                None => return vec![],
+            };
+            by_key
+                .range(..=lsn)
+                .map(|(_, v)| v)
+                .rev()
+                .cloned()
+                .collect()
+        }
+    }
+
+    #[test]
+    fn basic() {
+        let lm = TestHistoricStuff::default();
+
+        let (r, mut rw) = super::empty::<TestTypes>(UsizeCounter::new(0), lm);
+
+        let r = Arc::new(r);
+        let r2 = Arc::clone(&r);
+
+        let rt = tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap();
+
+        let read_jh = rt.spawn(async move { r.get(0, 10).await });
+
+        let mut rw = rt.block_on(async move {
+            rw.put(0, 1, "foo").await.unwrap();
+            rw.put(1, 1, "bar").await.unwrap();
+            rw.put(0, 10, "baz").await.unwrap();
+            rw
+        });
+
+        let read_res = rt.block_on(read_jh).unwrap().unwrap();
+        assert!(
+            read_res.historic_path.is_empty(),
+            "we have pushed less than needed for flush"
+        );
+        assert_eq!(read_res.inmem_records, vec!["baz", "foo"]);
+
+        let rw = rt.block_on(async move {
+            rw.put(0, 11, "blup").await.unwrap();
+            rw
+        });
+        let read_res = rt.block_on(async move { r2.get(0, 11).await.unwrap() });
+        assert_eq!(read_res.historic_path.len(), 0);
+        assert_eq!(read_res.inmem_records, vec!["blup", "baz", "foo"]);
+
+        drop(rw);
+    }
+}
diff --git a/libs/layer_map/src/inmem_shared_historic_imm.rs b/libs/layer_map/src/inmem_shared_historic_imm.rs
index 5b411bf7b1..49f4a7e3a5 100644
--- a/libs/layer_map/src/inmem_shared_historic_imm.rs
+++ b/libs/layer_map/src/inmem_shared_historic_imm.rs
@@ -161,7 +161,7 @@ impl<T: Types> ReadWriter<T> {
         lsn: T::Lsn,
         delta: T::DeltaRecord,
     ) -> Result<(), PutError<T>> {
-        let shared = self.shared.get_current_data();
+        let (_, shared) = self.shared.get_current_data();
         let mut inmem_guard = shared
             .inmem
             .try_lock()
@@ -208,7 +208,7 @@ impl<T: Types> ReadWriter<T> {
     }
 
     pub async fn force_flush(&mut self) -> tokio::io::Result<()> {
-        let shared = self.shared.get_current_data();
+        let (_, shared) = self.shared.get_current_data();
         let mut inmem_guard = shared
             .inmem
             .try_lock()
diff --git a/libs/layer_map/src/lib.rs b/libs/layer_map/src/lib.rs
index e18615e43f..48ccea78fe 100644
--- a/libs/layer_map/src/lib.rs
+++ b/libs/layer_map/src/lib.rs
@@ -1,3 +1,4 @@
+mod inmem_imm_historic_imm;
 mod inmem_shared_historic_imm;
 
 #[cfg(test)]