diff --git a/src/external_documents_ids.rs b/src/external_documents_ids.rs
new file mode 100644
index 000000000..f8765b57e
--- /dev/null
+++ b/src/external_documents_ids.rs
@@ -0,0 +1,149 @@
+use std::borrow::Cow;
+use std::convert::TryInto;
+use fst::{Streamer, IntoStreamer};
+
+pub struct ExternalDocumentsIds<'a> {
+    pub(crate) hard: fst::Map<Cow<'a, [u8]>>,
+    pub(crate) soft: fst::Map<Cow<'a, [u8]>>,
+}
+
+impl<'a> ExternalDocumentsIds<'a> {
+    pub fn new(hard: fst::Map<Cow<'a, [u8]>>, soft: fst::Map<Cow<'a, [u8]>>) -> ExternalDocumentsIds<'a> {
+        ExternalDocumentsIds { hard, soft }
+    }
+
+    pub fn get<A: AsRef<str>>(&self, external_id: A) -> Option<u32> {
+        let external_id = external_id.as_ref();
+        match self.soft.get(external_id).or_else(|| self.hard.get(external_id)) {
+            // u64 MAX means deleted in the soft fst map
+            Some(id) if id != u64::MAX => Some(id.try_into().unwrap()),
+            _otherwise => None
+        }
+    }
+
+    pub fn delete_ids<A: AsRef<[u8]>>(&mut self, other: fst::Set<A>) -> fst::Result<()> {
+        let other = fst::Map::from(other.into_fst());
+        let union_op = self.soft.op().add(&other).r#union();
+
+        let mut iter = union_op.into_stream();
+        let mut new_soft_builder = fst::MapBuilder::memory();
+        while let Some((external_id, docids)) = iter.next() {
+            if docids.iter().any(|v| v.index == 1) {
+                // If the `other` set returns a value here it means
+                // that it must be marked as deleted.
+                new_soft_builder.insert(external_id, u64::MAX)?;
+            } else {
+                new_soft_builder.insert(external_id, docids[0].value)?;
+            }
+        }
+
+        drop(iter);
+
+        // We save this new map as the new soft map.
+        self.soft = new_soft_builder.into_map().map_data(Cow::Owned)?;
+        self.merge_soft_into_hard()
+    }
+
+    pub fn insert_ids<A: AsRef<[u8]>>(&mut self, other: &fst::Map<A>) -> fst::Result<()> {
+        let union_op = self.soft.op().add(other).r#union();
+
+        let mut new_soft_builder = fst::MapBuilder::memory();
+        let mut iter = union_op.into_stream();
+        while let Some((external_id, docids)) = iter.next() {
+            let id = docids.last().unwrap().value;
+            new_soft_builder.insert(external_id, id)?;
+        }
+
+        drop(iter);
+
+        // We save the new map as the new soft map.
+        self.soft = new_soft_builder.into_map().map_data(Cow::Owned)?;
+        self.merge_soft_into_hard()
+    }
+
+    fn merge_soft_into_hard(&mut self) -> fst::Result<()> {
+        if self.soft.len() >= self.hard.len() / 2 {
+            let union_op = self.hard.op().add(&self.soft).r#union();
+
+            let mut iter = union_op.into_stream();
+            let mut new_hard_builder = fst::MapBuilder::memory();
+            while let Some((external_id, docids)) = iter.next() {
+                if docids.len() == 2 {
+                    if docids[1].value != u64::MAX {
+                        new_hard_builder.insert(external_id, docids[1].value)?;
+                    }
+                } else {
+                    new_hard_builder.insert(external_id, docids[0].value)?;
+                }
+            }
+
+            drop(iter);
+
+            self.hard = new_hard_builder.into_map().map_data(Cow::Owned)?;
+            self.soft = fst::Map::default().map_data(Cow::Owned)?;
+        }
+
+        Ok(())
+    }
+}
+
+impl Default for ExternalDocumentsIds<'static> {
+    fn default() -> Self {
+        ExternalDocumentsIds {
+            hard: fst::Map::default().map_data(Cow::Owned).unwrap(),
+            soft: fst::Map::default().map_data(Cow::Owned).unwrap(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn simple_insert_delete_ids() {
+        let mut external_documents_ids = ExternalDocumentsIds::default();
+
+        let new_ids = fst::Map::from_iter(vec![("a", 1), ("b", 2), ("c", 3), ("d", 4)]).unwrap();
+        external_documents_ids.insert_ids(&new_ids).unwrap();
+
+        assert_eq!(external_documents_ids.get("a"), Some(1));
+        assert_eq!(external_documents_ids.get("b"), Some(2));
+        assert_eq!(external_documents_ids.get("c"), Some(3));
+        assert_eq!(external_documents_ids.get("d"), Some(4));
+
+        let new_ids = fst::Map::from_iter(vec![("e", 5), ("f", 6), ("g", 7)]).unwrap();
+        external_documents_ids.insert_ids(&new_ids).unwrap();
+
+        assert_eq!(external_documents_ids.get("a"), Some(1));
+        assert_eq!(external_documents_ids.get("b"), Some(2));
+        assert_eq!(external_documents_ids.get("c"), Some(3));
+        assert_eq!(external_documents_ids.get("d"), Some(4));
+        assert_eq!(external_documents_ids.get("e"), Some(5));
+        assert_eq!(external_documents_ids.get("f"), Some(6));
+        assert_eq!(external_documents_ids.get("g"), Some(7));
+
+        let del_ids = fst::Set::from_iter(vec!["a", "c", "f"]).unwrap();
+        external_documents_ids.delete_ids(del_ids).unwrap();
+
+        assert_eq!(external_documents_ids.get("a"), None);
+        assert_eq!(external_documents_ids.get("b"), Some(2));
+        assert_eq!(external_documents_ids.get("c"), None);
+        assert_eq!(external_documents_ids.get("d"), Some(4));
+        assert_eq!(external_documents_ids.get("e"), Some(5));
+        assert_eq!(external_documents_ids.get("f"), None);
+        assert_eq!(external_documents_ids.get("g"), Some(7));
+
+        let new_ids = fst::Map::from_iter(vec![("a", 5), ("b", 6), ("h", 8)]).unwrap();
+        external_documents_ids.insert_ids(&new_ids).unwrap();
+
+        assert_eq!(external_documents_ids.get("a"), Some(5));
+        assert_eq!(external_documents_ids.get("b"), Some(6));
+        assert_eq!(external_documents_ids.get("c"), None);
+        assert_eq!(external_documents_ids.get("d"), Some(4));
+        assert_eq!(external_documents_ids.get("e"), Some(5));
+        assert_eq!(external_documents_ids.get("f"), None);
+        assert_eq!(external_documents_ids.get("g"), Some(7));
+        assert_eq!(external_documents_ids.get("h"), Some(8));
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 808c54a4a..fd0156bfa 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,5 @@
 mod criterion;
+mod external_documents_ids;
 mod fields_ids_map;
 mod index;
 mod mdfs;