diff --git a/Cargo.lock b/Cargo.lock
index 899893044..3c0f0456e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1624,7 +1624,6 @@ dependencies = [
  "sdset",
  "serde",
  "serde_json",
- "siphasher",
  "slice-group-by",
  "structopt",
  "tempfile",
diff --git a/meilisearch-core/Cargo.toml b/meilisearch-core/Cargo.toml
index d9b88f89b..341c6336a 100644
--- a/meilisearch-core/Cargo.toml
+++ b/meilisearch-core/Cargo.toml
@@ -35,7 +35,6 @@ regex = "1.3.6"
 sdset = "0.4.0"
 serde = { version = "1.0.105", features = ["derive"] }
 serde_json = { version = "1.0.50", features = ["preserve_order"] }
-siphasher = "0.3.2"
 slice-group-by = "0.2.6"
 unicase = "2.6.0"
 zerocopy = "0.3.0"
diff --git a/meilisearch-core/src/store/main.rs b/meilisearch-core/src/store/main.rs
index 9c3f89f39..0a23b82cb 100644
--- a/meilisearch-core/src/store/main.rs
+++ b/meilisearch-core/src/store/main.rs
@@ -85,10 +85,36 @@ impl Main {
         }
     }
 
+    pub fn merge_internal_ids(self, writer: &mut heed::RwTxn<MainT>, new_ids: &sdset::Set<DocumentId>) -> ZResult<()> {
+        use sdset::SetOperation;
+
+        // We do an union of the old and new internal ids.
+        let internal_ids = self.internal_ids(writer)?;
+        let internal_ids = sdset::duo::Union::new(&new_ids, &internal_ids).into_set_buf();
+        self.put_internal_ids(writer, &internal_ids)
+    }
+
     pub fn put_user_ids(self, writer: &mut heed::RwTxn<MainT>, ids: &fst::Map) -> ZResult<()> {
         self.main.put::<_, Str, ByteSlice>(writer, USER_IDS_KEY, ids.as_fst().as_bytes())
     }
 
+    pub fn merge_user_ids(self, writer: &mut heed::RwTxn<MainT>, new_ids: &fst::Map) -> ZResult<()> {
+        use fst::{Streamer, IntoStreamer};
+
+        let user_ids = self.user_ids(writer)?;
+
+        // Do an union of the old and the new set of user ids.
+        let mut op = user_ids.op().add(new_ids.into_stream()).r#union();
+        let mut build = fst::MapBuilder::memory();
+        while let Some((userid, values)) = op.next() {
+            build.insert(userid, values[0].value).unwrap();
+        }
+        let user_ids = build.into_inner().unwrap();
+
+        // TODO prefer using self.put_user_ids
+        self.main.put::<_, Str, ByteSlice>(writer, USER_IDS_KEY, user_ids.as_slice())
+    }
+
     pub fn user_ids(self, reader: &heed::RoTxn<MainT>) -> ZResult<fst::Map> {
         match self.main.get::<_, Str, ByteSlice>(reader, USER_IDS_KEY)? {
             Some(bytes) => {
diff --git a/meilisearch-core/src/update/documents_addition.rs b/meilisearch-core/src/update/documents_addition.rs
index d9d1af328..efafe3e1c 100644
--- a/meilisearch-core/src/update/documents_addition.rs
+++ b/meilisearch-core/src/update/documents_addition.rs
@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std::collections::{HashMap, BTreeMap};
 
 use fst::{set::OpBuilder, SetBuilder};
 use indexmap::IndexMap;
@@ -13,7 +13,7 @@ use crate::database::{UpdateEvent, UpdateEventsEmitter};
 use crate::facets;
 use crate::raw_indexer::RawIndexer;
 use crate::serde::Deserializer;
-use crate::store::{self, DocumentsFields, DocumentsFieldsCounts};
+use crate::store::{self, DocumentsFields, DocumentsFieldsCounts, DiscoverIds};
 use crate::update::helpers::{index_value, value_to_number, extract_document_id};
 use crate::update::{apply_documents_deletion, compute_short_prefixes, next_update_id, Update};
 use crate::{Error, MResult, RankedMap};
@@ -150,17 +150,26 @@ pub fn apply_addition<'a, 'b>(
     partial: bool
 ) -> MResult<()> {
     let mut documents_additions = HashMap::new();
+    let mut new_user_ids = BTreeMap::new();
+    let mut new_internal_ids = Vec::with_capacity(new_documents.len());
 
     let mut schema = match index.main.schema(writer)? {
         Some(schema) => schema,
         None => return Err(Error::SchemaMissing),
     };
 
+    // Retrieve the documents ids related structures
+    let user_ids = index.main.user_ids(writer)?;
+    let internal_ids = index.main.internal_ids(writer)?;
+    let mut available_ids = DiscoverIds::new(&internal_ids);
+
     let primary_key = schema.primary_key().ok_or(Error::MissingPrimaryKey)?;
 
     // 1. store documents ids for future deletion
     for mut document in new_documents {
-        let document_id = extract_document_id(&primary_key, &document)?;
+        let (document_id, userid) = extract_document_id(&primary_key, &document, &user_ids, &mut available_ids)?;
+        new_user_ids.insert(userid, document_id.0);
+        new_internal_ids.push(document_id);
 
         if partial {
             let mut deserializer = Deserializer {
@@ -233,6 +242,11 @@ pub fn apply_addition<'a, 'b>(
 
     index.main.put_schema(writer, &schema)?;
 
+    let new_user_ids = fst::Map::from_iter(new_user_ids)?;
+    let new_internal_ids = sdset::SetBuf::from_dirty(new_internal_ids);
+    index.main.merge_user_ids(writer, &new_user_ids)?;
+    index.main.merge_internal_ids(writer, &new_internal_ids)?;
+
     Ok(())
 }
 
diff --git a/meilisearch-core/src/update/documents_deletion.rs b/meilisearch-core/src/update/documents_deletion.rs
index 4526d053d..bfca8b360 100644
--- a/meilisearch-core/src/update/documents_deletion.rs
+++ b/meilisearch-core/src/update/documents_deletion.rs
@@ -71,7 +71,10 @@ pub fn apply_documents_deletion(
     writer: &mut heed::RwTxn<MainT>,
     index: &store::Index,
     deletion: Vec<DocumentId>,
-) -> MResult<()> {
+) -> MResult<()>
+{
+    unimplemented!("When we delete documents we must ask for user ids instead of internal ones");
+
     let schema = match index.main.schema(writer)? {
         Some(schema) => schema,
         None => return Err(Error::SchemaMissing),
diff --git a/meilisearch-core/src/update/helpers.rs b/meilisearch-core/src/update/helpers.rs
index d17bea3b2..7f9a7b634 100644
--- a/meilisearch-core/src/update/helpers.rs
+++ b/meilisearch-core/src/update/helpers.rs
@@ -1,16 +1,15 @@
 use std::fmt::Write as _;
-use std::hash::{Hash, Hasher};
 
 use indexmap::IndexMap;
 use meilisearch_schema::IndexedPos;
 use meilisearch_types::DocumentId;
 use ordered_float::OrderedFloat;
 use serde_json::Value;
-use siphasher::sip::SipHasher;
 
+use crate::Number;
 use crate::raw_indexer::RawIndexer;
 use crate::serde::SerializerError;
-use crate::Number;
+use crate::store::DiscoverIds;
 
 /// Returns the number of words indexed or `None` if the type is unindexable.
 pub fn index_value(
@@ -96,28 +95,43 @@ pub fn value_to_number(value: &Value) -> Option<Number> {
     }
 }
 
-/// Validates a string representation to be a correct document id and
-/// returns the hash of the given type, this is the way we produce documents ids.
-pub fn compute_document_id(string: &str) -> Result<DocumentId, SerializerError> {
-    if string.chars().all(|x| x.is_ascii_alphanumeric() || x == '-' || x == '_') {
-        let mut s = SipHasher::new();
-        string.hash(&mut s);
-        Ok(DocumentId(s.finish()))
+/// Validates a string representation to be a correct document id and returns
+/// the corresponding id or generate a new one, this is the way we produce documents ids.
+pub fn discover_document_id(
+    userid: &str,
+    user_ids: &fst::Map,
+    available_ids: &mut DiscoverIds<'_>,
+) -> Result<DocumentId, SerializerError>
+{
+    if userid.chars().all(|x| x.is_ascii_alphanumeric() || x == '-' || x == '_') {
+        match user_ids.get(userid) {
+            Some(internal_id) => Ok(DocumentId(internal_id)),
+            None => {
+                let internal_id = available_ids.next().expect("no more ids available");
+                Ok(internal_id)
+            },
+        }
     } else {
         Err(SerializerError::InvalidDocumentIdFormat)
     }
 }
 
 /// Extracts and validates the document id of a document.
-pub fn extract_document_id(primary_key: &str, document: &IndexMap<String, Value>) -> Result<DocumentId, SerializerError> {
+pub fn extract_document_id(
+    primary_key: &str,
+    document: &IndexMap<String, Value>,
+    user_ids: &fst::Map,
+    available_ids: &mut DiscoverIds<'_>,
+) -> Result<(DocumentId, String), SerializerError>
+{
     match document.get(primary_key) {
         Some(value) => {
-            let string = match value {
+            let userid = match value {
                 Value::Number(number) => number.to_string(),
                 Value::String(string) => string.clone(),
                 _ => return Err(SerializerError::InvalidDocumentIdFormat),
             };
-            compute_document_id(&string)
+            discover_document_id(&userid, user_ids, available_ids).map(|id| (id, userid))
         }
         None => Err(SerializerError::DocumentIdNotFound),
     }
diff --git a/meilisearch-core/src/update/mod.rs b/meilisearch-core/src/update/mod.rs
index 5b7c33c9d..55bdc9edc 100644
--- a/meilisearch-core/src/update/mod.rs
+++ b/meilisearch-core/src/update/mod.rs
@@ -9,7 +9,7 @@ pub use self::clear_all::{apply_clear_all, push_clear_all};
 pub use self::customs_update::{apply_customs_update, push_customs_update};
 pub use self::documents_addition::{apply_documents_addition, apply_documents_partial_addition, DocumentsAddition};
 pub use self::documents_deletion::{apply_documents_deletion, DocumentsDeletion};
-pub use self::helpers::{index_value, value_to_string, value_to_number, compute_document_id, extract_document_id};
+pub use self::helpers::{index_value, value_to_string, value_to_number, discover_document_id, extract_document_id};
 pub use self::settings_update::{apply_settings_update, push_settings_update};
 
 use std::cmp;