diff --git a/src/data/updates.rs b/src/data/updates.rs
index 9fd2015f6..e2bcba9d2 100644
--- a/src/data/updates.rs
+++ b/src/data/updates.rs
@@ -16,6 +16,7 @@ impl Data {
         method: IndexDocumentsMethod,
         format: UpdateFormat,
         mut stream: impl futures::Stream<Item=Result<B, E>> + Unpin,
+        primary_key: Option<String>,
     ) -> anyhow::Result<UpdateStatus>
     where
         B: Deref<Target = [u8]>,
@@ -47,7 +48,7 @@ impl Data {
                 mmap = unsafe { memmap::Mmap::map(&file)? };
                 &mmap
             };
-            index_controller.add_documents(index, method, format, &bytes)
+            index_controller.add_documents(index, method, format, &bytes, primary_key)
         }).await??;
         Ok(update.into())
     }
diff --git a/src/index_controller/local_index_controller/mod.rs b/src/index_controller/local_index_controller/mod.rs
index c0903a5e1..186ed3ddd 100644
--- a/src/index_controller/local_index_controller/mod.rs
+++ b/src/index_controller/local_index_controller/mod.rs
@@ -40,9 +40,10 @@ impl IndexController for LocalIndexController {
         method: milli::update::IndexDocumentsMethod,
         format: milli::update::UpdateFormat,
         data: &[u8],
+        primary_key: Option<String>,
     ) -> anyhow::Result<UpdateStatus<UpdateMeta, UpdateResult, String>> {
         let (_, update_store) = self.indexes.get_or_create_index(&index, self.update_db_size, self.index_db_size)?;
-        let meta = UpdateMeta::DocumentsAddition { method, format };
+        let meta = UpdateMeta::DocumentsAddition { method, format, primary_key };
         let pending = update_store.register_update(meta, data)?;
         Ok(pending.into())
     }
diff --git a/src/index_controller/local_index_controller/update_handler.rs b/src/index_controller/local_index_controller/update_handler.rs
index 7b7487ffa..7102f6f50 100644
--- a/src/index_controller/local_index_controller/update_handler.rs
+++ b/src/index_controller/local_index_controller/update_handler.rs
@@ -1,19 +1,19 @@
+use std::collections::HashMap;
 use std::io;
 use std::sync::Arc;
-use std::collections::HashMap;
 
 use anyhow::Result;
 use flate2::read::GzDecoder;
 use grenad::CompressionType;
 use log::info;
+use milli::update::{IndexDocumentsMethod, UpdateBuilder, UpdateFormat};
 use milli::Index;
-use milli::update::{UpdateBuilder, UpdateFormat, IndexDocumentsMethod};
 use rayon::ThreadPool;
 
-use crate::index_controller::updates::{Processing, Processed, Failed};
-use crate::index_controller::{UpdateResult, UpdateMeta, Settings, Facets};
-use crate::option::IndexerOpts;
 use super::update_store::HandleUpdate;
+use crate::index_controller::updates::{Failed, Processed, Processing};
+use crate::index_controller::{Facets, Settings, UpdateMeta, UpdateResult};
+use crate::option::IndexerOpts;
 
 pub struct UpdateHandler {
     index: Arc<Index>,
@@ -70,9 +70,19 @@ impl UpdateHandler {
         method: IndexDocumentsMethod,
         content: &[u8],
         update_builder: UpdateBuilder,
+        primary_key: Option<&str>,
     ) -> anyhow::Result<UpdateResult> {
         // We must use the write transaction of the update here.
         let mut wtxn = self.index.write_txn()?;
+
+        // Set the primary key if not set already, ignore if already set.
+        match (self.index.primary_key(&wtxn)?, primary_key) {
+            (None, Some(ref primary_key)) => {
+                self.index.put_primary_key(&mut wtxn, primary_key)?;
+            }
+            _ => (),
+        }
+
         let mut builder = update_builder.index_documents(&mut wtxn, &self.index);
         builder.update_format(format);
         builder.index_documents_method(method);
@@ -84,14 +94,16 @@ impl UpdateHandler {
             Box::new(content) as Box<dyn io::Read>
         };
 
-        let result = builder.execute(reader, |indexing_step, update_id| info!("update {}: {:?}", update_id, indexing_step));
+        let result = builder.execute(reader, |indexing_step, update_id| {
+            info!("update {}: {:?}", update_id, indexing_step)
+        });
 
         match result {
             Ok(addition_result) => wtxn
                 .commit()
                 .and(Ok(UpdateResult::DocumentsAddition(addition_result)))
                 .map_err(Into::into),
-            Err(e) => Err(e.into())
+            Err(e) => Err(e.into()),
         }
     }
 
@@ -105,11 +117,15 @@ impl UpdateHandler {
                 .commit()
                 .and(Ok(UpdateResult::Other))
                 .map_err(Into::into),
-            Err(e) => Err(e.into())
+            Err(e) => Err(e.into()),
         }
     }
 
-    fn update_settings(&self, settings: &Settings, update_builder: UpdateBuilder) -> anyhow::Result<UpdateResult> {
+    fn update_settings(
+        &self,
+        settings: &Settings,
+        update_builder: UpdateBuilder,
+    ) -> anyhow::Result<UpdateResult> {
         // We must use the write transaction of the update here.
         let mut wtxn = self.index.write_txn()?;
         let mut builder = update_builder.settings(&mut wtxn, &self.index);
@@ -144,21 +160,22 @@ impl UpdateHandler {
             }
         }
 
-        let result = builder.execute(|indexing_step, update_id| info!("update {}: {:?}", update_id, indexing_step));
+        let result = builder
+            .execute(|indexing_step, update_id| info!("update {}: {:?}", update_id, indexing_step));
 
         match result {
             Ok(()) => wtxn
                 .commit()
                 .and(Ok(UpdateResult::Other))
                 .map_err(Into::into),
-            Err(e) => Err(e.into())
+            Err(e) => Err(e.into()),
         }
     }
 
     fn update_facets(
         &self,
         levels: &Facets,
-        update_builder: UpdateBuilder
+        update_builder: UpdateBuilder,
     ) -> anyhow::Result<UpdateResult> {
         // We must use the write transaction of the update here.
         let mut wtxn = self.index.write_txn()?;
@@ -174,7 +191,7 @@ impl UpdateHandler {
                 .commit()
                 .and(Ok(UpdateResult::Other))
                 .map_err(Into::into),
-            Err(e) => Err(e.into())
+            Err(e) => Err(e.into()),
         }
     }
 
@@ -204,7 +221,7 @@ impl HandleUpdate<UpdateMeta, UpdateResult, String> for UpdateHandler {
     fn handle_update(
         &mut self,
         meta: Processing<UpdateMeta>,
-        content: &[u8]
+        content: &[u8],
     ) -> Result<Processed<UpdateMeta, UpdateResult>, Failed<UpdateMeta, String>> {
         use UpdateMeta::*;
 
@@ -213,7 +230,17 @@ impl HandleUpdate<UpdateMeta, UpdateResult, String> for UpdateHandler {
         let update_builder = self.update_buidler(update_id);
 
         let result = match meta.meta() {
-            DocumentsAddition { method, format } => self.update_documents(*format, *method, content, update_builder),
+            DocumentsAddition {
+                method,
+                format,
+                primary_key,
+            } => self.update_documents(
+                *format,
+                *method,
+                content,
+                update_builder,
+                primary_key.as_deref(),
+            ),
             ClearDocuments => self.clear_documents(update_builder),
             DeleteDocuments => self.delete_documents(content, update_builder),
             Settings(settings) => self.update_settings(settings, update_builder),
diff --git a/src/index_controller/mod.rs b/src/index_controller/mod.rs
index b881e3268..5bdaf8737 100644
--- a/src/index_controller/mod.rs
+++ b/src/index_controller/mod.rs
@@ -31,7 +31,11 @@ pub struct IndexMetadata {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(tag = "type")]
 pub enum UpdateMeta {
-    DocumentsAddition { method: IndexDocumentsMethod, format: UpdateFormat },
+    DocumentsAddition {
+        method: IndexDocumentsMethod,
+        format: UpdateFormat,
+        primary_key: Option<String>,
+    },
     ClearDocuments,
     DeleteDocuments,
     Settings(Settings),
@@ -128,6 +132,7 @@ pub trait IndexController {
         method: IndexDocumentsMethod,
         format: UpdateFormat,
         data: &[u8],
+        primary_key: Option<String>,
     ) -> anyhow::Result<UpdateStatus>;
 
     /// Clear all documents in the given index.
diff --git a/src/routes/document.rs b/src/routes/document.rs
index 9daaff6c8..0eb2bd463 100644
--- a/src/routes/document.rs
+++ b/src/routes/document.rs
@@ -127,17 +127,7 @@ async fn get_all_documents(
 #[derive(Deserialize)]
 #[serde(rename_all = "camelCase", deny_unknown_fields)]
 struct UpdateDocumentsQuery {
-    _primary_key: Option<String>,
-}
-
-async fn update_multiple_documents(
-    _data: web::Data<Data>,
-    _path: web::Path<IndexParam>,
-    _params: web::Query<UpdateDocumentsQuery>,
-    _body: web::Json<Vec<Document>>,
-    _is_partial: bool,
-) -> Result<HttpResponse, ResponseError> {
-    todo!()
+    primary_key: Option<String>,
 }
 
 /// Route used when the payload type is "application/json"
@@ -149,15 +139,16 @@ async fn update_multiple_documents(
 async fn add_documents_json(
     data: web::Data<Data>,
     path: web::Path<IndexParam>,
-    _params: web::Query<UpdateDocumentsQuery>,
+    params: web::Query<UpdateDocumentsQuery>,
     body: Payload,
 ) -> Result<HttpResponse, ResponseError> {
     let addition_result = data
         .add_documents(
             path.into_inner().index_uid,
-            IndexDocumentsMethod::UpdateDocuments,
+            IndexDocumentsMethod::ReplaceDocuments,
             UpdateFormat::Json,
-            body
+            body,
+            params.primary_key.clone(),
         ).await;
 
     match addition_result {
@@ -174,7 +165,7 @@ async fn add_documents_json(
 }
 
 
-/// Default route for addign documents, this should return an error en redirect to the docuentation
+/// Default route for adding documents, this should return an error and redirect to the documentation
 #[post("/indexes/{index_uid}/documents", wrap = "Authentication::Private")]
 async fn add_documents_default(
     _data: web::Data<Data>,
@@ -186,14 +177,49 @@ async fn add_documents_default(
     todo!()
 }
 
+/// Default route for adding documents, this should return an error and redirect to the documentation
 #[put("/indexes/{index_uid}/documents", wrap = "Authentication::Private")]
+async fn update_documents_default(
+    _data: web::Data<Data>,
+    _path: web::Path<IndexParam>,
+    _params: web::Query<UpdateDocumentsQuery>,
+    _body: web::Json<Vec<Document>>,
+) -> Result<HttpResponse, ResponseError> {
+    error!("Unknown document type");
+    todo!()
+}
+
+#[put(
+    "/indexes/{index_uid}/documents",
+    wrap = "Authentication::Private",
+    guard = "guard_json",
+)]
 async fn update_documents(
     data: web::Data<Data>,
     path: web::Path<IndexParam>,
     params: web::Query<UpdateDocumentsQuery>,
-    body: web::Json<Vec<Document>>,
+    body: web::Payload,
 ) -> Result<HttpResponse, ResponseError> {
-    update_multiple_documents(data, path, params, body, true).await
+    let addition_result = data
+        .add_documents(
+            path.into_inner().index_uid,
+            IndexDocumentsMethod::UpdateDocuments,
+            UpdateFormat::Json,
+            body,
+            params.primary_key.clone(),
+        ).await;
+
+    match addition_result {
+        Ok(update) => {
+            let value = serde_json::to_string(&update).unwrap();
+            let response = HttpResponse::Ok().body(value);
+            Ok(response)
+        }
+        Err(e) => {
+            error!("{}", e);
+            todo!()
+        }
+    }
 }
 
 #[post(