diff --git a/Cargo.lock b/Cargo.lock
index c3e9532e2..a21cbc007 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3858,7 +3858,7 @@ checksum = "a2e27bcfe835a379d32352112f6b8dbae2d99d16a5fff42abe6e5ba5386c1e5a"
 [[package]]
 name = "obkv"
 version = "0.3.0"
-source = "git+https://github.com/kerollmops/obkv?branch=unsized-kvreader#d248eb7edd3453ff758afc2883f6ae25684eb69e"
+source = "git+https://github.com/kerollmops/obkv?branch=unsized-kvreader#5289a6658cd471f4212c1edc1a40b2a3c3d11fe0"
 
 [[package]]
 name = "once_cell"
diff --git a/milli/src/update/new/channel.rs b/milli/src/update/new/channel.rs
new file mode 100644
index 000000000..0dd2d9935
--- /dev/null
+++ b/milli/src/update/new/channel.rs
@@ -0,0 +1,93 @@
+use crossbeam_channel::{Receiver, RecvError, SendError, Sender};
+use heed::types::Bytes;
+
+use super::indexer::KvReaderFieldId;
+use super::StdResult;
+use crate::{DocumentId, Index};
+
+/// The capacity of the channel is currently in number of messages.
+pub fn merge_writer_channel(cap: usize) -> WriterChannels {
+    let (sender, receiver) = crossbeam_channel::bounded(cap);
+
+    WriterChannels {
+        writer_receiver: WriterReceiver(receiver),
+        merger_sender: MergerSender(sender.clone()),
+        document_sender: DocumentSender(sender),
+    }
+}
+
+pub struct WriterChannels {
+    pub writer_receiver: WriterReceiver,
+    pub merger_sender: MergerSender,
+    pub document_sender: DocumentSender,
+}
+
+pub struct KeyValueEntry {
+    pub key_length: u16,
+    pub data: Box<[u8]>,
+}
+
+impl KeyValueEntry {
+    pub fn entry(&self) -> (&[u8], &[u8]) {
+        self.data.split_at(self.key_length as usize)
+    }
+}
+
+pub struct DocumentEntry {
+    docid: DocumentId,
+    content: Box<[u8]>,
+}
+
+impl DocumentEntry {
+    pub fn new_uncompressed(docid: DocumentId, content: Box<KvReaderFieldId>) -> Self {
+        DocumentEntry { docid, content: content.into() }
+    }
+
+    pub fn new_compressed(docid: DocumentId, content: Box<[u8]>) -> Self {
+        DocumentEntry { docid, content }
+    }
+
+    pub fn entry(&self) -> ([u8; 4], &[u8]) {
+        let docid = self.docid.to_be_bytes();
+        (docid, &self.content)
+    }
+}
+
+pub enum WriterOperation {
+    WordDocIds(KeyValueEntry),
+    Document(DocumentEntry),
+}
+
+impl WriterOperation {
+    pub fn database(&self, index: &Index) -> heed::Database<Bytes, Bytes> {
+        match self {
+            WriterOperation::WordDocIds(_) => index.word_docids.remap_types(),
+            WriterOperation::Document(_) => index.documents.remap_types(),
+        }
+    }
+}
+
+pub struct WriterReceiver(Receiver<WriterOperation>);
+
+impl WriterReceiver {
+    pub fn recv(&self) -> StdResult<WriterOperation, RecvError> {
+        self.0.recv()
+    }
+}
+
+pub struct MergerSender(Sender<WriterOperation>);
+
+#[derive(Clone)]
+pub struct DocumentSender(Sender<WriterOperation>);
+
+impl DocumentSender {
+    pub fn send(&self, document: DocumentEntry) -> StdResult<(), SendError<DocumentEntry>> {
+        match self.0.send(WriterOperation::Document(document)) {
+            Ok(()) => Ok(()),
+            Err(SendError(wop)) => match wop {
+                WriterOperation::Document(entry) => Err(SendError(entry)),
+                _ => unreachable!(),
+            },
+        }
+    }
+}
diff --git a/milli/src/update/new/mod.rs b/milli/src/update/new/mod.rs
index 20266267d..726153c53 100644
--- a/milli/src/update/new/mod.rs
+++ b/milli/src/update/new/mod.rs
@@ -1,9 +1,12 @@
 mod document_change;
 // mod extract;
+mod channel;
 mod items_pool;
 
 mod global_fields_ids_map;
 
+pub type StdResult<T, E> = std::result::Result<T, E>;
+
 mod indexer {
     use std::borrow::Cow;
     use std::collections::{BTreeMap, HashMap};
@@ -352,11 +355,13 @@ mod indexer {
 
     pub struct UpdateByFunctionIndexer;
 
+    // fn
+
     /// Reads the previous version of a document from the database, the new versions
     /// in the grenad update files and merges them to generate a new boxed obkv.
     ///
     /// This function is only meant to be used when doing an update and not a replacement.
-    pub fn merge_document_for_updates(
+    fn merge_document_for_updates(
         rtxn: &RoTxn,
         index: &Index,
         fields_ids_map: &FieldsIdsMap,
@@ -431,7 +436,7 @@ mod indexer {
     /// Returns only the most recent version of a document based on the updates from the payloads.
     ///
     /// This function is only meant to be used when doing a replacement and not an update.
-    pub fn merge_document_for_replacements(
+    fn merge_document_for_replacements(
         rtxn: &RoTxn,
         index: &Index,
         fields_ids_map: &FieldsIdsMap,