diff --git a/services/blob/src/constants.rs b/services/blob/src/constants.rs
--- a/services/blob/src/constants.rs
+++ b/services/blob/src/constants.rs
@@ -3,6 +3,26 @@
 pub const GRPC_SERVER_DEFAULT_PORT: u64 = 50051;
 pub const AWS_REGION: &str = "us-east-2";
 pub const LOCALSTACK_URL: &str = "http://localhost:4566";
+pub const MPSC_CHANNEL_BUFFER_CAPACITY: usize = 1;
+
+/// 4MB limit
+///
+/// WARNING: use keeping in mind that grpc adds its own headers to messages
+/// https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md
+/// so the message that actually is being sent over the network looks like this
+/// ```
+/// [Compressed-Flag] [Message-Length] [Message]
+///    [Compressed-Flag]   1 byte  - added by grpc
+///    [Message-Length]    4 bytes - added by grpc
+///    [Message]           N bytes - actual data
+/// ```
+/// so for every message we get 5 additional bytes of data
+/// as [mentioned here](https://github.com/grpc/grpc/issues/15734#issuecomment-396962671),
+/// gRPC stream may contain more than one message
+pub const GRPC_CHUNK_SIZE_LIMIT: u64 = 4 * 1024 * 1024;
+
+/// See [`GRPC_CHUNK_SIZE_LIMIT`] description for details
+pub const GRPC_METADATA_SIZE_PER_MESSAGE: u64 = 5;
 
 // DynamoDB constants
 
diff --git a/services/blob/src/service.rs b/services/blob/src/service.rs
--- a/services/blob/src/service.rs
+++ b/services/blob/src/service.rs
@@ -4,7 +4,10 @@
 use tokio_stream::Stream;
 use tonic::{Request, Response, Status};
 
-use crate::database::DatabaseClient;
+use crate::{
+  database::{BlobItem, DatabaseClient, ReverseIndexItem},
+  s3::S3Path,
+};
 
 pub mod blob {
   tonic::include_proto!("blob");
@@ -22,6 +25,31 @@
       s3: Arc::new(s3_client),
     }
   }
+
+  async fn find_s3_path_by_reverse_index(
+    &self,
+    reverse_index_item: &ReverseIndexItem,
+  ) -> Result<S3Path, Status> {
+    let blob_hash = &reverse_index_item.blob_hash;
+    match self.db.find_blob_item(&blob_hash).await {
+      Ok(Some(BlobItem { s3_path, .. })) => Ok(s3_path),
+      Ok(None) => Err(Status::not_found("blob not found")),
+      Err(_) => Err(Status::aborted("internal error")),
+    }
+  }
+
+  async fn find_s3_path_by_holder(
+    &self,
+    holder: &str,
+  ) -> Result<S3Path, Status> {
+    match self.db.find_reverse_index_by_holder(holder).await {
+      Ok(Some(reverse_index)) => {
+        self.find_s3_path_by_reverse_index(&reverse_index).await
+      }
+      Ok(None) => Err(Status::not_found("blob not found")),
+      Err(_) => Err(Status::aborted("internal error")),
+    }
+  }
 }
 
 // gRPC implementation