fix(search): persist FT.SYNUPDATE synonyms across server restarts

src/server/rdb_load.cc

@@ -2825,11 +2825,28 @@ void RdbLoader::LoadSearchIndexDefFromAux(string&& def) {
     return;
   }
 
-  // Prepend FT.CREATE to index definiton
+  // Determine command type and prepend appropriate prefix
   CmdArgVec arg_vec;
   facade::RespExpr::VecToArgList(resp_vec, &arg_vec);
-  string ft_create = "FT.CREATE";
-  arg_vec.insert(arg_vec.begin(), MutableSlice{ft_create.data(), ft_create.size()});
+
+  // Check if this is a SYNUPDATE command or a CREATE command
+  string command_name;
+  if (!arg_vec.empty()) {
+    string_view first_token = facade::ToSV(arg_vec[0]);
+    if (first_token == "SYNUPDATE") {
+      // Replace first token with FT.SYNUPDATE
+      command_name = "FT.SYNUPDATE";
+      arg_vec[0] = MutableSlice{command_name.data(), command_name.size()};
+    } else {
+      // Prepend FT.CREATE for index definition
+      command_name = "FT.CREATE";
+      arg_vec.insert(arg_vec.begin(), MutableSlice{command_name.data(), command_name.size()});
+    }
+  } else {
+    // Default: prepend FT.CREATE
+    command_name = "FT.CREATE";
+    arg_vec.insert(arg_vec.begin(), MutableSlice{command_name.data(), command_name.size()});
+  }
 
   service_->DispatchCommand(absl::MakeSpan(arg_vec), &crb, &cntx);
 

src/server/rdb_save.cc

@@ -1367,9 +1367,27 @@ RdbSaver::GlobalData RdbSaver::GetGlobalData(const Service* service) {
     if (shard->shard_id() == 0) {
       auto* indices = shard->search_indices();
       for (const auto& index_name : indices->GetIndexNames()) {
-        auto index_info = indices->GetIndex(index_name)->GetInfo();
+        auto* index = indices->GetIndex(index_name);
+        auto index_info = index->GetInfo();
+
+        // Save index definition
         search_indices.emplace_back(
             absl::StrCat(index_name, " ", index_info.BuildRestoreCommand()));
+
+        // Save synonym groups for this index
+        const auto& synonym_groups = index->GetSynonyms().GetGroups();
+        for (const auto& [group_id, terms] : synonym_groups) {
+          if (!terms.empty()) {
+            // Convert set<string> to vector for joining
+            std::vector<std::string_view> terms_vec(terms.begin(), terms.end());
+
+            // Format: "SYNUPDATE index_name group_id term1 term2 term3"
+            std::string syn_cmd = absl::StrCat("SYNUPDATE ", index_name, " ", group_id, " ",
+                                               absl::StrJoin(terms_vec, " "));
+
+            search_indices.emplace_back(std::move(syn_cmd));
+          }
+        }
       }
     }
 #endif

src/server/search/doc_index.cc

@@ -274,8 +274,11 @@ void ShardDocIndex::Rebuild(const OpArgs& op_args, PMR_NS::memory_resource* mr)
 
 void ShardDocIndex::RebuildForGroup(const OpArgs& op_args, const std::string_view& group_id,
                                     const std::vector<std::string_view>& terms) {
-  if (!indices_)
+  // Always update synonyms, even if indices are not yet built (during RDB load)
+  if (!indices_) {
+    synonyms_.UpdateGroup(group_id, terms);
     return;
+  }
 
   absl::flat_hash_set<DocId> docs_to_rebuild;
   std::vector<search::TextIndex*> text_indices = indices_->GetAllTextIndices();

tests/dragonfly/search_test.py

@@ -572,3 +572,29 @@ def make_car(producer, description, speed):
 
     for index in client.execute_command("FT._LIST"):
         client.ft(index.decode()).dropindex()
+
+
+@dfly_args({"proactor_threads": 4, "dbfilename": "synonym-persistence"})
+async def test_synonym_persistence(df_server):
+    """Test that synonyms are persisted across server restarts"""
+    client = aioredis.Redis(port=df_server.port)
+
+    # Create index and add documents
+    idx = client.ft("idx")
+    await idx.create_index([TextField("txt")], definition=IndexDefinition(prefix=["d:"]))
+    await client.hset("d:1", mapping={"txt": "car"})
+    await client.hset("d:2", mapping={"txt": "automobile"})
+
+    # Add synonyms and verify they work
+    await client.execute_command("FT.SYNUPDATE", "idx", "grp", "car", "automobile")
+    assert (await idx.search(Query("car"))).total == 2
+
+    # Restart server
+    df_server.stop()
+    df_server.start()
+    client = aioredis.Redis(port=df_server.port)
+    await wait_available_async(client)
+    idx = client.ft("idx")
+
+    # Verify synonyms still work after restart
+    assert (await idx.search(Query("car"))).total == 2