diff --git a/src/facade/reply_builder.cc b/src/facade/reply_builder.cc
index 822aa1c5c..7e10978d0 100644
--- a/src/facade/reply_builder.cc
+++ b/src/facade/reply_builder.cc
@@ -162,8 +162,14 @@ char* RedisReplyBuilder::FormatDouble(double val, char* dest, unsigned dest_len)
 RedisReplyBuilder::RedisReplyBuilder(::io::Sink* sink) : SinkReplyBuilder(sink) {
 }
 
-void RedisReplyBuilder::SendError(string_view str, std::string_view type) {
-  err_count_[type.empty() ? str : type]++;
+void RedisReplyBuilder::SendError(string_view str, string_view err_type) {
+  if (err_type.empty()) {
+    err_type = str;
+    if (err_type == kSyntaxErr)
+      err_type = kSyntaxErrType;
+  }
+
+  err_count_[err_type]++;
 
   if (str[0] == '-') {
     iovec v[] = {IoVec(str), IoVec(kCRLF)};
diff --git a/src/server/main_service.cc b/src/server/main_service.cc
index 6a1c126f0..fd3ceb720 100644
--- a/src/server/main_service.cc
+++ b/src/server/main_service.cc
@@ -486,12 +486,11 @@ void Service::DispatchCommand(CmdArgList args, facade::ConnectionContext* cntx)
 
     if (IsTransactional(cid)) {
       dist_trans.reset(new Transaction{cid, &shard_set_});
-      dfly_cntx->transaction = dist_trans.get();
-
       OpStatus st = dist_trans->InitByArgs(dfly_cntx->conn_state.db_index, args);
       if (st != OpStatus::OK)
         return (*cntx)->SendError(st);
 
+      dfly_cntx->transaction = dist_trans.get();
       dfly_cntx->last_command_debug.shards_count = dfly_cntx->transaction->unique_shard_cnt();
     } else {
       dfly_cntx->transaction = nullptr;
diff --git a/src/server/set_family.cc b/src/server/set_family.cc
index af74f4a44..6d1345b9f 100644
--- a/src/server/set_family.cc
+++ b/src/server/set_family.cc
@@ -305,8 +305,9 @@ OpResult<uint32_t> OpAdd(const OpArgs& op_args, std::string_view key, ArgSlice v
   auto* es = op_args.shard;
   auto& db_slice = es->db_slice();
 
-  // overwrite - meaning we run in the context of 2-hop operation and we had already
-  // ensured that the key exists.
+  // overwrite - meaning we run in the context of 2-hop operation and we want
+  // to overwrite the key. However, if the set is empty it means we should delete the
+  // key if it exists.
   if (overwrite && vals.empty()) {
     auto it = db_slice.FindExt(op_args.db_ind, key).first;
     db_slice.Del(op_args.db_ind, it);
diff --git a/src/server/set_family_test.cc b/src/server/set_family_test.cc
index d2c8fcb74..79b3a95bb 100644
--- a/src/server/set_family_test.cc
+++ b/src/server/set_family_test.cc
@@ -90,6 +90,8 @@ TEST_F(SetFamilyTest, SInter) {
   resp = Run({"sinter", "x", "y"});
   ASSERT_EQ(1, GetDebugInfo("IO0").shards_count);
   EXPECT_THAT(resp, ErrArg("WRONGTYPE Operation against a key"));
+  resp = Run({"sinterstore", "none1", "none2"});
+  EXPECT_THAT(resp, IntArg(0));
 }
 
 TEST_F(SetFamilyTest, SMove) {
diff --git a/src/server/transaction.cc b/src/server/transaction.cc
index fa53c89ab..7a812c584 100644
--- a/src/server/transaction.cc
+++ b/src/server/transaction.cc
@@ -103,6 +103,7 @@ OpStatus Transaction::InitByArgs(DbIndex index, CmdArgList args) {
 
   bool incremental_locking = multi_ && multi_->incremental;
   bool single_key = !multi_ && key_index.HasSingleKey();
+  bool needs_reverse_mapping = cid_->opt_mask() & CO::REVERSE_MAPPING;
 
   if (single_key) {
     DCHECK_GT(key_index.step, 0u);
@@ -118,6 +119,12 @@ OpStatus Transaction::InitByArgs(DbIndex index, CmdArgList args) {
     unique_shard_cnt_ = 1;
     unique_shard_id_ = Shard(key, ess_->size());
 
+    if (needs_reverse_mapping) {
+      reverse_index_.resize(args_.size());
+      for (unsigned j = 0; j < reverse_index_.size(); ++j) {
+        reverse_index_[j] = j + key_index.start - 1;
+      }
+    }
     return OpStatus::OK;
   }
 
@@ -137,7 +144,6 @@ OpStatus Transaction::InitByArgs(DbIndex index, CmdArgList args) {
   // and regular commands.
   IntentLock::Mode mode = IntentLock::EXCLUSIVE;
   bool should_record_locks = false;
-  bool needs_reverse_mapping = cid_->opt_mask() & CO::REVERSE_MAPPING;
 
   if (multi_) {
     mode = Mode();
@@ -148,11 +154,12 @@ OpStatus Transaction::InitByArgs(DbIndex index, CmdArgList args) {
 
   if (key_index.bonus) {  // additional one-of key.
     DCHECK(key_index.step == 1);
-    DCHECK(!needs_reverse_mapping);
 
     string_view key = ArgS(args, key_index.bonus);
     uint32_t sid = Shard(key, shard_data_.size());
     shard_index[sid].args.push_back(key);
+    if (needs_reverse_mapping)
+      shard_index[sid].original_index.push_back(key_index.bonus - 1);
   }
 
   for (unsigned i = key_index.start; i < key_index.end; ++i) {
@@ -183,7 +190,7 @@ OpStatus Transaction::InitByArgs(DbIndex index, CmdArgList args) {
 
   args_.resize(key_index.num_args());
 
-  // we need reverse index only for blocking commands or commands like MSET.
+  // we need reverse index only for some commands (MSET etc).
   if (needs_reverse_mapping)
     reverse_index_.resize(args_.size());
 
@@ -213,20 +220,25 @@ OpStatus Transaction::InitByArgs(DbIndex index, CmdArgList args) {
 
     ++unique_shard_cnt_;
     unique_shard_id_ = i;
-    uint32_t orig_indx = 0;
     for (size_t j = 0; j < si.args.size(); ++j) {
       *next_arg = si.args[j];
       if (needs_reverse_mapping) {
-        *rev_indx_it++ = si.original_index[orig_indx];
+        *rev_indx_it++ = si.original_index[j];
       }
       ++next_arg;
-      ++orig_indx;
     }
   }
 
   CHECK(next_arg == args_.end());
   DVLOG(1) << "InitByArgs " << DebugId() << " " << args_.front();
 
+  // validation
+  if (needs_reverse_mapping) {
+    for (size_t i = 0; i < args_.size(); ++i) {
+      DCHECK_EQ(args_[i], ArgS(args, 1 + reverse_index_[i]));  // 1 for the commandname.
+    }
+  }
+
   if (unique_shard_cnt_ == 1) {
     PerShardData* sd;
     if (multi_) {
@@ -892,11 +904,14 @@ ArgSlice Transaction::ShardArgsInShard(ShardId sid) const {
   return ArgSlice{args_.data() + sd.arg_start, sd.arg_count};
 }
 
+// from local index back to original arg index skipping the command.
+// i.e. returns (first_key_pos -1) or bigger.
 size_t Transaction::ReverseArgIndex(ShardId shard_id, size_t arg_index) const {
-  if (unique_shard_cnt_ == 1)
-    return arg_index;
+  if (unique_shard_cnt_ == 1)  // mget: 0->0, 1->1. zunionstore has 0->2
+    return reverse_index_[arg_index];
 
-  return reverse_index_[shard_data_[shard_id].arg_start + arg_index];
+  const auto& sd = shard_data_[shard_id];
+  return reverse_index_[sd.arg_start + arg_index];
 }
 
 bool Transaction::WaitOnWatch(const time_point& tp) {
@@ -1164,10 +1179,13 @@ OpResult<KeyIndex> DetermineKeys(const CommandId* cid, CmdArgList args) {
     if (args.size() < 3) {
       return OpStatus::SYNTAX_ERR;
     }
+
     string_view num(ArgS(args, 2));
-    if (!absl::SimpleAtoi(num, &num_custom_keys) || num_custom_keys < 0 ||
-        size_t(num_custom_keys) + 3 > args.size())
+    if (!absl::SimpleAtoi(num, &num_custom_keys) || num_custom_keys < 0)
       return OpStatus::INVALID_INT;
+
+    if (size_t(num_custom_keys) + 3 > args.size())
+      return OpStatus::SYNTAX_ERR;
   }
 
   if (cid->first_key_pos() > 0) {
diff --git a/src/server/transaction.h b/src/server/transaction.h
index bc0331c6a..7a72eb12a 100644
--- a/src/server/transaction.h
+++ b/src/server/transaction.h
@@ -70,9 +70,8 @@ class Transaction {
   // Runs in engine thread
   ArgSlice ShardArgsInShard(ShardId sid) const;
 
-  // Maps the index in ShardKeys(shard_id) slice back to the index in the original array passed to
-  // InitByArgs.
-  // Runs in the coordinator thread.
+  // Maps the index in ShardArgsInShard(shard_id) slice back to the index
+  // in the original array passed to InitByArgs.
   size_t ReverseArgIndex(ShardId shard_id, size_t arg_index) const;
 
   //! Returns true if the transaction spans this shard_id.
diff --git a/src/server/zset_family.cc b/src/server/zset_family.cc
index aa2ac87e0..e05596d92 100644
--- a/src/server/zset_family.cc
+++ b/src/server/zset_family.cc
@@ -14,6 +14,7 @@ extern "C" {
 #include <double-conversion/double-to-string.h>
 
 #include "base/logging.h"
+#include "base/stl_util.h"
 #include "facade/error.h"
 #include "server/command_registry.h"
 #include "server/conn_context.h"
@@ -78,15 +79,21 @@ zlexrangespec GetLexRange(bool reverse, const ZSetFamily::LexInterval& li) {
   return range;
 }
 
-OpResult<PrimeIterator> FindZEntry(unsigned flags, const OpArgs& op_args, string_view key,
+struct ZParams {
+  unsigned flags = 0;  // mask of ZADD_IN_ macros.
+  bool ch = false;     // Corresponds to CH option.
+  bool override = false;
+};
+
+OpResult<PrimeIterator> FindZEntry(const ZParams& zparams, const OpArgs& op_args, string_view key,
                                    size_t member_len) {
   auto& db_slice = op_args.shard->db_slice();
-  if (flags & ZADD_IN_XX) {
+  if (zparams.flags & ZADD_IN_XX) {
     return db_slice.Find(op_args.db_ind, key, OBJ_ZSET);
   }
 
   auto [it, inserted] = db_slice.AddOrFind(op_args.db_ind, key);
-  if (inserted) {
+  if (inserted || zparams.override) {
     robj* zobj = nullptr;
 
     if (member_len > kMaxListPackValue) {
@@ -96,12 +103,16 @@ OpResult<PrimeIterator> FindZEntry(unsigned flags, const OpArgs& op_args, string
     }
 
     DVLOG(2) << "Created zset " << zobj->ptr;
+    if (!inserted) {
+      db_slice.PreUpdate(op_args.db_ind, it);
+    }
     it->second.ImportRObj(zobj);
   } else {
     if (it->second.ObjType() != OBJ_ZSET)
       return OpStatus::WRONG_TYPE;
     db_slice.PreUpdate(op_args.db_ind, it);
   }
+
   return it;
 }
 
@@ -562,6 +573,193 @@ bool ParseLexBound(string_view src, ZSetFamily::LexBound* bound) {
   return true;
 }
 
+void SendAtLeastOneKeyError(ConnectionContext* cntx) {
+  string name = cntx->cid->name();
+  absl::AsciiStrToLower(&name);
+  (*cntx)->SendError(absl::StrCat("at least 1 input key is needed for ", name));
+}
+
+enum class AggType : uint8_t { SUM, MIN, MAX };
+using ScoredMap = absl::flat_hash_map<std::string, double>;
+
+ScoredMap FromObject(const CompactObj& co, double weight) {
+  robj* obj = co.AsRObj();
+  ZSetFamily::RangeParams params;
+  params.with_scores = true;
+  IntervalVisitor vis(Action::RANGE, params, obj);
+  vis(ZSetFamily::IndexInterval(0, -1));
+
+  ZSetFamily::ScoredArray arr = vis.PopResult();
+  ScoredMap res;
+  res.reserve(arr.size());
+
+  for (auto& elem : arr) {
+    elem.second *= weight;
+    res.emplace(move(elem));
+  }
+
+  return res;
+}
+
+double Aggregate(double v1, double v2, AggType atype) {
+  switch (atype) {
+    case AggType::SUM:
+      return v1 + v2;
+    case AggType::MAX:
+      return max(v1, v2);
+    case AggType::MIN:
+      return min(v1, v2);
+  }
+  return 0;
+}
+
+// the result is in the destination.
+void UnionScoredMap(ScoredMap* dest, ScoredMap* src, AggType agg_type) {
+  ScoredMap* target = dest;
+  ScoredMap* iter = src;
+
+  if (iter->size() > target->size())
+    swap(target, iter);
+
+  for (const auto& elem : *iter) {
+    auto [it, inserted] = target->emplace(elem);
+    if (!inserted) {
+      it->second = Aggregate(it->second, elem.second, agg_type);
+    }
+  }
+
+  if (target != dest)
+    dest->swap(*src);
+}
+
+OpResult<ScoredMap> OpUnion(EngineShard* shard, Transaction* t, string_view dest, AggType agg_type,
+                            const vector<double>& weights, bool store) {
+  ArgSlice keys = t->ShardArgsInShard(shard->shard_id());
+  DVLOG(1) << "shard:" << shard->shard_id() << ", keys " << vector(keys.begin(), keys.end());
+  DCHECK(!keys.empty());
+
+  unsigned start = 0;
+
+  if (keys.front() == dest) {
+    ++start;
+  }
+
+  auto& db_slice = shard->db_slice();
+  vector<pair<PrimeIterator, double>> it_arr(keys.size() - start);
+  if (it_arr.empty())     // could be when only the dest key is hosted in this shard
+    return OpStatus::OK;  // return empty map
+
+  for (unsigned j = start; j < keys.size(); ++j) {
+    auto it_res = db_slice.Find(t->db_index(), keys[j], OBJ_ZSET);
+    if (it_res == OpStatus::WRONG_TYPE)  // TODO: support sets with default score 1.
+      return it_res.status();
+    if (!it_res)
+      continue;
+
+    // first global index is 2 after {destkey, numkeys}
+    unsigned src_indx = j - start;
+    unsigned windex = t->ReverseArgIndex(shard->shard_id(), j) - 2;
+    DCHECK_LT(windex, weights.size());
+    it_arr[src_indx] = {*it_res, weights[windex]};
+  }
+
+  ScoredMap result;
+  for (auto it = it_arr.begin(); it != it_arr.end(); ++it) {
+    if (it->first.is_done())
+      continue;
+
+    ScoredMap sm = FromObject(it->first->second, it->second);
+    if (result.empty())
+      result.swap(sm);
+    else
+      UnionScoredMap(&result, &sm, agg_type);
+  }
+
+  return result;
+}
+
+using ScoredMemberView = std::pair<double, std::string_view>;
+using ScoredMemberSpan = absl::Span<ScoredMemberView>;
+
+struct AddResult {
+  double new_score;
+  unsigned num_updated = 0;
+
+  bool is_nan = false;
+};
+
+OpResult<AddResult> OpAdd(const OpArgs& op_args, const ZParams& zparams, string_view key,
+                          ScoredMemberSpan members) {
+  DCHECK(!members.empty() || zparams.override);
+  auto& db_slice = op_args.shard->db_slice();
+
+  if (zparams.override && members.empty()) {
+    auto it = db_slice.FindExt(op_args.db_ind, key).first;
+    db_slice.Del(op_args.db_ind, it);
+    return OpStatus::OK;
+  }
+
+  OpResult<PrimeIterator> res_it = FindZEntry(zparams, op_args, key, members.front().second.size());
+
+  if (!res_it)
+    return res_it.status();
+
+  robj* zobj = res_it.value()->second.AsRObj();
+
+  unsigned added = 0;
+  unsigned updated = 0;
+  unsigned processed = 0;
+
+  sds& tmp_str = op_args.shard->tmp_str1;
+  double new_score = 0;
+  int retflags = 0;
+
+  OpStatus op_status = OpStatus::OK;
+  AddResult aresult;
+
+  for (size_t j = 0; j < members.size(); j++) {
+    const auto& m = members[j];
+    tmp_str = sdscpylen(tmp_str, m.second.data(), m.second.size());
+
+    int retval = zsetAdd(zobj, m.first, tmp_str, zparams.flags, &retflags, &new_score);
+
+    if (zparams.flags & ZADD_IN_INCR) {
+      if (retval == 0) {
+        CHECK_EQ(1u, members.size());
+
+        aresult.is_nan = true;
+        break;
+      }
+
+      if (retflags & ZADD_OUT_NOP) {
+        op_status = OpStatus::SKIPPED;
+      }
+    }
+
+    if (retflags & ZADD_OUT_ADDED)
+      added++;
+    if (retflags & ZADD_OUT_UPDATED)
+      updated++;
+    if (!(retflags & ZADD_OUT_NOP))
+      processed++;
+  }
+
+  DVLOG(2) << "ZAdd " << zobj->ptr;
+
+  res_it.value()->second.SyncRObj();
+  op_args.shard->db_slice().PostUpdate(op_args.db_ind, *res_it);
+
+  if (zparams.flags & ZADD_IN_INCR) {
+    aresult.new_score = new_score;
+  } else {
+    aresult.num_updated = zparams.ch ? added + updated : added;
+  }
+
+  if (op_status != OpStatus::OK)
+    return op_status;
+  return aresult;
+}
+
 }  // namespace
 
 void ZSetFamily::ZAdd(CmdArgList args, ConnectionContext* cntx) {
@@ -631,33 +829,31 @@ void ZSetFamily::ZAdd(CmdArgList args, ConnectionContext* cntx) {
   DCHECK(cntx->transaction);
 
   absl::Span memb_sp{members.data(), members.size()};
-  AddResult add_result;
-
-  auto cb = [&](Transaction* t, EngineShard* shard) -> OpStatus {
+  auto cb = [&](Transaction* t, EngineShard* shard) {
     OpArgs op_args{shard, t->db_index()};
-    return OpAdd(zparams, op_args, key, memb_sp, &add_result);
+    return OpAdd(op_args, zparams, key, memb_sp);
   };
 
-  OpStatus status = cntx->transaction->ScheduleSingleHop(std::move(cb));
-  if (status == OpStatus::WRONG_TYPE) {
+  OpResult<AddResult> add_result = cntx->transaction->ScheduleSingleHopT(std::move(cb));
+  if (add_result.status() == OpStatus::WRONG_TYPE) {
     return (*cntx)->SendError(kWrongTypeErr);
   }
 
   // KEY_NOTFOUND may happen in case of XX flag.
-  if (status == OpStatus::KEY_NOTFOUND) {
+  if (add_result.status() == OpStatus::KEY_NOTFOUND) {
     if (zparams.flags & ZADD_IN_INCR)
       (*cntx)->SendNull();
     else
       (*cntx)->SendLong(0);
-  } else if (status == OpStatus::SKIPPED) {
+  } else if (add_result.status() == OpStatus::SKIPPED) {
     (*cntx)->SendNull();
-  } else if (add_result.is_nan) {
+  } else if (add_result->is_nan) {
     (*cntx)->SendError(kScoreNaN);
   } else {
     if (zparams.flags & ZADD_IN_INCR) {
-      (*cntx)->SendDouble(add_result.new_score);
+      (*cntx)->SendDouble(add_result->new_score);
     } else {
-      (*cntx)->SendLong(add_result.num_updated);
+      (*cntx)->SendLong(add_result->num_updated);
     }
   }
 }
@@ -725,31 +921,28 @@ void ZSetFamily::ZIncrBy(CmdArgList args, ConnectionContext* cntx) {
   ZParams zparams;
   zparams.flags = ZADD_IN_INCR;
 
-  AddResult add_result;
-
-  auto cb = [&](Transaction* t, EngineShard* shard) -> OpStatus {
+  auto cb = [&](Transaction* t, EngineShard* shard) {
     OpArgs op_args{shard, t->db_index()};
-    return OpAdd(zparams, op_args, key, ScoredMemberSpan{&scored_member, 1}, &add_result);
+    return OpAdd(op_args, zparams, key, ScoredMemberSpan{&scored_member, 1});
   };
 
-  OpStatus status = cntx->transaction->ScheduleSingleHop(std::move(cb));
-  if (status == OpStatus::WRONG_TYPE) {
+  OpResult<AddResult> add_result = cntx->transaction->ScheduleSingleHopT(std::move(cb));
+  if (add_result.status() == OpStatus::WRONG_TYPE) {
     return (*cntx)->SendError(kWrongTypeErr);
   }
 
-  if (status == OpStatus::SKIPPED) {
+  if (add_result.status() == OpStatus::SKIPPED) {
     return (*cntx)->SendNull();
   }
 
-  if (add_result.is_nan) {
+  if (add_result->is_nan) {
     return (*cntx)->SendError(kScoreNaN);
   }
 
-  (*cntx)->SendDouble(add_result.new_score);
+  (*cntx)->SendDouble(add_result->new_score);
 }
 
 void ZSetFamily::ZInterStore(CmdArgList args, ConnectionContext* cntx) {
-
 }
 
 void ZSetFamily::ZLexCount(CmdArgList args, ConnectionContext* cntx) {
@@ -984,16 +1177,93 @@ void ZSetFamily::ZScan(CmdArgList args, ConnectionContext* cntx) {
 }
 
 void ZSetFamily::ZUnionStore(CmdArgList args, ConnectionContext* cntx) {
-  auto cb = [&](Transaction* t, EngineShard* es) {
-    auto args = t->ShardArgsInShard(es->shard_id());
-    for (auto x : args) {
-      LOG(INFO) << "arg " << x;
+  string_view dest_key = ArgS(args, 1);
+  string_view num_str = ArgS(args, 2);
+  uint32_t num_keys;
+  AggType agg_type = AggType::SUM;
+
+  // we parsed the structure before, when transaction has been initialized.
+  CHECK(absl::SimpleAtoi(num_str, &num_keys));
+  if (num_keys == 0) {
+    return SendAtLeastOneKeyError(cntx);
+  }
+
+  DCHECK_GE(args.size(), 3 + num_keys);
+
+  vector<double> weights(num_keys, 1);
+  for (size_t i = 3 + num_keys; i < args.size(); ++i) {
+    ToUpper(&args[i]);
+    string_view arg = ArgS(args, i);
+    if (arg == "WEIGHTS") {
+      if (args.size() <= i + num_keys) {
+        return (*cntx)->SendError(kSyntaxErr);
+      }
+      for (unsigned j = 0; j < num_keys; ++j) {
+        string_view weight = ArgS(args, i + j + 1);
+        if (!absl::SimpleAtod(weight, &weights[j])) {
+          return (*cntx)->SendError("weight value is not a float", kSyntaxErrType);
+        }
+      }
+      i += num_keys;
+    } else if (arg == "AGGREGATE") {
+      if (i + 2 != args.size()) {
+        return (*cntx)->SendError(kSyntaxErr);
+      }
+      ToUpper(&args[i + 1]);
+
+      string_view agg = ArgS(args, i + 1);
+      if (agg == "SUM") {
+        agg_type = AggType::SUM;
+      } else if (agg == "MIN") {
+        agg_type = AggType::MIN;
+      } else if (agg == "MAX") {
+        agg_type = AggType::MAX;
+      } else {
+        return (*cntx)->SendError(kSyntaxErr);
+      }
+      break;
+    } else {
+      return (*cntx)->SendError(kSyntaxErr);
+    }
+  }
+
+  vector<OpResult<ScoredMap>> maps(cntx->shard_set->size());
+
+  auto cb = [&](Transaction* t, EngineShard* shard) {
+    maps[shard->shard_id()] = OpUnion(shard, t, dest_key, agg_type, weights, false);
+    return OpStatus::OK;
+  };
+
+  cntx->transaction->Schedule();
+
+  cntx->transaction->Execute(std::move(cb), false);
+  ScoredMap result;
+
+  for (auto& op_res : maps) {
+    if (!op_res)
+      return (*cntx)->SendError(op_res.status());
+    UnionScoredMap(&result, &op_res.value(), agg_type);
+  }
+  ShardId dest_shard = Shard(dest_key, maps.size());
+  AddResult add_result;
+  vector<ScoredMemberView> smvec;
+  for (const auto& elem : result) {
+    smvec.emplace_back(elem.second, elem.first);
+  }
+
+  auto store_cb = [&](Transaction* t, EngineShard* shard) {
+    if (shard->shard_id() == dest_shard) {
+      ZParams zparams;
+      zparams.override = true;
+      add_result =
+          OpAdd(OpArgs{shard, t->db_index()}, zparams, dest_key, ScoredMemberSpan{smvec}).value();
     }
     return OpStatus::OK;
   };
 
-  OpStatus result = cntx->transaction->ScheduleSingleHop(std::move(cb));
-  (*cntx)->SendOk();
+  cntx->transaction->Execute(std::move(store_cb), true);
+
+  (*cntx)->SendLong(smvec.size());
 }
 
 void ZSetFamily::ZRangeByScoreInternal(string_view key, string_view min_s, string_view max_s,
@@ -1202,68 +1472,6 @@ OpResult<StringVec> ZSetFamily::OpScan(const OpArgs& op_args, std::string_view k
   return res;
 }
 
-OpStatus ZSetFamily::OpAdd(const ZParams& zparams, const OpArgs& op_args, string_view key,
-                           ScoredMemberSpan members, AddResult* add_result) {
-  DCHECK(!members.empty());
-  OpResult<PrimeIterator> res_it =
-      FindZEntry(zparams.flags, op_args, key, members.front().second.size());
-
-  if (!res_it)
-    return res_it.status();
-
-  robj* zobj = res_it.value()->second.AsRObj();
-
-  unsigned added = 0;
-  unsigned updated = 0;
-  unsigned processed = 0;
-
-  sds& tmp_str = op_args.shard->tmp_str1;
-  double new_score = 0;
-  int retflags = 0;
-
-  OpStatus res = OpStatus::OK;
-
-  for (size_t j = 0; j < members.size(); j++) {
-    const auto& m = members[j];
-    tmp_str = sdscpylen(tmp_str, m.second.data(), m.second.size());
-
-    int retval = zsetAdd(zobj, m.first, tmp_str, zparams.flags, &retflags, &new_score);
-
-    if (zparams.flags & ZADD_IN_INCR) {
-      if (retval == 0) {
-        CHECK_EQ(1u, members.size());
-
-        add_result->is_nan = true;
-        break;
-      }
-
-      if (retflags & ZADD_OUT_NOP) {
-        res = OpStatus::SKIPPED;
-      }
-    }
-
-    if (retflags & ZADD_OUT_ADDED)
-      added++;
-    if (retflags & ZADD_OUT_UPDATED)
-      updated++;
-    if (!(retflags & ZADD_OUT_NOP))
-      processed++;
-  }
-
-  DVLOG(2) << "ZAdd " << zobj->ptr;
-
-  res_it.value()->second.SyncRObj();
-  op_args.shard->db_slice().PostUpdate(op_args.db_ind, *res_it);
-
-  if (zparams.flags & ZADD_IN_INCR) {
-    add_result->new_score = new_score;
-  } else {
-    add_result->num_updated = zparams.ch ? added + updated : added;
-  }
-
-  return res;
-}
-
 OpResult<unsigned> ZSetFamily::OpRem(const OpArgs& op_args, string_view key, ArgSlice members) {
   auto& db_slice = op_args.shard->db_slice();
   OpResult<PrimeIterator> res_it = db_slice.Find(op_args.db_ind, key, OBJ_ZSET);
@@ -1496,11 +1704,13 @@ OpResult<unsigned> ZSetFamily::OpLexCount(const OpArgs& op_args, string_view key
 #define HFUNC(x) SetHandler(&ZSetFamily::x)
 
 void ZSetFamily::Register(CommandRegistry* registry) {
+  constexpr uint32_t kUnionMask = CO::WRITE | CO::DESTINATION_KEY | CO::REVERSE_MAPPING;
+
   *registry << CI{"ZADD", CO::FAST | CO::WRITE | CO::DENYOOM, -4, 1, 1, 1}.HFUNC(ZAdd)
             << CI{"ZCARD", CO::FAST | CO::READONLY, 2, 1, 1, 1}.HFUNC(ZCard)
             << CI{"ZCOUNT", CO::FAST | CO::READONLY, 4, 1, 1, 1}.HFUNC(ZCount)
             << CI{"ZINCRBY", CO::FAST | CO::WRITE | CO::DENYOOM, 4, 1, 1, 1}.HFUNC(ZIncrBy)
-            << CI{"ZINTERSTORE", CO::WRITE | CO::DESTINATION_KEY, -4, 1, 1, 1}.HFUNC(ZInterStore)
+            << CI{"ZINTERSTORE", kUnionMask, -4, 3, 3, 1}.HFUNC(ZInterStore)
             << CI{"ZLEXCOUNT", CO::READONLY, 4, 1, 1, 1}.HFUNC(ZLexCount)
             << CI{"ZREM", CO::FAST | CO::WRITE, -3, 1, 1, 1}.HFUNC(ZRem)
             << CI{"ZRANGE", CO::READONLY, -4, 1, 1, 1}.HFUNC(ZRange)
@@ -1515,7 +1725,7 @@ void ZSetFamily::Register(CommandRegistry* registry) {
             << CI{"ZREVRANGEBYSCORE", CO::READONLY, -4, 1, 1, 1}.HFUNC(ZRevRangeByScore)
             << CI{"ZREVRANK", CO::READONLY | CO::FAST, 3, 1, 1, 1}.HFUNC(ZRevRank)
             << CI{"ZSCAN", CO::READONLY | CO::RANDOM, -3, 1, 1, 1}.HFUNC(ZScan)
-            << CI{"ZUNIONSTORE", CO::WRITE | CO::DESTINATION_KEY, -4, 3, 3, 1}.HFUNC(ZUnionStore);
+            << CI{"ZUNIONSTORE", kUnionMask, -4, 3, 3, 1}.HFUNC(ZUnionStore);
 }
 
 }  // namespace dfly
diff --git a/src/server/zset_family.h b/src/server/zset_family.h
index dda433e7c..801e3561e 100644
--- a/src/server/zset_family.h
+++ b/src/server/zset_family.h
@@ -86,23 +86,6 @@ class ZSetFamily {
 
   static OpResult<StringVec> OpScan(const OpArgs& op_args, std::string_view key, uint64_t* cursor);
 
-  struct ZParams {
-    unsigned flags = 0;  // mask of ZADD_IN_ macros.
-    bool ch = false;     // Corresponds to CH option.
-  };
-
-  using ScoredMemberView = std::pair<double, std::string_view>;
-  using ScoredMemberSpan = absl::Span<ScoredMemberView>;
-
-  struct AddResult {
-    double new_score;
-    unsigned num_updated = 0;
-
-    bool is_nan = false;
-  };
-
-  static facade::OpStatus OpAdd(const ZParams& zparams, const OpArgs& op_args, std::string_view key,
-                                ScoredMemberSpan members, AddResult* add_result);
   static OpResult<unsigned> OpRem(const OpArgs& op_args, std::string_view key, ArgSlice members);
   static OpResult<double> OpScore(const OpArgs& op_args, std::string_view key,
                                   std::string_view member);
diff --git a/src/server/zset_family_test.cc b/src/server/zset_family_test.cc
index 233c4a698..1acdf426e 100644
--- a/src/server/zset_family_test.cc
+++ b/src/server/zset_family_test.cc
@@ -167,4 +167,63 @@ TEST_F(ZSetFamilyTest, ZScan) {
   EXPECT_EQ(100 * 2, scan_len);
 }
 
+TEST_F(ZSetFamilyTest, ZUnionStore) {
+  RespExpr resp;
+
+  resp = Run({"zunionstore", "key", "0"});
+  EXPECT_THAT(resp, ErrArg("wrong number of arguments"));
+
+  resp = Run({"zunionstore", "key", "0", "aggregate", "sum"});
+  EXPECT_THAT(resp, ErrArg("at least 1 input key is needed"));
+  resp = Run({"zunionstore", "key", "-1", "aggregate", "sum"});
+  EXPECT_THAT(resp, ErrArg("out of range"));
+  resp = Run({"zunionstore", "key", "2", "foo", "bar", "weights", "1"});
+  EXPECT_THAT(resp, ErrArg("syntax error"));
+
+  EXPECT_EQ(2, CheckedInt({"zadd", "z1", "1", "a", "2", "b"}));
+  EXPECT_EQ(2, CheckedInt({"zadd", "z2", "3", "c", "2", "b"}));
+
+  resp = Run({"zunionstore", "key", "2", "z1", "z2"});
+  EXPECT_THAT(resp, IntArg(3));
+  resp = Run({"zrange", "key", "0", "-1", "withscores"});
+  EXPECT_THAT(resp.GetVec(), ElementsAre("a", "1", "c", "3", "b", "4"));
+
+  resp = Run({"zunionstore", "z1", "1", "z1"});
+  EXPECT_THAT(resp, IntArg(2));
+
+  resp = Run({"zunionstore", "z1", "2", "z1", "z2"});
+  EXPECT_THAT(resp, IntArg(3));
+  resp = Run({"zrange", "z1", "0", "-1", "withscores"});
+  EXPECT_THAT(resp.GetVec(), ElementsAre("a", "1", "c", "3", "b", "4"));
+
+  Run({"set", "foo", "bar"});
+  resp = Run({"zunionstore", "foo", "1", "z2"});
+  EXPECT_THAT(resp, IntArg(2));
+  resp = Run({"zrange", "foo", "0", "-1", "withscores"});
+  EXPECT_THAT(resp.GetVec(), ElementsAre("b", "2", "c", "3"));
+}
+
+TEST_F(ZSetFamilyTest, ZUnionStoreOpts) {
+  EXPECT_EQ(2, CheckedInt({"zadd", "z1", "1", "a", "2", "b"}));
+  EXPECT_EQ(2, CheckedInt({"zadd", "z2", "3", "c", "2", "b"}));
+  RespExpr resp;
+
+  EXPECT_EQ(3, CheckedInt({"zunionstore", "a", "2", "z1",  "z2", "weights", "1", "3"}));
+  resp = Run({"zrange", "a", "0", "-1", "withscores"});
+  EXPECT_THAT(resp.GetVec(), ElementsAre("a", "1", "b", "8", "c", "9"));
+
+  resp = Run({"zunionstore", "a", "2", "z1",  "z2", "weights", "1"});
+  EXPECT_THAT(resp, ErrArg("syntax error"));
+
+  resp = Run({"zunionstore", "z1", "1", "z1",  "weights", "2"});
+  EXPECT_THAT(resp, IntArg(2));
+  resp = Run({"zrange", "z1", "0", "-1", "withscores"});
+  EXPECT_THAT(resp.GetVec(), ElementsAre("a", "2", "b", "4"));
+
+  resp = Run({"zunionstore", "max", "2", "z1", "z2", "weights", "1", "0", "aggregate", "max"});
+  ASSERT_THAT(resp, IntArg(3));
+  resp = Run({"zrange", "max", "0", "-1", "withscores"});
+  EXPECT_THAT(resp.GetVec(), ElementsAre("c", "0", "a", "2", "b", "4"));
+}
+
 }  // namespace dfly