diff --git a/etcd/v3/action_constants.hpp b/etcd/v3/action_constants.hpp
index 5c3fdc5..faa77c2 100644
--- a/etcd/v3/action_constants.hpp
+++ b/etcd/v3/action_constants.hpp
@@ -13,6 +13,17 @@ namespace etcdv3
   extern char const * LOCK_ACTION;
   extern char const * UNLOCK_ACTION;
   extern char const * TXN_ACTION;
+
+  extern char const * NUL;
+
+  extern char const * KEEPALIVE_CREATE;
+  extern char const * KEEPALIVE_WRITE;
+  extern char const * KEEPALIVE_READ;
+  extern char const * KEEPALIVE_DONE;
+
+  extern char const * WATCH_CREATE;
+  extern char const * WATCH_WRITE;
+  extern char const * WATCH_WRITES_DONE;
 }
 
 #endif
diff --git a/src/v3/Action.cpp b/src/v3/Action.cpp
index 558e9fa..43d2ef2 100644
--- a/src/v3/Action.cpp
+++ b/src/v3/Action.cpp
@@ -1,4 +1,5 @@
 #include <grpc/support/log.h>
+#include "etcd/v3/action_constants.hpp"
 #include "etcd/v3/Action.hpp"
 
 etcdv3::Action::Action(etcdv3::ActionParameters params)
@@ -50,7 +51,5 @@ std::string etcdv3::detail::string_plus_one(std::string const &value) {
       return ret;
     }
   }
-  // see: noPrefixEnd in etcd, however c++ doesn't allows '\0' inside a string, thus we use
-  // the UTF-8 char U+0000 (i.e., "\xC0\x80").
-  return {"\xC0\x80"};
+  return {etcdv3::NUL};
 }
diff --git a/src/v3/AsyncDeleteAction.cpp b/src/v3/AsyncDeleteAction.cpp
index 8bf0933..4e93e62 100644
--- a/src/v3/AsyncDeleteAction.cpp
+++ b/src/v3/AsyncDeleteAction.cpp
@@ -12,7 +12,7 @@ etcdv3::AsyncDeleteAction::AsyncDeleteAction(ActionParameters param)
   if(parameters.withPrefix)
   {
     if (parameters.key.empty()) {
-      del_request.set_range_end(detail::string_plus_one("\0"));
+      del_request.set_range_end(detail::string_plus_one(etcdv3::NUL));
     } else {
       del_request.set_range_end(detail::string_plus_one(parameters.key));
     }
diff --git a/src/v3/AsyncGetAction.cpp b/src/v3/AsyncGetAction.cpp
index bed840e..0d3ba7e 100644
--- a/src/v3/AsyncGetAction.cpp
+++ b/src/v3/AsyncGetAction.cpp
@@ -11,7 +11,7 @@ etcdv3::AsyncGetAction::AsyncGetAction(etcdv3::ActionParameters param)
 {
   RangeRequest get_request;
   if (parameters.key.empty()) {
-    get_request.set_key("\0");
+    get_request.set_key(etcdv3::NUL);
   } else {
     get_request.set_key(parameters.key);
   }
@@ -19,7 +19,7 @@ etcdv3::AsyncGetAction::AsyncGetAction(etcdv3::ActionParameters param)
   if(parameters.withPrefix)
   {
     if (parameters.key.empty()) {
-      get_request.set_range_end(detail::string_plus_one("\0"));
+      get_request.set_range_end(detail::string_plus_one(etcdv3::NUL));
     } else {
       get_request.set_range_end(detail::string_plus_one(parameters.key));
     }
diff --git a/src/v3/AsyncLeaseAction.cpp b/src/v3/AsyncLeaseAction.cpp
index 2ef12aa..62e57ba 100644
--- a/src/v3/AsyncLeaseAction.cpp
+++ b/src/v3/AsyncLeaseAction.cpp
@@ -59,11 +59,11 @@ etcdv3::AsyncLeaseKeepAliveAction::AsyncLeaseKeepAliveAction(etcdv3::ActionParam
   : etcdv3::Action(param)
 {
   isCancelled = false;
-  stream = parameters.lease_stub->AsyncLeaseKeepAlive(&context, &cq_, (void*)"keepalive create");
+  stream = parameters.lease_stub->AsyncLeaseKeepAlive(&context, &cq_, (void*)etcdv3::KEEPALIVE_CREATE);
 
   void *got_tag = nullptr;
   bool ok = false;
-  if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)"keepalive create") {
+  if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)etcdv3::KEEPALIVE_CREATE) {
     // ok
   } else {
     throw std::runtime_error("Failed to create a lease keep-alive connection");
@@ -100,12 +100,12 @@ etcd::Response etcdv3::AsyncLeaseKeepAliveAction::Refresh()
   void *got_tag = nullptr;
   bool ok = false;
 
-  stream->Write(leasekeepalive_request, (void *)"keepalive write");
+  stream->Write(leasekeepalive_request, (void *)etcdv3::KEEPALIVE_WRITE);
   // wait write finish
-  if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)"keepalive write") {
-    stream->Read(&reply, (void*)"keepalive read");
+  if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)etcdv3::KEEPALIVE_WRITE) {
+    stream->Read(&reply, (void*)etcdv3::KEEPALIVE_READ);
     // wait read finish
-    if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)"keepalive read") {
+    if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)etcdv3::KEEPALIVE_READ) {
       auto resp = ParseResponse();
       auto duration = std::chrono::duration_cast<std::chrono::microseconds>(
             std::chrono::high_resolution_clock::now() - start_timepoint);
@@ -121,7 +121,7 @@ void etcdv3::AsyncLeaseKeepAliveAction::CancelKeepAlive()
   if(isCancelled == false)
   {
     isCancelled = true;
-    stream->WritesDone((void*)"keepalive done");
+    stream->WritesDone((void*)etcdv3::KEEPALIVE_DONE);
     grpc::Status status;
     stream->Finish(&status, (void *)this);
     cq_.Shutdown();
diff --git a/src/v3/AsyncWatchAction.cpp b/src/v3/AsyncWatchAction.cpp
index 0c4a77e..6454993 100644
--- a/src/v3/AsyncWatchAction.cpp
+++ b/src/v3/AsyncWatchAction.cpp
@@ -10,7 +10,7 @@ etcdv3::AsyncWatchAction::AsyncWatchAction(etcdv3::ActionParameters param)
   : etcdv3::Action(param) 
 {
   isCancelled.store(false);
-  stream = parameters.watch_stub->AsyncWatch(&context,&cq_,(void*)"create");
+  stream = parameters.watch_stub->AsyncWatch(&context,&cq_,(void*)etcdv3::WATCH_CREATE);
 
   WatchRequest watch_req;
   WatchCreateRequest watch_create_req;
@@ -21,7 +21,7 @@ etcdv3::AsyncWatchAction::AsyncWatchAction(etcdv3::ActionParameters param)
   if(parameters.withPrefix)
   {
     if (parameters.key.empty()) {
-      watch_create_req.set_range_end(detail::string_plus_one("\0"));
+      watch_create_req.set_range_end(detail::string_plus_one(etcdv3::NUL));
     } else {
       watch_create_req.set_range_end(detail::string_plus_one(parameters.key));
     }
@@ -35,14 +35,14 @@ etcdv3::AsyncWatchAction::AsyncWatchAction(etcdv3::ActionParameters param)
   // wait "create" success (the stream becomes ready)
   void *got_tag;
   bool ok = false;
-  if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)"create") {
-    stream->Write(watch_req, (void *)"write");
+  if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)etcdv3::WATCH_CREATE) {
+    stream->Write(watch_req, (void *)etcdv3::WATCH_WRITE);
   } else {
     throw std::runtime_error("failed to create a watch connection");
   }
 
   // wait "write" (WatchCreateRequest) success, and start to read the first reply
-  if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)"write") {
+  if (cq_.Next(&got_tag, &ok) && ok && got_tag == (void *)etcdv3::WATCH_WRITE) {
     stream->Read(&reply, (void*)this);
   } else {
     throw std::runtime_error("failed to write WatchCreateRequest to server");
@@ -60,7 +60,7 @@ void etcdv3::AsyncWatchAction::waitForResponse()
     {
       break;
     }
-    if(got_tag == (void*)"writes done") {
+    if(got_tag == (void*)etcdv3::WATCH_WRITES_DONE) {
       isCancelled.store(true);
       cq_.Shutdown();
       break;
@@ -78,7 +78,7 @@ void etcdv3::AsyncWatchAction::waitForResponse()
         // 1. watch for a future revision, return immediately with empty events set
         // 2. receive any effective events.
         isCancelled.store(true);
-        stream->WritesDone((void*)"writes done");
+        stream->WritesDone((void*)etcdv3::WATCH_WRITES_DONE);
         grpc::Status status;
         stream->Finish(&status, (void *)this);
         cq_.Shutdown();
@@ -101,7 +101,7 @@ void etcdv3::AsyncWatchAction::CancelWatch()
 {
   std::lock_guard<std::mutex> scope_lock(this->protect_is_cancalled);
   if (!isCancelled.exchange(true)) {
-    stream->WritesDone((void*)"writes done");
+    stream->WritesDone((void*)etcdv3::WATCH_WRITES_DONE);
     grpc::Status status;
     stream->Finish(&status, (void *)this);
     cq_.Shutdown();
@@ -123,7 +123,7 @@ void etcdv3::AsyncWatchAction::waitForResponse(std::function<void(etcd::Response
     {
       break;
     }
-    if(got_tag == (void*)"writes done")
+    if(got_tag == (void*)etcdv3::WATCH_WRITES_DONE)
     {
       isCancelled.store(true);
       cq_.Shutdown();
diff --git a/src/v3/action_constants.cpp b/src/v3/action_constants.cpp
index 891ac1e..abbbdeb 100644
--- a/src/v3/action_constants.cpp
+++ b/src/v3/action_constants.cpp
@@ -10,3 +10,16 @@ char const * etcdv3::COMPAREDELETE_ACTION = "compareAndDelete";
 char const * etcdv3::LOCK_ACTION = "lock";
 char const * etcdv3::UNLOCK_ACTION = "unlock";
 char const * etcdv3::TXN_ACTION = "txn";
+
+// see: noPrefixEnd in etcd, however c++ doesn't allows '\0' inside a string, thus we use
+// the UTF-8 char U+0000 (i.e., "\xC0\x80").
+char const * etcdv3::NUL = "\xC0\x80";
+
+char const * etcdv3::KEEPALIVE_CREATE = "keepalive create";
+char const * etcdv3::KEEPALIVE_WRITE = "keepalive write";
+char const * etcdv3::KEEPALIVE_READ = "keepalive read";
+char const * etcdv3::KEEPALIVE_DONE = "keepalive done";
+
+char const * etcdv3::WATCH_CREATE = "watch create";
+char const * etcdv3::WATCH_WRITE = "watch write";
+char const * etcdv3::WATCH_WRITES_DONE = "watch writes done";