diff --git a/example/rest_book_client/main.cc b/example/rest_book_client/main.cc
index 4cfae18..72ebe9e 100644
--- a/example/rest_book_client/main.cc
+++ b/example/rest_book_client/main.cc
@@ -29,7 +29,7 @@ class BookClientBase {
   BookClientBase(const std::string& host, const std::string& port,
                  int timeout_seconds)
       : rest_client_(host, port) {
-    rest_client_.set_timeout_seconds(timeout_seconds);
+    rest_client_.SetTimeout(timeout_seconds);
   }
 
   virtual ~BookClientBase() = default;
diff --git a/example/soap_book_client/book_client.h b/example/soap_book_client/book_client.h
index 203a83d..92da8b5 100644
--- a/example/soap_book_client/book_client.h
+++ b/example/soap_book_client/book_client.h
@@ -4,6 +4,7 @@
 #include <functional>
 #include <string>
 
+#include "pugixml/pugixml.hpp"
 #include "webcc/soap_client.h"
 
 #include "example/common/book.h"
diff --git a/example/soap_calc_client/main.cc b/example/soap_calc_client/main.cc
index b60e288..8ef54d3 100644
--- a/example/soap_calc_client/main.cc
+++ b/example/soap_calc_client/main.cc
@@ -9,7 +9,7 @@ class CalcClient {
  public:
   CalcClient(const std::string& host, const std::string& port)
       : soap_client_(host, port) {
-    soap_client_.set_timeout_seconds(5);
+    soap_client_.SetTimeout(5);
 
     soap_client_.set_url("/calculator");
     soap_client_.set_service_ns({
diff --git a/example/soap_calc_client_parasoft/main.cc b/example/soap_calc_client_parasoft/main.cc
index e912395..47638ca 100644
--- a/example/soap_calc_client_parasoft/main.cc
+++ b/example/soap_calc_client_parasoft/main.cc
@@ -9,7 +9,7 @@ class CalcClient {
  public:
   CalcClient(const std::string& host, const std::string& port)
       : soap_client_(host, port) {
-    soap_client_.set_timeout_seconds(5);
+    soap_client_.SetTimeout(5);
 
     soap_client_.set_url("/glue/calculator");
     soap_client_.set_service_ns({
diff --git a/webcc/http_async_client.cc b/webcc/http_async_client.cc
index 04b68ad..3cb87c5 100644
--- a/webcc/http_async_client.cc
+++ b/webcc/http_async_client.cc
@@ -11,7 +11,7 @@ namespace webcc {
 
 HttpAsyncClient::HttpAsyncClient(boost::asio::io_context& io_context)
     : socket_(io_context),
-      resolver_(new tcp::resolver(io_context)),
+      resolver_(io_context),
       buffer_(kBufferSize),
       deadline_(io_context),
       timeout_seconds_(kMaxReadSeconds),
@@ -34,11 +34,11 @@ void HttpAsyncClient::Request(std::shared_ptr<HttpRequest> request,
   request_ = request;
   response_handler_ = response_handler;
 
-  resolver_->async_resolve(tcp::v4(), request->host(), request->port(kHttpPort),
-                           std::bind(&HttpAsyncClient::ResolveHandler,
-                                     shared_from_this(),
-                                     std::placeholders::_1,
-                                     std::placeholders::_2));
+  resolver_.async_resolve(tcp::v4(), request->host(), request->port(kHttpPort),
+                          std::bind(&HttpAsyncClient::ResolveHandler,
+                                    shared_from_this(),
+                                    std::placeholders::_1,
+                                    std::placeholders::_2));
 }
 
 void HttpAsyncClient::Stop() {
@@ -65,11 +65,8 @@ void HttpAsyncClient::ResolveHandler(boost::system::error_code ec,
              request_->host().c_str(), request_->port().c_str());
     response_handler_(response_, kHostResolveError, timed_out_);
   } else {
-    // Start the connect actor.
-    endpoints_ = endpoints;
-
     // ConnectHandler: void(boost::system::error_code, tcp::endpoint)
-    boost::asio::async_connect(socket_, endpoints_,
+    boost::asio::async_connect(socket_, endpoints,
                                std::bind(&HttpAsyncClient::ConnectHandler,
                                          shared_from_this(),
                                          std::placeholders::_1,
diff --git a/webcc/http_async_client.h b/webcc/http_async_client.h
index ec74a5e..e4e4a6f 100644
--- a/webcc/http_async_client.h
+++ b/webcc/http_async_client.h
@@ -19,6 +19,10 @@ namespace webcc {
 // Response handler/callback.
 typedef std::function<void(HttpResponsePtr, Error, bool)> HttpResponseHandler;
 
+// HTTP client session in asynchronous mode.
+// A request will return without waiting for the response, the callback handler
+// will be invoked when the response is received or timeout occurs.
+// Don't use the same HttpAsyncClient object in multiple threads.
 class HttpAsyncClient : public std::enable_shared_from_this<HttpAsyncClient> {
  public:
   explicit HttpAsyncClient(boost::asio::io_context& io_context);
@@ -55,9 +59,8 @@ class HttpAsyncClient : public std::enable_shared_from_this<HttpAsyncClient> {
   void AsyncWaitDeadline();
   void DeadlineHandler(boost::system::error_code ec);
 
+  tcp::resolver resolver_;
   tcp::socket socket_;
-  std::unique_ptr<tcp::resolver> resolver_;
-  tcp::resolver::results_type endpoints_;
 
   std::shared_ptr<HttpRequest> request_;
   std::vector<char> buffer_;
diff --git a/webcc/http_client.cc b/webcc/http_client.cc
index 192a82a..656b647 100644
--- a/webcc/http_client.cc
+++ b/webcc/http_client.cc
@@ -27,11 +27,19 @@ HttpClient::HttpClient()
       error_(kNoError) {
 }
 
+void HttpClient::SetTimeout(int seconds) {
+  if (seconds > 0) {
+    timeout_seconds_ = seconds;
+  }
+}
+
 bool HttpClient::Request(const HttpRequest& request) {
   response_.reset(new HttpResponse());
   response_parser_.reset(new HttpResponseParser(response_.get()));
 
-  stopped_ = timed_out_ = false;
+  stopped_ = false;
+  timed_out_ = false;
+  error_ = kNoError;
 
   if ((error_ = Connect(request)) != kNoError) {
     return false;
diff --git a/webcc/http_client.h b/webcc/http_client.h
index 061e521..cc0de64 100644
--- a/webcc/http_client.h
+++ b/webcc/http_client.h
@@ -16,6 +16,9 @@
 
 namespace webcc {
 
+// HTTP client session in synchronous mode.
+// A request will not return until the response is received or timeout occurs.
+// Don't use the same HttpClient object in multiple threads.
 class HttpClient {
  public:
   HttpClient();
@@ -23,10 +26,9 @@ class HttpClient {
 
   DELETE_COPY_AND_ASSIGN(HttpClient);
 
-  void set_timeout_seconds(int timeout_seconds) {
-    assert(timeout_seconds > 0);
-    timeout_seconds_ = timeout_seconds;
-  }
+  // Set the timeout seconds for reading response.
+  // The |seconds| is only effective when greater than 0.
+  void SetTimeout(int seconds);
 
   // Connect to server, send request, wait until response is received.
   bool Request(const HttpRequest& request);
diff --git a/webcc/rest_client.cc b/webcc/rest_client.cc
index 747adab..db8f599 100644
--- a/webcc/rest_client.cc
+++ b/webcc/rest_client.cc
@@ -1,14 +1,11 @@
 #include "webcc/rest_client.h"
 
-#include "webcc/http_client.h"
 #include "webcc/http_request.h"
 
 namespace webcc {
 
 RestClient::RestClient(const std::string& host, const std::string& port)
-    : host_(host), port_(port),
-      timeout_seconds_(0), timed_out_(false),
-      error_(kNoError) {
+    : host_(host), port_(port) {
   if (port_.empty()) {
     std::size_t i = host_.find_last_of(':');
     if (i != std::string::npos) {
@@ -20,11 +17,6 @@ RestClient::RestClient(const std::string& host, const std::string& port)
 
 bool RestClient::Request(const std::string& method, const std::string& url,
                          std::string&& content) {
-  response_.reset();
-
-  error_ = kNoError;
-  timed_out_ = false;
-
   HttpRequest http_request;
 
   http_request.set_method(method);
@@ -38,19 +30,10 @@ bool RestClient::Request(const std::string& method, const std::string& url,
 
   http_request.UpdateStartLine();
 
-  HttpClient http_client;
-
-  if (timeout_seconds_ > 0) {
-    http_client.set_timeout_seconds(timeout_seconds_);
-  }
-
-  if (!http_client.Request(http_request)) {
-    error_ = http_client.error();
-    timed_out_ = http_client.timed_out();
+  if (!http_client_.Request(http_request)) {
     return false;
   }
 
-  response_ = http_client.response();
   return true;
 }
 
diff --git a/webcc/rest_client.h b/webcc/rest_client.h
index 11dabeb..f20a6b1 100644
--- a/webcc/rest_client.h
+++ b/webcc/rest_client.h
@@ -6,6 +6,7 @@
 #include <utility>  // for move()
 
 #include "webcc/globals.h"
+#include "webcc/http_client.h"
 #include "webcc/http_response.h"
 
 namespace webcc {
@@ -14,20 +15,23 @@ class RestClient {
  public:
   // If |port| is empty, |host| will be checked to see if it contains port or
   // not (separated by ':').
-  RestClient(const std::string& host, const std::string& port = "");
+  explicit RestClient(const std::string& host, const std::string& port = "");
 
   ~RestClient() = default;
 
   DELETE_COPY_AND_ASSIGN(RestClient);
 
-  void set_timeout_seconds(int timeout_seconds) {
-    timeout_seconds_ = timeout_seconds;
+  void SetTimeout(int seconds) {
+    http_client_.SetTimeout(seconds);
   }
 
+  // NOTE:
+  // The return value of the following methods (Get, Post, etc.) only indicates
+  // if the socket communication is successful or not. Check error() and
+  // timed_out() for more information if it's failed. Check response_status()
+  // instead for the HTTP status code.
+
   // HTTP GET request.
-  // The return value indicates if the socket communication is successful
-  // or not. If it's failed, check error() and timed_out() for more details.
-  // For HTTP status, check response_status() instead.
   inline bool Get(const std::string& url) {
     return Request(kHttpGet, url, "");
   }
@@ -52,21 +56,27 @@ class RestClient {
     return Request(kHttpDelete, url, "");
   }
 
-  HttpResponsePtr response() const { return response_; }
+  HttpResponsePtr response() const {
+    return http_client_.response();
+  }
 
   int response_status() const {
-    assert(response_);
-    return response_->status();
+    assert(response());
+    return response()->status();
   }
 
   const std::string& response_content() const {
-    assert(response_);
-    return response_->content();
+    assert(response());
+    return response()->content();
   }
 
-  bool timed_out() const { return timed_out_; }
+  bool timed_out() const {
+    return http_client_.timed_out();
+  }
 
-  Error error() const { return error_; }
+  Error error() const {
+    return http_client_.error();
+  }
 
  private:
   bool Request(const std::string& method, const std::string& url,
@@ -75,15 +85,7 @@ class RestClient {
   std::string host_;
   std::string port_;
 
-  // Timeout in seconds; only effective when > 0.
-  int timeout_seconds_;
-
-  HttpResponsePtr response_;
-
-  // If the error was caused by timeout or not.
-  bool timed_out_;
-
-  Error error_;
+  HttpClient http_client_;
 };
 
 }  // namespace webcc
diff --git a/webcc/soap_client.cc b/webcc/soap_client.cc
index 4bc50a2..6080f50 100644
--- a/webcc/soap_client.cc
+++ b/webcc/soap_client.cc
@@ -3,8 +3,6 @@
 #include <cassert>
 #include <utility>  // for move()
 
-#include "webcc/http_client.h"
-#include "webcc/soap_globals.h"
 #include "webcc/soap_request.h"
 #include "webcc/soap_response.h"
 
@@ -13,8 +11,7 @@ namespace webcc {
 SoapClient::SoapClient(const std::string& host, const std::string& port)
     : host_(host), port_(port),
       soapenv_ns_(kSoapEnvNamespace),
-      format_raw_(true), timeout_seconds_(0), timed_out_(false),
-      error_(kNoError) {
+      format_raw_(true), error_(kNoError) {
   if (port_.empty()) {
     std::size_t i = host_.find_last_of(':');
     if (i != std::string::npos) {
@@ -31,6 +28,8 @@ bool SoapClient::Request(const std::string& operation,
   assert(!url_.empty() && !host_.empty());
   assert(!result_name_.empty());
 
+  error_ = kNoError;
+
   SoapRequest soap_request;
 
   soap_request.set_soapenv_ns(soapenv_ns_);
@@ -55,22 +54,15 @@ bool SoapClient::Request(const std::string& operation,
   http_request.SetHeader(kSoapAction, operation);
   http_request.UpdateStartLine();
 
-  HttpClient http_client;
-
-  if (timeout_seconds_ > 0) {
-    http_client.set_timeout_seconds(timeout_seconds_);
-  }
-
-  if (!http_client.Request(http_request)) {
-    error_ = http_client.error();
-    timed_out_ = http_client.timed_out();
+  if (!http_client_.Request(http_request)) {
+    error_ = http_client_.error();
     return false;
   }
 
   SoapResponse soap_response;
   soap_response.set_result_name(result_name_);
 
-  if (!soap_response.FromXml(http_client.response()->content())) {
+  if (!soap_response.FromXml(http_client_.response()->content())) {
     error_ = kXmlError;
     return false;
   }
diff --git a/webcc/soap_client.h b/webcc/soap_client.h
index c90d431..8ed3067 100644
--- a/webcc/soap_client.h
+++ b/webcc/soap_client.h
@@ -4,7 +4,8 @@
 #include <string>
 #include <vector>
 
-#include "webcc/soap_message.h"
+#include "webcc/http_client.h"
+#include "webcc/soap_globals.h"
 #include "webcc/soap_parameter.h"
 
 namespace webcc {
@@ -13,14 +14,14 @@ class SoapClient {
  public:
   // If |port| is empty, |host| will be checked to see if it contains port or
   // not (separated by ':').
-  SoapClient(const std::string& host, const std::string& port = "");
+  explicit SoapClient(const std::string& host, const std::string& port = "");
 
   ~SoapClient() = default;
 
   DELETE_COPY_AND_ASSIGN(SoapClient);
 
-  void set_timeout_seconds(int timeout_seconds) {
-    timeout_seconds_ = timeout_seconds;
+  void SetTimeout(int seconds) {
+    http_client_.SetTimeout(seconds);
   }
 
   void set_url(const std::string& url) { url_ = url; }
@@ -33,7 +34,9 @@ class SoapClient {
     result_name_ = result_name;
   }
 
-  void set_format_raw(bool format_raw) { format_raw_ = format_raw; }
+  void set_format_raw(bool format_raw) {
+    format_raw_ = format_raw;
+  }
 
   void set_indent_str(const std::string& indent_str) {
     indent_str_ = indent_str;
@@ -43,7 +46,9 @@ class SoapClient {
                std::vector<SoapParameter>&& parameters,
                std::string* result);
 
-  bool timed_out() const { return timed_out_; }
+  bool timed_out() const {
+    return http_client_.timed_out();
+  }
 
   Error error() const { return error_; }
 
@@ -68,11 +73,7 @@ class SoapClient {
   // Applicable when |format_raw_| is false.
   std::string indent_str_;
 
-  // Timeout in seconds; only effective when > 0.
-  int timeout_seconds_;
-
-  // If the error was caused by timeout or not.
-  bool timed_out_;
+  HttpClient http_client_;
 
   Error error_;
 };
diff --git a/webcc/soap_globals.cc b/webcc/soap_globals.cc
index 7ba5cc3..377d5f1 100644
--- a/webcc/soap_globals.cc
+++ b/webcc/soap_globals.cc
@@ -10,4 +10,9 @@ const std::string kSoapAction = "SOAPAction";
 // See: https://www.w3.org/TR/2007/REC-soap12-part0-20070427/#L26854
 const std::string kTextXmlUtf8 = "text/xml; charset=utf-8";
 
+const SoapNamespace kSoapEnvNamespace{
+  "soap",
+  "http://schemas.xmlsoap.org/soap/envelope/"
+};
+
 }  // namespace webcc
diff --git a/webcc/soap_globals.h b/webcc/soap_globals.h
index 166f2be..5d4195b 100644
--- a/webcc/soap_globals.h
+++ b/webcc/soap_globals.h
@@ -12,6 +12,22 @@ extern const std::string kSoapAction;
 
 extern const std::string kTextXmlUtf8;
 
+// -----------------------------------------------------------------------------
+
+// XML namespace name/url pair.
+// E.g., { "soap", "http://schemas.xmlsoap.org/soap/envelope/" }
+struct SoapNamespace {
+  std::string name;
+  std::string url;
+
+  bool IsValid() const {
+    return !name.empty() && !url.empty();
+  }
+};
+
+// CSoap's default namespace for SOAP Envelope.
+extern const SoapNamespace kSoapEnvNamespace;
+
 }  // namespace webcc
 
 #endif  // WEBCC_SOAP_GLOBALS_H_
diff --git a/webcc/soap_message.cc b/webcc/soap_message.cc
index b5e928e..5ff29c4 100644
--- a/webcc/soap_message.cc
+++ b/webcc/soap_message.cc
@@ -6,11 +6,6 @@
 
 namespace webcc {
 
-const SoapNamespace kSoapEnvNamespace{
-  "soap",
-  "http://schemas.xmlsoap.org/soap/envelope/"
-};
-
 void SoapMessage::ToXml(bool format_raw, const std::string& indent,
                         std::string* xml_string) {
   assert(soapenv_ns_.IsValid() && service_ns_.IsValid() && !operation_.empty());
diff --git a/webcc/soap_message.h b/webcc/soap_message.h
index 71dfa8c..a2fc235 100644
--- a/webcc/soap_message.h
+++ b/webcc/soap_message.h
@@ -5,28 +5,14 @@
 
 #include "pugixml/pugixml.hpp"
 
-#include "webcc/globals.h"
+#include "webcc/soap_globals.h"
 
 namespace webcc {
 
-// XML namespace name/url pair.
-// E.g., { "soap", "http://schemas.xmlsoap.org/soap/envelope/" }
-struct SoapNamespace {
-  std::string name;
-  std::string url;
-
-  bool IsValid() const {
-    return !name.empty() && !url.empty();
-  }
-};
-
-// CSoap's default namespace for SOAP Envelope.
-extern const SoapNamespace kSoapEnvNamespace;
-
 // Base class for SOAP request and response.
 class SoapMessage {
  public:
-  virtual ~SoapMessage() {}
+  virtual ~SoapMessage() = default;
 
   // E.g., set as kSoapEnvNamespace.
   void set_soapenv_ns(const SoapNamespace& soapenv_ns) {