diff --git a/rclcpp/executors/executor_comparison/CMakeLists.txt b/rclcpp/executors/executor_comparison/CMakeLists.txt
new file mode 100644
index 00000000..ec92a0f7
--- /dev/null
+++ b/rclcpp/executors/executor_comparison/CMakeLists.txt
@@ -0,0 +1,34 @@
+cmake_minimum_required(VERSION 3.5)
+project(simple_thread_tester)
+
+find_package(ament_cmake REQUIRED)
+find_package(rclcpp REQUIRED)
+find_package(std_msgs REQUIRED)
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_BUILD_TYPE Debug)
+
+add_executable(publisher_node publisher_node.cc)
+ament_target_dependencies(publisher_node rclcpp std_msgs)
+install(TARGETS publisher_node DESTINATION lib/simple_thread_tester)
+
+add_executable(single_thread_subscriber_node single_thread_subscriber_node.cc)
+ament_target_dependencies(single_thread_subscriber_node rclcpp std_msgs)
+install(TARGETS single_thread_subscriber_node
+        DESTINATION lib/simple_thread_tester)
+
+add_executable(multi_thread_reentrant_subscriber_node
+               multi_thread_reentrant_subscriber_node.cc)
+ament_target_dependencies(multi_thread_reentrant_subscriber_node rclcpp
+                          std_msgs)
+install(TARGETS multi_thread_reentrant_subscriber_node
+        DESTINATION lib/simple_thread_tester)
+
+add_executable(multi_thread_mutually_exclusive_subscriber_node
+               multi_thread_mutually_exclusive_subscriber_node.cc)
+ament_target_dependencies(multi_thread_mutually_exclusive_subscriber_node
+                          rclcpp std_msgs)
+install(TARGETS multi_thread_mutually_exclusive_subscriber_node
+        DESTINATION lib/simple_thread_tester)
+
+ament_package()
diff --git a/rclcpp/executors/executor_comparison/README.md b/rclcpp/executors/executor_comparison/README.md
new file mode 100644
index 00000000..c9663401
--- /dev/null
+++ b/rclcpp/executors/executor_comparison/README.md
@@ -0,0 +1,31 @@
+# Simple ros2 package to understand threads
+
+Visit [here](https://motion-boseong.vercel.app/threading-ros2) for explanations.
+
+## Running node
+
+### 1. Single-thread test  
+In terminal A (also for the two belows)
+
+```
+ros2 run simple_thread_tester publisher_node
+```
+
+In terminal B
+
+```
+ros2 run simple_thread_tester single_thread_subscriber_node
+```
+### 2. Multi-thread reentrant callback group test 
+
+```
+ros2 run simple_thread_tester multi_thread_reentrant_subscriber_node
+```
+
+### 3. Multi-thread mutually exclusive callback group test
+
+```
+ros2 run simple_thread_tester multi_thread_reentrant_subscriber_node
+# If simulate mutex 
+ros2 param set /subscriber_node use_mutex true
+```
diff --git a/rclcpp/executors/executor_comparison/multi_thread_mutually_exclusive_subscriber_node.cc b/rclcpp/executors/executor_comparison/multi_thread_mutually_exclusive_subscriber_node.cc
new file mode 100644
index 00000000..b93b68a0
--- /dev/null
+++ b/rclcpp/executors/executor_comparison/multi_thread_mutually_exclusive_subscriber_node.cc
@@ -0,0 +1,138 @@
+#include <chrono>
+#include <random>
+#include <thread>
+
+#include <rclcpp/rclcpp.hpp>
+#include <std_msgs/msg/string.hpp>
+
+using namespace std::chrono_literals;
+
+class MultiThreadMutuallyExclusiveSubscriber : public rclcpp::Node {
+
+public:
+  MultiThreadMutuallyExclusiveSubscriber(rclcpp::NodeOptions node_options)
+      : Node("subscriber_node",
+             node_options.allow_undeclared_parameters(true)) {
+    rclcpp::SubscriptionOptions options;
+    options.callback_group =
+        create_callback_group(rclcpp::CallbackGroupType::MutuallyExclusive);
+
+    short_subscriber_ = create_subscription<std_msgs::msg::String>(
+        "/short_topic", rclcpp::QoS(10),
+        std::bind(&MultiThreadMutuallyExclusiveSubscriber::ShortTopicCallback,
+                  this, std::placeholders::_1),
+        options);
+
+    options.callback_group =
+        create_callback_group(rclcpp::CallbackGroupType::MutuallyExclusive);
+    long_subscriber_ = create_subscription<std_msgs::msg::String>(
+        "/long_topic", rclcpp::QoS(10),
+        std::bind(&MultiThreadMutuallyExclusiveSubscriber::LongTopicCallback,
+                  this, std::placeholders::_1),
+        options);
+
+    timer_ = create_wall_timer(
+        1s, std::bind(&MultiThreadMutuallyExclusiveSubscriber::TimerCallback,
+                      this));
+
+    auto param_change_callback =
+        [this](std::vector<rclcpp::Parameter> parameters) {
+          for (auto parameter : parameters) {
+            if (parameter.get_name() == "use_mutex") {
+              use_mutex = parameter.as_bool();
+              if (use_mutex)
+                RCLCPP_INFO(get_logger(), "will use mutex");
+              else
+                RCLCPP_INFO(get_logger(), "will not mutex");
+            }
+          }
+
+          auto result = rcl_interfaces::msg::SetParametersResult();
+          result.successful = true;
+          return result;
+        };
+
+    callback_handler =
+        this->add_on_set_parameters_callback(param_change_callback);
+  }
+
+private:
+  bool use_mutex{false};
+  struct {
+    std::mutex short_topic_mutex;
+    std::mutex long_topic_mutex;
+  } mutex_list;
+  std::string processed_short_string_;
+  std::string processed_long_string_;
+  void ProcessString(const std::string &raw_string, std::string &output);
+
+  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr short_subscriber_;
+  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr long_subscriber_;
+  rclcpp::TimerBase::SharedPtr timer_;
+  OnSetParametersCallbackHandle::SharedPtr callback_handler;
+
+  void ShortTopicCallback(const std_msgs::msg::String::SharedPtr msg);
+  void LongTopicCallback(const std_msgs::msg::String::SharedPtr msg);
+  void TimerCallback();
+};
+
+void MultiThreadMutuallyExclusiveSubscriber::ProcessString(
+    const std::string &raw_string, std::string &output) {
+  output = "** ";
+
+  for (char c : raw_string) {
+    output.push_back(c);
+    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+  }
+  output += " **";
+};
+
+void MultiThreadMutuallyExclusiveSubscriber::ShortTopicCallback(
+    const std_msgs::msg::String::SharedPtr msg) {
+  std::shared_ptr<std::unique_lock<std::mutex>> lock;
+  if (use_mutex)
+    mutex_list.short_topic_mutex.lock();
+  ProcessString(msg->data, processed_short_string_);
+  RCLCPP_INFO(get_logger(), "Setting processed:  %s",
+              processed_short_string_.c_str());
+  if (use_mutex)
+    mutex_list.short_topic_mutex.unlock();
+}
+
+void MultiThreadMutuallyExclusiveSubscriber::LongTopicCallback(
+    const std_msgs::msg::String::SharedPtr msg) {
+
+  if (use_mutex)
+    mutex_list.long_topic_mutex.lock();
+  ProcessString(msg->data, processed_long_string_);
+  RCLCPP_INFO(get_logger(), "Setting processed:  %s",
+              processed_long_string_.c_str());
+  if (use_mutex)
+    mutex_list.long_topic_mutex.unlock();
+}
+
+void MultiThreadMutuallyExclusiveSubscriber::TimerCallback() {
+  if (use_mutex) {
+    mutex_list.short_topic_mutex.lock();
+    mutex_list.long_topic_mutex.lock();
+  }
+  RCLCPP_INFO(get_logger(),
+              "Getting processed strings: \n  [long] %s \n [short] %s",
+              processed_long_string_.c_str(), processed_short_string_.c_str());
+
+  if (use_mutex) {
+    mutex_list.short_topic_mutex.unlock();
+    mutex_list.long_topic_mutex.unlock();
+  }
+}
+
+int main(int argc, char **argv) {
+  rclcpp::init(argc, argv);
+  rclcpp::NodeOptions option;
+  auto node = std::make_shared<MultiThreadMutuallyExclusiveSubscriber>(option);
+  rclcpp::executors::MultiThreadedExecutor executor;
+  executor.add_node(node);
+  executor.spin();
+  rclcpp::shutdown();
+  return 0;
+}
diff --git a/rclcpp/executors/executor_comparison/multi_thread_reentrant_subscriber_node.cc b/rclcpp/executors/executor_comparison/multi_thread_reentrant_subscriber_node.cc
new file mode 100644
index 00000000..2d62b925
--- /dev/null
+++ b/rclcpp/executors/executor_comparison/multi_thread_reentrant_subscriber_node.cc
@@ -0,0 +1,76 @@
+#include <chrono>
+#include <random>
+#include <thread>
+
+#include <rclcpp/rclcpp.hpp>
+#include <std_msgs/msg/string.hpp>
+
+using namespace std::chrono_literals;
+
+class MultiThreadReentrantSubscriber : public rclcpp::Node {
+
+public:
+  MultiThreadReentrantSubscriber() : Node("subscriber_node") {
+    rclcpp::SubscriptionOptions options;
+    options.callback_group =
+        create_callback_group(rclcpp::CallbackGroupType::Reentrant);
+
+    short_subscriber_ = create_subscription<std_msgs::msg::String>(
+        "/short_topic", rclcpp::QoS(10),
+        std::bind(&MultiThreadReentrantSubscriber::ShortTopicCallback, this,
+                  std::placeholders::_1),
+        options);
+
+    long_subscriber_ = create_subscription<std_msgs::msg::String>(
+        "/long_topic", rclcpp::QoS(10),
+        std::bind(&MultiThreadReentrantSubscriber::LongTopicCallback, this,
+                  std::placeholders::_1),
+        options);
+  }
+
+private:
+  std::string processed_short_string_;
+  std::string processed_long_string_;
+  std::string ProcessString(const std::string &raw_string);
+
+  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr short_subscriber_;
+  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr long_subscriber_;
+  void ShortTopicCallback(const std_msgs::msg::String::SharedPtr msg);
+  void LongTopicCallback(const std_msgs::msg::String::SharedPtr msg);
+};
+
+std::string
+MultiThreadReentrantSubscriber::ProcessString(const std::string &raw_string) {
+  std::thread::id this_id = std::this_thread::get_id();
+  std::ostringstream oss;
+  oss << std::this_thread::get_id();
+
+  std::this_thread::sleep_for(std::chrono::seconds(2));
+  return "** " + raw_string + " **" + " ( by " + oss.str() + ") ";
+};
+
+void MultiThreadReentrantSubscriber::ShortTopicCallback(
+    const std_msgs::msg::String::SharedPtr msg) {
+  auto processed_string = ProcessString(msg->data);
+
+  RCLCPP_INFO(get_logger(), "Setting processed:  %s", processed_string.c_str());
+  processed_short_string_ = processed_string;
+}
+
+void MultiThreadReentrantSubscriber::LongTopicCallback(
+    const std_msgs::msg::String::SharedPtr msg) {
+  auto processed_string = ProcessString(msg->data);
+
+  RCLCPP_INFO(get_logger(), "Setting processed:  %s", processed_string.c_str());
+  processed_long_string_ = processed_string;
+}
+
+int main(int argc, char **argv) {
+  rclcpp::init(argc, argv);
+  auto node = std::make_shared<MultiThreadReentrantSubscriber>();
+  rclcpp::executors::MultiThreadedExecutor executor;
+  executor.add_node(node);
+  executor.spin();
+  rclcpp::shutdown();
+  return 0;
+}
diff --git a/rclcpp/executors/executor_comparison/package.xml b/rclcpp/executors/executor_comparison/package.xml
new file mode 100644
index 00000000..113288fb
--- /dev/null
+++ b/rclcpp/executors/executor_comparison/package.xml
@@ -0,0 +1,19 @@
+<?xml version="1.0"?>
+<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
+<package format="3">
+  <name>simple_thread_tester</name>
+  <version>0.0.0</version>
+  <description>TODO: Package description</description>
+  <maintainer email="jbs@todo.todo">jbs</maintainer>
+  <license>TODO: License declaration</license>
+
+  <buildtool_depend>ament_cmake</buildtool_depend>
+
+
+  <test_depend>ament_lint_auto</test_depend>
+  <test_depend>ament_lint_common</test_depend>
+
+  <export>
+    <build_type>ament_cmake</build_type>
+  </export>
+</package>
\ No newline at end of file
diff --git a/rclcpp/executors/executor_comparison/publisher_node.cc b/rclcpp/executors/executor_comparison/publisher_node.cc
new file mode 100644
index 00000000..5644b590
--- /dev/null
+++ b/rclcpp/executors/executor_comparison/publisher_node.cc
@@ -0,0 +1,54 @@
+#include <random>
+
+#include <rclcpp/rclcpp.hpp>
+#include <std_msgs/msg/string.hpp>
+
+using namespace std::chrono_literals;
+
+class PublisherNode : public rclcpp::Node {
+public:
+  PublisherNode() : Node("publisher_node") {
+    short_publisher_ = create_publisher<std_msgs::msg::String>(
+        "/short_topic", rclcpp::SystemDefaultsQoS());
+    long_publisher_ = create_publisher<std_msgs::msg::String>(
+        "/long_topic", rclcpp::SystemDefaultsQoS());
+
+    short_timer_ =
+        create_wall_timer(100ms, [this]() { PublishShortMessage(); });
+
+    long_timer_ = create_wall_timer(1s, [this]() { PublishLongMessage(); });
+  }
+
+private:
+  size_t short_seq_{0};
+  size_t long_seq_{0};
+
+  rclcpp::Publisher<std_msgs::msg::String>::SharedPtr short_publisher_;
+  rclcpp::Publisher<std_msgs::msg::String>::SharedPtr long_publisher_;
+
+  rclcpp::TimerBase::SharedPtr short_timer_;
+  rclcpp::TimerBase::SharedPtr long_timer_;
+
+  void PublishShortMessage() {
+    std_msgs::msg::String message;
+    message.data = "Short message (seq=" + std::to_string(short_seq_++) + ")";
+    RCLCPP_INFO(get_logger(), message.data, " published.");
+    short_publisher_->publish(message);
+  }
+
+  void PublishLongMessage() {
+    std_msgs::msg::String message;
+    message.data = "Long message with more characters (seq=" +
+                   std::to_string(long_seq_++) + ")";
+    RCLCPP_INFO(get_logger(), message.data, " published.");
+    long_publisher_->publish(message);
+  }
+};
+
+int main(int argc, char **argv) {
+  rclcpp::init(argc, argv);
+  auto node = std::make_shared<PublisherNode>();
+  rclcpp::spin(node);
+  rclcpp::shutdown();
+  return 0;
+}
diff --git a/rclcpp/executors/executor_comparison/single_thread_subscriber_node.cc b/rclcpp/executors/executor_comparison/single_thread_subscriber_node.cc
new file mode 100644
index 00000000..6ddb0042
--- /dev/null
+++ b/rclcpp/executors/executor_comparison/single_thread_subscriber_node.cc
@@ -0,0 +1,63 @@
+#include <chrono>
+#include <random>
+
+#include <rclcpp/rclcpp.hpp>
+#include <std_msgs/msg/string.hpp>
+
+using namespace std::chrono_literals;
+
+class SingleThreadSubscriber : public rclcpp::Node {
+
+public:
+  SingleThreadSubscriber() : Node("subscriber_node") {
+    short_subscriber_ = create_subscription<std_msgs::msg::String>(
+        "/short_topic", rclcpp::QoS(10),
+        std::bind(&SingleThreadSubscriber::ShortTopicCallback, this,
+                  std::placeholders::_1));
+
+    long_subscriber_ = create_subscription<std_msgs::msg::String>(
+        "/long_topic", rclcpp::QoS(10),
+        std::bind(&SingleThreadSubscriber::LongTopicCallback, this,
+                  std::placeholders::_1));
+  }
+
+private:
+  std::string processed_short_string_;
+  std::string processed_long_string_;
+  std::string ProcessString(const std::string &raw_string);
+
+  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr short_subscriber_;
+  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr long_subscriber_;
+  void ShortTopicCallback(const std_msgs::msg::String::SharedPtr msg);
+  void LongTopicCallback(const std_msgs::msg::String::SharedPtr msg);
+};
+
+std::string
+SingleThreadSubscriber::ProcessString(const std::string &raw_string) {
+  std::this_thread::sleep_for(std::chrono::seconds(2));
+  return "** " + raw_string + " **";
+};
+
+void SingleThreadSubscriber::ShortTopicCallback(
+    const std_msgs::msg::String::SharedPtr msg) {
+  auto processed_string = ProcessString(msg->data);
+
+  RCLCPP_INFO(get_logger(), "Setting processed:  %s", processed_string.c_str());
+  processed_short_string_ = processed_string;
+}
+
+void SingleThreadSubscriber::LongTopicCallback(
+    const std_msgs::msg::String::SharedPtr msg) {
+  auto processed_string = ProcessString(msg->data);
+
+  RCLCPP_INFO(get_logger(), "Setting processed:  %s", processed_string.c_str());
+  processed_long_string_ = processed_string;
+}
+
+int main(int argc, char **argv) {
+  rclcpp::init(argc, argv);
+  auto node = std::make_shared<SingleThreadSubscriber>();
+  rclcpp::spin(node);
+  rclcpp::shutdown();
+  return 0;
+}