diff --git a/src/common/expression/test/AggregateExpressionBenchmark.cpp b/src/common/expression/test/AggregateExpressionBenchmark.cpp
index 2e7f32fa7cb..f79cc2fc106 100644
--- a/src/common/expression/test/AggregateExpressionBenchmark.cpp
+++ b/src/common/expression/test/AggregateExpressionBenchmark.cpp
@@ -5,8 +5,6 @@
 
 #include <folly/Benchmark.h>
 
-#include <memory>
-
 #include "common/base/ObjectPool.h"
 #include "common/expression/AggregateExpression.h"
 #include "common/expression/ConstantExpression.h"
diff --git a/src/common/fs/FileUtils.cpp b/src/common/fs/FileUtils.cpp
index f5293de8303..b1183d25055 100644
--- a/src/common/fs/FileUtils.cpp
+++ b/src/common/fs/FileUtils.cpp
@@ -92,8 +92,8 @@ StatusOr<std::string> FileUtils::readLink(const char* path) {
 }
 
 StatusOr<std::string> FileUtils::realPath(const char* path) {
-  char* buffer = ::realpath(path, NULL);
-  if (buffer == NULL) {
+  char* buffer = ::realpath(path, nullptr);
+  if (buffer == nullptr) {
     return Status::Error("realpath %s: %s", path, ::strerror(errno));
   }
   std::string truePath(buffer);
diff --git a/src/common/memory/MemoryUtils.cpp b/src/common/memory/MemoryUtils.cpp
index 7e0bb7db7cc..ede9599c230 100644
--- a/src/common/memory/MemoryUtils.cpp
+++ b/src/common/memory/MemoryUtils.cpp
@@ -5,13 +5,10 @@
 
 #include "common/memory/MemoryUtils.h"
 
-#include <folly/String.h>
 #include <gflags/gflags.h>
 
 #include <algorithm>
-#include <cstdio>
 #include <fstream>
-#include <regex>
 
 #include "common/fs/FileUtils.h"
 
@@ -42,7 +39,7 @@ StatusOr<bool> MemoryUtils::hitsHighWatermark() {
     uint64_t cacheSize = 0;
     for (; iter.valid(); ++iter) {
       auto& sm = iter.matched();
-      cacheSize += std::stoul(sm[2].str(), NULL);
+      cacheSize += std::stoul(sm[2].str(), nullptr);
     }
 
     std::string limitPath =
@@ -64,7 +61,7 @@ StatusOr<bool> MemoryUtils::hitsHighWatermark() {
     std::vector<uint64_t> memorySize;
     for (; iter.valid(); ++iter) {
       auto& sm = iter.matched();
-      memorySize.emplace_back(std::stoul(sm[2].str(), NULL) << 10);
+      memorySize.emplace_back(std::stoul(sm[2].str(), nullptr) << 10);
     }
     std::sort(memorySize.begin(), memorySize.end());
     if (memorySize.size() >= 2u) {
diff --git a/src/common/network/NetworkUtils.cpp b/src/common/network/NetworkUtils.cpp
index 5891ebef09c..937d2f34085 100644
--- a/src/common/network/NetworkUtils.cpp
+++ b/src/common/network/NetworkUtils.cpp
@@ -112,7 +112,7 @@ std::unordered_set<uint16_t> NetworkUtils::getPortsInUse() {
     fs::FileUtils::FileLineIterator iter("/proc/net/tcp", &regex);
     while (iter.valid()) {
       auto& sm = iter.matched();
-      inUse.emplace(std::stoul(sm[1].str(), NULL, 16));
+      inUse.emplace(std::stoul(sm[1].str(), nullptr, 16));
       ++iter;
     }
   }
@@ -120,7 +120,7 @@ std::unordered_set<uint16_t> NetworkUtils::getPortsInUse() {
     fs::FileUtils::FileLineIterator iter("/proc/net/tcp6", &regex);
     while (iter.valid()) {
       auto& sm = iter.matched();
-      inUse.emplace(std::stoul(sm[1].str(), NULL, 16));
+      inUse.emplace(std::stoul(sm[1].str(), nullptr, 16));
       ++iter;
     }
   }
@@ -128,7 +128,7 @@ std::unordered_set<uint16_t> NetworkUtils::getPortsInUse() {
     fs::FileUtils::FileLineIterator iter("/proc/net/udp", &regex);
     while (iter.valid()) {
       auto& sm = iter.matched();
-      inUse.emplace(std::stoul(sm[1].str(), NULL, 16));
+      inUse.emplace(std::stoul(sm[1].str(), nullptr, 16));
       ++iter;
     }
   }
@@ -136,7 +136,7 @@ std::unordered_set<uint16_t> NetworkUtils::getPortsInUse() {
     fs::FileUtils::FileLineIterator iter("/proc/net/udp6", &regex);
     while (iter.valid()) {
       auto& sm = iter.matched();
-      inUse.emplace(std::stoul(sm[1].str(), NULL, 16));
+      inUse.emplace(std::stoul(sm[1].str(), nullptr, 16));
       ++iter;
     }
   }
@@ -144,7 +144,7 @@ std::unordered_set<uint16_t> NetworkUtils::getPortsInUse() {
     fs::FileUtils::FileLineIterator iter("/proc/net/raw", &regex);
     while (iter.valid()) {
       auto& sm = iter.matched();
-      inUse.emplace(std::stoul(sm[1].str(), NULL, 16));
+      inUse.emplace(std::stoul(sm[1].str(), nullptr, 16));
       ++iter;
     }
   }
@@ -152,7 +152,7 @@ std::unordered_set<uint16_t> NetworkUtils::getPortsInUse() {
     fs::FileUtils::FileLineIterator iter("/proc/net/raw6", &regex);
     while (iter.valid()) {
       auto& sm = iter.matched();
-      inUse.emplace(std::stoul(sm[1].str(), NULL, 16));
+      inUse.emplace(std::stoul(sm[1].str(), nullptr, 16));
       ++iter;
     }
   }
@@ -209,7 +209,7 @@ StatusOr<std::vector<HostAddr>> NetworkUtils::resolveHost(const std::string& hos
         continue;
     }
 
-    auto address = ((struct sockaddr_in*)rp->ai_addr)->sin_addr.s_addr;
+    auto address = (reinterpret_cast<struct sockaddr_in*>(rp->ai_addr))->sin_addr.s_addr;
     // We need to match the integer byte order generated by ipv4ToInt,
     // so we need to convert here.
     addrs.emplace_back(intToIPv4(htonl(std::move(address))), port);
diff --git a/src/common/time/test/WallClockBenchmark.cpp b/src/common/time/test/WallClockBenchmark.cpp
index 87c1b0b5f04..9bae3a0c371 100644
--- a/src/common/time/test/WallClockBenchmark.cpp
+++ b/src/common/time/test/WallClockBenchmark.cpp
@@ -16,7 +16,7 @@ using nebula::time::WallClock;
 BENCHMARK(gettimeofday_get_msec, iters) {
   for (uint32_t i = 0; i < iters; i++) {
     struct timeval tp;
-    gettimeofday(&tp, NULL);
+    gettimeofday(&tp, nullptr);
     auto ts = tp.tv_sec * 1000 + tp.tv_usec / 1000;
     folly::doNotOptimizeAway(ts);
   }
@@ -46,7 +46,7 @@ BENCHMARK_DRAW_LINE();
 BENCHMARK(gettimeofday_get_sec, iters) {
   for (uint32_t i = 0; i < iters; i++) {
     struct timeval tp;
-    gettimeofday(&tp, NULL);
+    gettimeofday(&tp, nullptr);
     auto ts = tp.tv_sec;
     folly::doNotOptimizeAway(ts);
   }
diff --git a/src/graph/service/GraphFlags.h b/src/graph/service/GraphFlags.h
index f92ff0a2965..7719e0dd6e2 100644
--- a/src/graph/service/GraphFlags.h
+++ b/src/graph/service/GraphFlags.h
@@ -45,7 +45,7 @@ DECLARE_uint32(failed_login_attempts);
 // The deault value is 0. A value of 0 disables the option.
 DECLARE_uint32(password_lock_time_in_secs);
 
-// optimizer
+// Optimizer
 DECLARE_bool(enable_optimizer);
 
 DECLARE_int64(max_allowed_connections);
diff --git a/src/graph/service/GraphServer.cpp b/src/graph/service/GraphServer.cpp
index c9254d35295..897133a905e 100644
--- a/src/graph/service/GraphServer.cpp
+++ b/src/graph/service/GraphServer.cpp
@@ -40,6 +40,7 @@ bool GraphServer::start() {
     return false;
   }
 
+  // Init worker id for snowflake generating unique id
   nebula::Snowflake::initWorkerId(interface->metaClient_.get());
 
   graphThread_ = std::make_unique<std::thread>([&] {
@@ -90,6 +91,7 @@ void GraphServer::notifyStop() {
   }
 }
 
+// Stop the server.
 void GraphServer::stop() {
   if (serverStatus_.load() == ServiceStatus::STATUS_STOPPED) {
     LOG(INFO) << "The graph server has been stopped";
diff --git a/src/graph/service/GraphServer.h b/src/graph/service/GraphServer.h
index 671c0720605..6c2b2696dde 100644
--- a/src/graph/service/GraphServer.h
+++ b/src/graph/service/GraphServer.h
@@ -24,7 +24,7 @@ class GraphServer {
 
   void stop();
 
-  // used for signal handler to set an internal stop flag
+  // Used for signal handler to set an internal stop flag
   void notifyStop();
 
   void waitUntilStop();
diff --git a/src/graph/service/GraphService.cpp b/src/graph/service/GraphService.cpp
index eddeddf1030..a89fc66c7a1 100644
--- a/src/graph/service/GraphService.cpp
+++ b/src/graph/service/GraphService.cpp
@@ -49,7 +49,7 @@ Status GraphService::init(std::shared_ptr<folly::IOThreadPoolExecutor> ioExecuto
 
   metaClient_ = std::make_unique<meta::MetaClient>(ioExecutor, std::move(addrs.value()), options);
 
-  // load data try 3 time
+  // Load data try 3 time
   bool loadDataOk = metaClient_->waitForMetadReady(3);
   if (!loadDataOk) {
     // Resort to retrying in the background
diff --git a/src/graph/service/PermissionCheck.cpp b/src/graph/service/PermissionCheck.cpp
index d39f88019eb..4dc28ee1dcf 100644
--- a/src/graph/service/PermissionCheck.cpp
+++ b/src/graph/service/PermissionCheck.cpp
@@ -28,11 +28,10 @@ namespace graph {
  * Special operation : kShow, kChangePassword
  */
 
-// static
-Status PermissionCheck::permissionCheck(ClientSession *session,
-                                        Sentence *sentence,
-                                        ValidateContext *vctx,
-                                        GraphSpaceID targetSpace) {
+/* static */ Status PermissionCheck::permissionCheck(ClientSession *session,
+                                                     Sentence *sentence,
+                                                     ValidateContext *vctx,
+                                                     GraphSpaceID targetSpace) {
   if (!FLAGS_enable_authorize) {
     return Status::OK();
   }
@@ -177,7 +176,7 @@ Status PermissionCheck::permissionCheck(ClientSession *session,
     case Sentence::Kind::kShowMetaLeader:
     case Sentence::Kind::kShowHosts: {
       /**
-       * all roles can be show for above operations.
+       * All roles can be show for above operations.
        */
       return Status::OK();
     }
@@ -218,7 +217,7 @@ Status PermissionCheck::permissionCheck(ClientSession *session,
       return Status::OK();
     }
     case Sentence::Kind::kExplain:
-      // everyone could explain
+      // Everyone could explain
       return Status::OK();
     case Sentence::Kind::kSequential: {
       // No permission checking for sequential sentence.
diff --git a/src/graph/service/PermissionManager.cpp b/src/graph/service/PermissionManager.cpp
index ab13091089e..08dd566aa38 100644
--- a/src/graph/service/PermissionManager.cpp
+++ b/src/graph/service/PermissionManager.cpp
@@ -10,8 +10,7 @@
 namespace nebula {
 namespace graph {
 
-// static
-Status PermissionManager::canReadSpace(ClientSession *session, GraphSpaceID spaceId) {
+/* static */ Status PermissionManager::canReadSpace(ClientSession *session, GraphSpaceID spaceId) {
   if (!FLAGS_enable_authorize) {
     return Status::OK();
   }
@@ -35,8 +34,8 @@ Status PermissionManager::canReadSpace(ClientSession *session, GraphSpaceID spac
   return Status::PermissionError("No permission to read space.");
 }
 
-// static
-Status PermissionManager::canReadSchemaOrData(ClientSession *session, ValidateContext *vctx) {
+/* static */ Status PermissionManager::canReadSchemaOrData(ClientSession *session,
+                                                           ValidateContext *vctx) {
   if (!FLAGS_enable_authorize) {
     return Status::OK();
   }
@@ -60,8 +59,7 @@ Status PermissionManager::canReadSchemaOrData(ClientSession *session, ValidateCo
   return Status::PermissionError("No permission to read schema/data.");
 }
 
-// static
-Status PermissionManager::canWriteSpace(ClientSession *session) {
+/* static */ Status PermissionManager::canWriteSpace(ClientSession *session) {
   if (!FLAGS_enable_authorize) {
     return Status::OK();
   }
@@ -71,8 +69,8 @@ Status PermissionManager::canWriteSpace(ClientSession *session) {
   return Status::PermissionError("No permission to write space.");
 }
 
-// static
-Status PermissionManager::canWriteSchema(ClientSession *session, ValidateContext *vctx) {
+/* static */ Status PermissionManager::canWriteSchema(ClientSession *session,
+                                                      ValidateContext *vctx) {
   if (!FLAGS_enable_authorize) {
     return Status::OK();
   }
@@ -100,8 +98,7 @@ Status PermissionManager::canWriteSchema(ClientSession *session, ValidateContext
   return Status::PermissionError("No permission to write schema.");
 }
 
-// static
-Status PermissionManager::canWriteUser(ClientSession *session) {
+/* static */ Status PermissionManager::canWriteUser(ClientSession *session) {
   if (!FLAGS_enable_authorize) {
     return Status::OK();
   }
@@ -116,8 +113,8 @@ Status PermissionManager::canWriteUser(ClientSession *session) {
   }
 }
 
-// static
-Status PermissionManager::canReadUser(ClientSession *session, const std::string &targetUser) {
+/* static */ Status PermissionManager::canReadUser(ClientSession *session,
+                                                   const std::string &targetUser) {
   if (!FLAGS_enable_authorize) {
     return Status::OK();
   }
@@ -180,10 +177,9 @@ Status PermissionManager::canWriteRole(ClientSession *session,
                                  targetUser.c_str());
 }
 
-// static
-Status PermissionManager::canWriteData(ClientSession *session,
-                                       ValidateContext *vctx,
-                                       bool isAdminJob) {
+/* static */ Status PermissionManager::canWriteData(ClientSession *session,
+                                                    ValidateContext *vctx,
+                                                    bool isAdminJob) {
   if (!FLAGS_enable_authorize) {
     return Status::OK();
   }
diff --git a/src/graph/service/PermissionManager.h b/src/graph/service/PermissionManager.h
index 8c013990e38..ebfea701fa3 100644
--- a/src/graph/service/PermissionManager.h
+++ b/src/graph/service/PermissionManager.h
@@ -16,6 +16,7 @@
 namespace nebula {
 namespace graph {
 
+// This module is responsible for checking the permission of the user
 class PermissionManager final {
  public:
   PermissionManager() = delete;
diff --git a/src/graph/service/QueryEngine.cpp b/src/graph/service/QueryEngine.cpp
index d3f4f54e609..477ad18a5f1 100644
--- a/src/graph/service/QueryEngine.cpp
+++ b/src/graph/service/QueryEngine.cpp
@@ -34,6 +34,7 @@ Status QueryEngine::init(std::shared_ptr<folly::IOThreadPoolExecutor> ioExecutor
 
   PlannersRegister::registerPlanners();
 
+  // Set default optimizer rules
   std::vector<const opt::RuleSet*> rulesets{&opt::RuleSet::DefaultRules()};
   if (FLAGS_enable_optimizer) {
     rulesets.emplace_back(&opt::RuleSet::QueryRules());
@@ -43,6 +44,7 @@ Status QueryEngine::init(std::shared_ptr<folly::IOThreadPoolExecutor> ioExecutor
   return setupMemoryMonitorThread();
 }
 
+// Create query context and query instance and execute it
 void QueryEngine::execute(RequestContextPtr rctx) {
   auto qctx = std::make_unique<QueryContext>(std::move(rctx),
                                              schemaManager_.get(),
diff --git a/src/graph/service/QueryInstance.cpp b/src/graph/service/QueryInstance.cpp
index c418dbd1f8d..e3e43aa6433 100644
--- a/src/graph/service/QueryInstance.cpp
+++ b/src/graph/service/QueryInstance.cpp
@@ -44,11 +44,14 @@ void QueryInstance::execute() {
     return;
   }
 
+  // Sentence is explain query, finish
   if (!explainOrContinue()) {
     onFinish();
     return;
   }
 
+  // The execution engine converts the physical execution plan generated by the Planner into a
+  // series of Executors through the Scheduler to drive the execution of the Executors.
   scheduler_->schedule()
       .thenValue([this](Status s) {
         if (s.ok()) {
@@ -67,6 +70,7 @@ Status QueryInstance::validateAndOptimize() {
   auto *rctx = qctx()->rctx();
   auto &spaceName = rctx->session()->space().name;
   VLOG(1) << "Parsing query: " << rctx->query();
+  // Result of parsing, get the parsing tree
   auto result = GQLParser(qctx()).parse(rctx->query());
   NG_RETURN_IF_ERROR(result);
   sentence_ = std::move(result).value();
@@ -85,7 +89,9 @@ Status QueryInstance::validateAndOptimize() {
     }
   }
 
+  // Validate the query, if failed, return
   NG_RETURN_IF_ERROR(Validator::validate(sentence_.get(), qctx()));
+  // Optimize the query, and get the execution plan
   NG_RETURN_IF_ERROR(findBestPlan());
   stats::StatsManager::addValue(kOptimizerLatencyUs, *(qctx_->plan()->optimizeTimeInUs()));
   if (FLAGS_enable_space_level_metrics && spaceName != "") {
@@ -214,6 +220,7 @@ void QueryInstance::addSlowQueryStats(uint64_t latency, const std::string &space
   }
 }
 
+// Get result from query context and fill the response
 void QueryInstance::fillRespData(ExecutionResponse *resp) {
   auto ectx = DCHECK_NOTNULL(qctx_->ectx());
   auto plan = DCHECK_NOTNULL(qctx_->plan());
@@ -223,7 +230,7 @@ void QueryInstance::fillRespData(ExecutionResponse *resp) {
   auto &&value = ectx->moveValue(name);
   if (!value.isDataSet()) return;
 
-  // fill dataset
+  // Fill dataset
   auto result = value.moveDataSet();
   if (!result.colNames.empty()) {
     resp->data = std::make_unique<DataSet>(std::move(result));
@@ -234,6 +241,7 @@ void QueryInstance::fillRespData(ExecutionResponse *resp) {
   }
 }
 
+// The entry point of the optimizer
 Status QueryInstance::findBestPlan() {
   auto plan = qctx_->plan();
   SCOPED_TIMER(plan->optimizeTimeInUs());
diff --git a/src/graph/service/QueryInstance.h b/src/graph/service/QueryInstance.h
index a702b41b7d9..5588999a4bb 100644
--- a/src/graph/service/QueryInstance.h
+++ b/src/graph/service/QueryInstance.h
@@ -29,6 +29,7 @@ class QueryInstance final : public boost::noncopyable, public cpp::NonMovable {
   QueryInstance(std::unique_ptr<QueryContext> qctx, opt::Optimizer* optimizer);
   ~QueryInstance() = default;
 
+  // Entrance of the Validate, Optimize, Schedule, Execute process
   void execute();
 
   QueryContext* qctx() const {
@@ -51,7 +52,7 @@ class QueryInstance final : public boost::noncopyable, public cpp::NonMovable {
   void onError(Status);
 
   Status validateAndOptimize();
-  // return true if continue to execute
+  // Return true if continue to execute
   bool explainOrContinue();
   void addSlowQueryStats(uint64_t latency, const std::string& spaceName) const;
   void fillRespData(ExecutionResponse* resp);
diff --git a/src/graph/service/RequestContext.h b/src/graph/service/RequestContext.h
index ca0eb571ae7..2028401f7e7 100644
--- a/src/graph/service/RequestContext.h
+++ b/src/graph/service/RequestContext.h
@@ -34,7 +34,7 @@ class RequestContext final : public boost::noncopyable, public cpp::NonMovable {
   RequestContext() = default;
   ~RequestContext() {
     if (session_ != nullptr) {
-      // keep the session active
+      // Keep the session active
       session_->charge();
     }
   }
@@ -58,7 +58,7 @@ class RequestContext final : public boost::noncopyable, public cpp::NonMovable {
   void setSession(std::shared_ptr<ClientSession> session) {
     session_ = std::move(session);
     if (session_ != nullptr) {
-      // keep the session active
+      // Keep the session active
       session_->charge();
     }
   }