document the RequestLanes, and "fix" the lanes of a few handlers (#6152)

arangod/ClusterEngine/ClusterRestExportHandler.cpp

@@ -23,18 +23,6 @@
 
 #include "ClusterRestExportHandler.h"
 #include "Basics/Exceptions.h"
-#include "Basics/MutexLocker.h"
-#include "Basics/VelocyPackHelper.h"
-#include "StorageEngine/EngineSelectorFeature.h"
-#include "Utils/Cursor.h"
-#include "Utils/CursorRepository.h"
-#include "VocBase/ticks.h"
-
-#include <velocypack/Builder.h>
-#include <velocypack/Dumper.h>
-#include <velocypack/Iterator.h>
-#include <velocypack/Slice.h>
-#include <velocypack/velocypack-aliases.h>
 
 using namespace arangodb;
 using namespace arangodb::rest;

arangod/ClusterEngine/ClusterRestExportHandler.h

@@ -36,7 +36,7 @@ class ClusterRestExportHandler : public RestVocbaseBaseHandler {
 
  public:
   char const* name() const override final { return "ClusterRestExportHandler"; }
-  RequestLane lane() const override final { return RequestLane::CLUSTER_INTERNAL; }
+  RequestLane lane() const override final { return RequestLane::CLIENT_SLOW; }
   RestStatus execute() override;
 };
 }

arangod/ClusterEngine/ClusterRestReplicationHandler.h

@@ -42,7 +42,7 @@ class ClusterRestReplicationHandler : public RestReplicationHandler {
     return "ClusterRestReplicationHandler";
   }
 
-  RequestLane lane() const override final { return RequestLane::CLUSTER_INTERNAL; }
+  RequestLane lane() const override final { return RequestLane::CLIENT_SLOW; }
 
  private:
 
@@ -76,8 +76,6 @@ class ClusterRestReplicationHandler : public RestReplicationHandler {
 
   /// @brief handle a dump command for a specific collection
   void handleCommandDump() override;
-
- private:
 };
 }
 

arangod/GeneralServer/RequestLane.h

@@ -27,17 +27,61 @@
 
 namespace arangodb {
 enum class RequestLane {
+  // For requests that do not block or wait for something.
+  // This ignores blocks that can occur when delivering
+  // a file from, for example, an NFS mount.
   CLIENT_FAST,
+
+  // For requests that execute an AQL query or tightly
+  // related like simple queries, but not V8 actions,
+  // that do AQL requests, user administrator that
+  // internally uses AQL.
   CLIENT_AQL,
+    
+  // For requests that are executed within an V8 context,
+  // but not for requests that might use a V8 context for
+  // user defined functions.
   CLIENT_V8,
+
+  // For requests that might block or wait for something,
+  // which are not CLIENT_AQL or CLIENT_V8.
   CLIENT_SLOW,
+
+  // For requests between agents. These are basically the
+  // requests used to implement RAFT.
   AGENCY_INTERNAL,
+   
+  // For requests from the DBserver or Coordinator accessing
+  // the agency.
   AGENCY_CLUSTER,
+   
+  // For requests from the DBserver to the Coordinator or
+  // from the Coordinator to the DBserver. But not using
+  // V8 or having high priority.
   CLUSTER_INTERNAL,
+    
+  // For requests from the from the Coordinator to the
+  // DBserver using V8.
   CLUSTER_V8,
+    
+  // For requests from the DBserver to the Coordinator or
+  // from the Coordinator to the DBserver for administration
+  // or diagnostic purpose. Should not block.
   CLUSTER_ADMIN,
+    
+  // For requests used between leader and follower for
+  // replication.
   SERVER_REPLICATION,
+  
+  // For periodic or one-off V8-based tasks executed by the
+  // Scheduler.
   TASK_V8
+
+  // Not yet used:
+  // For requests which go from the agency back to coordinators or
+  // DBservers to report about changes in the agency. They are fast
+  // and should have high prio. Will never block.
+  // AGENCY_CALLBACK`
 };
 
 enum class RequestPriority { HIGH, LOW, V8 };

arangod/RestHandler/RestAqlUserFunctionsHandler.h

@@ -33,7 +33,7 @@ class RestAqlUserFunctionsHandler : public arangodb::RestVocbaseBaseHandler {
 
  public:
   char const* name() const override final { return "RestAqlUserFunctionsHandler"; }
-  RequestLane lane() const override final { return RequestLane::CLIENT_SLOW; }
+  RequestLane lane() const override final { return RequestLane::CLIENT_V8; }
   RestStatus execute() override;
 };
 }

arangod/RestHandler/RestBatchHandler.h

@@ -63,7 +63,9 @@ class RestBatchHandler : public RestVocbaseBaseHandler {
  public:
   RestStatus execute() override;
   char const* name() const override final { return "RestBatchHandler"; }
-  RequestLane lane() const override final { return RequestLane::CLIENT_SLOW; }
+  // be pessimistic about what this handler does... it may invoke V8
+  // or not, but as we don't know where, we need to assume it
+  RequestLane lane() const override final { return RequestLane::CLIENT_V8; }
 
  private:
   RestStatus executeHttp();

arangod/RestHandler/RestTasksHandler.h

@@ -33,7 +33,7 @@ class RestTasksHandler : public arangodb::RestVocbaseBaseHandler {
 
  public:
   char const* name() const override final { return "RestTasksHandler"; }
-  RequestLane lane() const override final { return RequestLane::TASK_V8; }
+  RequestLane lane() const override final { return RequestLane::CLIENT_V8; }
   RestStatus execute() override;
 
  protected:

arangod/RestHandler/RestTransactionHandler.h

@@ -40,7 +40,7 @@ class RestTransactionHandler : public arangodb::RestVocbaseBaseHandler {
 
  public:
   char const* name() const override final { return "RestTransactionHandler"; }
-  RequestLane lane() const override final { return RequestLane::CLIENT_SLOW; }
+  RequestLane lane() const override final { return RequestLane::CLIENT_V8; }
   RestStatus execute() override;
   bool cancel() override final;
 

arangod/RestHandler/RestWalAccessHandler.h

@@ -36,7 +36,7 @@ class RestWalAccessHandler : public arangodb::RestVocbaseBaseHandler {
 
  public:
   char const* name() const override final { return "RestWalAccessHandler"; }
-  RequestLane lane() const override final { return RequestLane::CLUSTER_INTERNAL; }
+  RequestLane lane() const override final { return RequestLane::SERVER_REPLICATION; }
   RestStatus execute() override;
 
  private: