Index: clangd/BuildSystem.h
===================================================================
--- /dev/null
+++ clangd/BuildSystem.h
@@ -0,0 +1,91 @@
+//===--- BuildSystem.h --------------------------------------------*-C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Interfaces for buildsystem integration.
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_BUILD_SYSTEM_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_BUILD_SYSTEM_H
+
+#include "FSProvider.h"
+#include "Function.h"
+#include "GlobalCompilationDatabase.h"
+#include "clang/Tooling/CompilationDatabase.h"
+#include "llvm/ADT/FunctionExtras.h"
+#include "llvm/Support/VirtualFileSystem.h"
+#include <memory>
+#include <mutex>
+
+namespace clang {
+namespace clangd {
+
+class BuildSystem;
+
+/// Default compilation database used by clangd, based on the build system.
+class Integration : public GlobalCompilationDatabase {
+public:
+  /// Loads the build systems by consulting the registry of available
+  /// BuildSystemPlugin instances.
+  static std::unique_ptr<Integration> PluginBased(FileSystemProvider &FS);
+
+  using GlobalLoaderFunc =
+      llvm::unique_function<std::shared_ptr<BuildSystem>()>;
+  /// Will load a single build system once and reuse it for all files that are
+  /// required.
+  static std::unique_ptr<Integration> Global(FileSystemProvider &FS,
+                                             GlobalLoaderFunc Loader);
+
+  ~Integration();
+
+  FileSystemProvider &fs() const override;
+
+  llvm::Optional<CompileInputs> getCompileCommand(PathRef File) const override;
+
+  CommandChanged::Subscription watch(CommandChanged::Listener L) const override;
+
+  class Discovery;
+
+private:
+  Integration(std::unique_ptr<Discovery> D, FileSystemProvider &FS);
+
+  FileSystemProvider &FS;
+  std::unique_ptr<Discovery> D;
+
+  /// An event firing when the build system is loaded.
+  using BSLoadedEvent = Event<std::shared_ptr<BuildSystem>>;
+  BSLoadedEvent::Subscription Subscription;
+
+  mutable CommandChanged OnFilesAdded;
+};
+
+/// Handles interaction between clangd and a build system.
+/// Similar to a CompilationDatabase, but additionally supports change tracking
+/// facilities.
+/// The abstraction is highly experimental, we expect to change the interface in
+/// the future.
+class BuildSystem {
+public:
+  virtual ~BuildSystem() = default;
+
+  /// Provide compilation command for the provided \p Path, possibly building
+  /// all prerequisites if the build system supports it.
+  virtual llvm::Optional<CompileInputs> buildInputs(FileSystemProvider &FS,
+                                                    llvm::StringRef Path) = 0;
+
+  /// Notification that fires when inputs for a set of files might have changed.
+  virtual CommandChanged::Subscription watch(CommandChanged::Listener L) = 0;
+};
+
+/// Attempt to load a build system under \p Dir with a set of plugins.
+/// Does not traverse parent directories.
+std::unique_ptr<BuildSystem> loadWithPlugins(FileSystemProvider &FS,
+                                             llvm::StringRef Dir);
+
+} // namespace clangd
+} // namespace clang
+
+#endif
\ No newline at end of file
Index: clangd/BuildSystemPlugin.h
===================================================================
--- /dev/null
+++ clangd/BuildSystemPlugin.h
@@ -0,0 +1,43 @@
+//===--- BuildSystemPlugin.h --------------------------------------*-C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Interfaces for buildsystem integration.
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_BUILD_SYSTEM_PLUGIN_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_BUILD_SYSTEM_PLUGIN_H
+
+#include "FSProvider.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Registry.h"
+
+namespace clang {
+namespace clangd {
+
+class BuildSystem;
+
+/// A plugin to provide a build system integration for clangd.
+class BuildSystemPlugin {
+public:
+  virtual ~BuildSystemPlugin() = default;
+
+  /// Registered plugins are consulted in the decreasing order of their weights.
+  /// The first plugin that produces a non-null BuildSystem wins.
+  /// The 'compile_commands.json' has a weight of 1.
+  virtual float weight() = 0;
+  /// Attempt to detect and load the build system that resides in \p Dir.
+  /// Should return null if detection failed.
+  virtual std::unique_ptr<BuildSystem> tryLoad(FileSystemProvider &FS,
+                                               llvm::StringRef Dir) = 0;
+};
+
+using BuildSystemPluginRegistry = llvm::Registry<BuildSystemPlugin>;
+
+} // namespace clangd
+} // namespace clang
+
+#endif
\ No newline at end of file
Index: clangd/FSProvider.h
===================================================================
--- clangd/FSProvider.h
+++ clangd/FSProvider.h
@@ -10,6 +10,7 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_FSPROVIDER_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_FSPROVIDER_H
 
+#include "FSWatch.h"
 #include "llvm/ADT/IntrusiveRefCntPtr.h"
 #include "llvm/Support/VirtualFileSystem.h"
 
@@ -21,12 +22,15 @@
 class FileSystemProvider {
 public:
   virtual ~FileSystemProvider() = default;
+
   /// Called by ClangdServer to obtain a vfs::FileSystem to be used for parsing.
   /// Context::current() will be the context passed to the clang entrypoint,
   /// such as addDocument(), and will also be propagated to result callbacks.
   /// Embedders may use this to isolate filesystem accesses.
   virtual llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>
   getFileSystem() const = 0;
+
+  virtual FileSystemWatcher &watcher() = 0;
 };
 
 class RealFileSystemProvider : public FileSystemProvider {
Index: clangd/FSWatch.h
===================================================================
--- /dev/null
+++ clangd/FSWatch.h
@@ -0,0 +1,48 @@
+//===--- FSWatch.h ------------------------------------------------*-C++-*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_FS_WATCH_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANGD_FS_WATCH_H
+
+#include "Function.h"
+#include "llvm/ADT/FunctionExtras.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/VirtualFileSystem.h"
+#include <memory>
+
+namespace clang {
+namespace clangd {
+
+enum class FSEventKind { Added, Modified, Removed };
+
+/// An event that fires when the file has changed.
+using FileChangedEvent = Event<FSEventKind>;
+
+/// An interface to allow watching for changes in the filesystem.
+class FileSystemWatcher {
+public:
+  virtual ~FileSystemWatcher() = default;
+
+  /// Add a watch for a single file or directory.
+  virtual FileChangedEvent::Subscription
+  watchFile(llvm::StringRef Path, FileChangedEvent::Listener L) = 0;
+};
+
+/// Polls the underlying VFS for changes every \p PollIntervalSecs seconds on a
+/// separate thread. Should only be used when a small number of files.
+std::unique_ptr<FileSystemWatcher>
+createPollingWatcher(llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS,
+                     std::chrono::duration<double> PollIntervalSecs);
+
+/// Never reports any changes.
+std::unique_ptr<FileSystemWatcher> createSilentWatcher();
+} // namespace clangd
+} // namespace clang
+
+#endif
\ No newline at end of file
Index: clangd/GlobalCompilationDatabase.h
===================================================================
--- clangd/GlobalCompilationDatabase.h
+++ clangd/GlobalCompilationDatabase.h
@@ -10,8 +10,10 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_GLOBALCOMPILATIONDATABASE_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_GLOBALCOMPILATIONDATABASE_H
 
+#include "FSProvider.h"
 #include "Function.h"
 #include "Path.h"
+#include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/ADT/StringMap.h"
 #include <memory>
 #include <mutex>
@@ -34,15 +36,30 @@
   std::string SourceRoot;
 };
 
+/// Full information required to run clang frontend on the file.
+struct CompileInputs {
+  /// Command used to build the AST.
+  tooling::CompileCommand Command;
+  /// An instance of VFS that should be used for compilations. Must have proper
+  /// working directory set in advance, can contain overlays that include the
+  /// generated files.
+  IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS;
+  /// Information about the project that the file belongs to.
+  llvm::Optional<ProjectInfo> Project;
+};
+
+using CommandChanged = Event<std::vector<std::string>>;
+
 /// Provides compilation arguments used for parsing C and C++ files.
 class GlobalCompilationDatabase {
 public:
   virtual ~GlobalCompilationDatabase() = default;
 
+  virtual FileSystemProvider &fs() const = 0;
+
   /// If there are any known-good commands for building this file, returns one.
-  /// If the ProjectInfo pointer is set, it will also be populated.
-  virtual llvm::Optional<tooling::CompileCommand>
-  getCompileCommand(PathRef File, ProjectInfo * = nullptr) const = 0;
+  virtual llvm::Optional<CompileInputs>
+  getCompileCommand(PathRef File) const = 0;
 
   /// Makes a guess at how to build a file.
   /// The default implementation just runs clang on the file.
@@ -52,12 +69,7 @@
   using CommandChanged = Event<std::vector<std::string>>;
   /// The callback is notified when files may have new compile commands.
   /// The argument is a list of full file paths.
-  CommandChanged::Subscription watch(CommandChanged::Listener L) const {
-    return OnCommandChanged.observe(std::move(L));
-  }
-
-protected:
-  mutable CommandChanged OnCommandChanged;
+  virtual CommandChanged::Subscription watch(CommandChanged::Listener L) const;
 };
 
 /// Gets compile args from tooling::CompilationDatabases built for parent