Index: clangd/Trace.h
===================================================================
--- clangd/Trace.h
+++ clangd/Trace.h
@@ -27,12 +27,29 @@
 namespace clangd {
 namespace trace {
 
+/// A sink for consuming tracing events. Calls to begin_event and end_event with
+/// the same name are always properly nested by using a RAII object (Span).
+/// However, global tracers (i.e., returned from the global Context object) may
+/// be called concurrently and have to be properly synchronized. For per-request
+/// EventTracers (i.e., returned from the request-specific Context objects) the
+/// calls to begin_event and end_event must be properly synchronized and must
+/// never be called concurrently.
 class EventTracer {
 public:
+  /// A key used by trace::Span and trace::log to find EventTracer in a \p
+  /// Context.
   static PtrKey<EventTracer> CtxKey;
 
   virtual ~EventTracer() = default;
-  virtual void event(llvm::StringRef Phase, json::obj &&Contents) = 0;
+
+  /// Called when event with \p Name starts.
+  virtual void begin_event(Context &Ctx, llvm::StringRef Name) = 0;
+  /// Called when event with \p Name ends.
+  virtual void end_event(Context &Ctx, llvm::StringRef Name,
+                         json::obj &&Args) = 0;
+  /// Called for instant events.
+  virtual void instant_event(Context &Ctx, llvm::StringRef Name,
+                             json::obj &&Args) = 0;
 };
 
 /// Create an instance of EventTracer that produces an output in the Trace Event
@@ -66,6 +83,7 @@
 
 private:
   Context &Ctx;
+  std::string Name;
   std::unique_ptr<json::obj> Args;
 };
 
Index: clangd/Trace.cpp
===================================================================
--- clangd/Trace.cpp
+++ clangd/Trace.cpp
@@ -45,9 +45,23 @@
     Out.flush();
   }
 
+  void begin_event(Context &Ctx, llvm::StringRef Name) override {
+    jsonEvent("B", json::obj{{"name", Name}});
+  }
+
+  void end_event(Context &Ctx, llvm::StringRef Name,
+                 json::obj &&Args) override {
+    jsonEvent("E", json::obj{{"name", Name}, {"args", std::move(Args)}});
+  }
+
+  void instant_event(Context &Ctx, llvm::StringRef Name,
+                     json::obj &&Args) override {
+    jsonEvent("i", json::obj{{"name", Name}, {"args", std::move(Args)}});
+  }
+
   // Record an event on the current thread. ph, pid, tid, ts are set.
   // Contents must be a list of the other JSON key/values.
-  void event(StringRef Phase, json::obj &&Contents) override {
+  void jsonEvent(StringRef Phase, json::obj &&Contents) {
     uint64_t TID = get_threadid();
     std::lock_guard<std::mutex> Lock(Mu);
     // If we haven't already, emit metadata describing this thread.
@@ -103,17 +117,14 @@
   EventTracer *T = Ctx.get(EventTracer::CtxKey);
   if (!T)
     return;
-  T->event("i", json::obj{
-                    {"name", "Log"},
-                    {"args", json::obj{{"Message", Message.str()}}},
-                });
+  T->instant_event("Log", json::obj{{"Message", Message.str()}});
 }
 
-Span::Span(Context &Ctx, std::string Name) : Ctx(Ctx) {
-  EventTracer* T = Ctx.get(EventTracer::CtxKey);
+Span::Span(Context &Ctx, std::string Name) : Ctx(Ctx), Name(std::move(Name)) {
+  EventTracer *T = Ctx.get(EventTracer::CtxKey);
   if (!T)
     return;
-  T->event("B", json::obj{{"name", std::move(Name)}});
+  T->begin_event(this->Name);
   Args = llvm::make_unique<json::obj>();
 }
 
@@ -121,9 +132,8 @@
   auto T = Ctx.get(EventTracer::CtxKey);
   if (!T)
     return;
-  if (!Args)
-    Args = llvm::make_unique<json::obj>();
-  T->event("E", Args ? json::obj{{"args", std::move(*Args)}} : json::obj{});
+  assert(Args && "Args can't be null at this point");
+  T->end_event(Name, std::move(*Args));
 }
 
 } // namespace trace