Support for asynchronous HTTP response processing (#19)

This change introduces WriteResponseAsync which can be used to move potentially slow request handler code (like upstream lookups) off the I/O service thread to ensure we are always able to serve as many HTTP requests as possible. The current implementation defaults to 16 async worker threads and there is currently no back-pressure.

- Added RequestStats - Metrics for network requests. Aggregates tracking of duration, payload sizes into a single class for ease of use
- Added some metrics on upstream communication

Co-authored-by: Per Larsson <[email protected]>

1 files changed, 77 insertions, 0 deletions

diff --git a/zenhttp/workthreadpool.cpp b/zenhttp/workthreadpool.cpp
new file mode 100644
index 000000000..41eaaae94
--- /dev/null
+++ b/zenhttp/workthreadpool.cpp
@@ -0,0 +1,77 @@
+// Copyright Epic Games, Inc. All Rights Reserved.
+
+#include "workthreadpool.h"
+
+#include <zencore/logging.h>
+
+namespace zen {
+
+namespace detail {
+	struct LambdaWork : IWork
+	{
+		LambdaWork(auto Work) : WorkFunction(Work) {}
+		virtual void Execute() override { WorkFunction(); }
+
+		std::function<void()> WorkFunction;
+	};
+}  // namespace detail
+
+WorkerThreadPool::WorkerThreadPool(int InThreadCount)
+{
+	for (int i = 0; i < InThreadCount; ++i)
+	{
+		m_WorkerThreads.emplace_back(&WorkerThreadPool::WorkerThreadFunction, this);
+	}
+}
+
+WorkerThreadPool::~WorkerThreadPool()
+{
+	m_WorkQueue.CompleteAdding();
+
+	for (std::thread& Thread : m_WorkerThreads)
+	{
+		Thread.join();
+	}
+
+	m_WorkerThreads.clear();
+}
+
+void
+WorkerThreadPool::ScheduleWork(Ref<IWork> Work)
+{
+	m_WorkQueue.Enqueue(std::move(Work));
+}
+
+void
+WorkerThreadPool::ScheduleWork(std::function<void()>&& Work)
+{
+	m_WorkQueue.Enqueue(new detail::LambdaWork(Work));
+}
+
+void
+WorkerThreadPool::WorkerThreadFunction()
+{
+	do
+	{
+		Ref<IWork> Work;
+		if (m_WorkQueue.WaitAndDequeue(Work))
+		{
+			try
+			{
+				Work->Execute();
+			}
+			catch (std::exception& e)
+			{
+				Work->m_Exception = std::current_exception();
+
+				ZEN_WARN("Caught exception in worker thread: {}", e.what());
+			}
+		}
+		else
+		{
+			return;
+		}
+	} while (true);
+}
+
+}  // namespace zen
\ No newline at end of file